Road Log

0.0 (0.0)       Refer to Map 3B.1.  Junction of FS Rd 21 and FS Rd 18.  Turn left (north) onto FS Rd 18 to begin Field Trip 3B.  A right turn here heading south on FS 18 continues Field Trip 3A.

0.3 (0.3)       Junction of FS Rd 18 and FS Rd 1849.  Turn right (east) onto FS Rd 1849 which swings around the north side of China Hat rhyolite dome and over its associated lava flow (Map 3B.1).  The silicic China Hat Flow has been dated at about 780,000 years old.  The viscous nature of this lava flow is exhibited by its steep sides, thickness (roughly 300 feet thick), and limited aerial extent (about 1 mile wide and 1.5 miles long).

1.2 (0.9)       After crossing a draw, the road begins climbing the side of the China Hat Flow.  As you climb the outer margin of the flow, look to the west; the southeastern flank of Newberry Volcano, peppered with cinder cones, lies at about 9:00.

2.5 (1.3)       “Y” junction of FS Rd 1849 and 1849-500.  Take the left (northeast) fork in the “Y” onto FS Rd 1849-500.  This road crosses the top of the eastern lobe of the China Hat Flow and descends its steep eastern side.  A right fork here continues on FS Rd 1849 to the summit of China Hat’s         dome; unfortunately, the view from the summit is mostly obscured by trees.

3.6 (1.1)       The road cut to the left here exposes rhyolite of the China Hat Flow.

4.0 (0.4)       Junction of FS Rd 1849-500 and FS Rd 2510.  Turn left (northeast) onto FS Rd 2510.

5.0 (1.0)       Refer to Map 3B.2.  FS Rd 2510 climbs onto the northern lobe of the Red Hill Lava Flow here (Figure 3A.15).  This basaltic, aa lava flow originated at Red Hill cinder cone, now some five miles to the west on the upper, east flank of Newberry Volcano.  Drill hole data collected nearby suggests that the relatively fluid lava was channelized by occupying an old gully eroded into the pyroclastic materials on the volcano’s slopes (Jensen, 2006), allowing it to flow a considerable distance downslope and past this location.

5.1 (0.1)       The entrance to the Ground Hog OHV Play Area and Rock Quarry is on the right side of the road.  Turn in here, and drive about two-tenths of a mile to a wide flat staging area for OHVs and park.  Explore the quarry to the left of the staging area at your leisure, but be on the lookout for motorized vehicles.  In several places, the quarry walls nicely expose the stratigraphy of an aa lava flow, displaying a thick rubbly flow top, dense lava flow interior, and thin rubbly flow base (Jensen, 2006).  The rubbled surface of the Red Hill Lava Flow formed as movement of the hotter, more fluid interior of the lava continually broke up the cooling and congealing crust, rafting pieces about, and jamming them into random piles here and there.

When you leave the quarry area, notice the road cut near the junction with FS Rd 2510 exposing Newberry Pumice.  The tephra is still about 45 inches thick here (Figure 3A.6), although pumice lapilli are decreasing in size and abundance and the amount of ashy matrix is increasing.

5.4 (0.3)       Junction of the entrance road to the Ground Hog OHV Play Area and Quarry and FS Rd 2510.  Turn right (east) on FS Rd 2510.

6.5 (1.1)       The road drops off the northern edge of the Red Hill Lava Flow here and onto pyroclastic deposits (Figure 3A.15).

7.5 (1.0)       FS Rd 2510 begins to follow the margin of the younger, basaltic Pot Holes Flow at this point as it swings in from the right (Figure 3A.15).  Just south of this location, the pahoehoe lavas of the Pot Holes Flow bank around the blocky southern edge of the Red Hill Flow and spread downslope to the north and west.

8.5 (1.0)       Refer to Map 3B.3.  Pullout and OHV staging area on the right.  Park here and explore the margin of the Pot Holes Flow.  This widespread lava flow originated from a small cluster of cinder cones and spatter vents about seven miles south of here on the east side of East Butte (Figure 3A.15).  The pahoehoe lavas flowed northward almost eleven miles, spreading out and occupying much of the upper drainage of the major southern tributary of the Dry River system which is flanked by Newberry Volcano on the west and the Pine Mountain silicic dome complex on the east.  The presence of Newberry Pumice has been reduced to a sprinkling of loose pumice fragments, indicating that this location is near the northern margin of the tephra plume.

The Pot Holes Flow is comprised of multiple lobes, many showing signs of inflation (Chitwood, 1994).  Inflated pahoehoe lava flows typically develop where low slopes force the flow front to slow and fan outward, while constructing a complex lava tube system.  Eventually, forward progress of the flow front ceases as internal hydraulic pressures from upslope can no longer overcome the viscosity and strength of the congealing lava surface.  As lava continues to enter the tube-fed systems from upslope, the flow front inflates and forms multiple blisters on the lava crust.  As you carefully negotiate the lava flow margin, notice the inflation blisters, recognizable as portions of the flow front surface with a central fracture surrounded by outward-tilting slabs of crust; also note the flat, smooth- or ropy-textured surfaces on the tilted slabs of the pahoehoe lavas.  These surfaces form as the more solidified crust is buckled by continuing flow of the still hot, plastic-like lava within.

      Return to your vehicle and head northeast on FS Rd 2510.

9.2 (0.7)       FS Rd 2510 continues to follow closely along the western margin of the Pot Holes Flow with good views of Pine Mountain’s silicic dome complex to the northeast.  Pine Mountain is a 22 million year old silicic dome complex surrounded by younger lava flows of the High Lava Plains (Walker and Nolf, 1981).

At this location, the road approaches very near to the edge of the Pot Holes Flow, providing another point of easy access to the flow front and ample pahoehoe lava and inflation features

10.9 (1.7)     Refer to Map 3B.4.  The road crosses a gully here that is cut into the Tepee Draw Tuff (Figure 3.2).   This rhyolitic ash-flow tuff unit is about 300,000 years old and occurs as widespread, scattered outcrops between younger lava flows and within gullies eroded into the slopes of Newberry Volcano’s lower eastern flanks.  It is thought to represent pyroclastic deposition during the early phase of Newberry’s caldera-forming eruptions (MacLeod et al., 1995; Jensen, 2006).  A narrow tongue of the Pot Holes Flow occupies the floor of the gully.

11.4 (0.5)     The road crosses another draw eroded into the Tepee Draw Tuff at this point.  To the right, the leading edge of the Pot Holes Flow poured onto the gully floor (up the gully counter to slope).

11.7 (0.3)     Junction of FS Rd 2510 and FS Rd 25.  Turn right (north) onto FS Rd 25 and continue to follow the western margin of the Pot Holes Flow.

11.9 (0.2)     Good views ahead of Horse Ridge at 12:00 and the Millican Valley at 11:00.  Horse Ridge is formed of late Miocene age (roughly 7 million-year-old) basalts that have been chopped up by normal faulting related to the Brothers Fault Zone (Walker and Nolf, 1981).  The timing of activity on these faults is constrained by the age of the Horse Ridge basalts, and by young late Pleistocene sedimentary fill in Millican Valley that is not displaced.  Faulting associated with the Tumalo Fault Zone visited on Field Trip 1C that cuts Pleistocene lava flows in the Bend area may be an extension of the Brothers Fault Zone.

13.1 (1.2)     Just ahead, FS Rd 25 crosses a cattleguard and passes through a fence, Park in the pullout on the right-hand (east) side of the road, just before crossing the cattleguard.  Tepee Draw and fine exposures of the Tepee Draw Tuff (Figure 3.2) are directly west of the sign for the East Fort Rock OHV Trail System.  Tepee Draw represents what a geologist refers to as a “type locality”, the definitive location for the description of a rock unit, in this case the Tepee Draw Tuff (Figure 3B.1).

Figure 3B.1 - Tepee Draw copyrighted

Figure 3B.1.  A low cliff on the west side of Tepee Draw exposes the Tepee Draw Tuff; this is the type-locality of this geologic bedrock unit.

Cross the road and walk the fence line to the edge of Tepee Draw.  Carefully descend to the floor of Tepee Draw and cross to the low cliff on its far (western) side to examine the excellent exposures of the Tepee Draw Tuff (MacLeod et al., 1981).  This 300,000-year-old rhyolitic ash-flow tuff is exposed over a significant portion of the lower eastern and northeastern slopes of Newberry Volcano, but becomes buried by younger lava flows to the south and northwest and by younger pyroclastic material and slope colluvium higher on Newberry’s eastern flank.  It likely represents the tephra plume deposits associated with early stages of Newberry Volcano’s caldera formation.  The tuff is well jointed and composed of pumice lapilli and rock fragments in a buff-to-pinkish-colored, rhyolitic ashy matrix (Figure 3B.2a).  Pumice lapilli become progressively flattened upwards within the unit.  A greater degree of welding occurs internally upward and toward the caldera rim.  Look carefully for vertically-oriented fossil fumaroles scattered in the cliff-face outcrop, formed just after deposition from hot gases escaping the ash-flow (Figure 3B.2b).

Return to your vehicle and proceed north on FS Rd 25.

Figure 3B.2 - Tepee Draw Tuff

Figure 3B.2.  The Tepee Draw Tuff (A), an ash-flow tuff composed of pumice lapilli and rock fragments in a buff-to-pinkish-colored, rhyolitic ashy matrix; look carefully for the many fine examples of fossil fumaroles (B).

13.5 (0.4)     A good view of Pine Mountain to the east at 4:00.

13.9 (0.4)     Junction of FS Rd 25 (which ends here), FS Rd 23 to the right (southeast), and Deschutes County Rd 23 to the left (north).  Turn right (north) on County Rd 23 for now; you’ll return to this junction momentarily.

14.1 (0.2)     The road crosses Tepee Draw here.

14.2 (0.1)     Junction of Deschutes County Rd 23 and County Rd 2016.  Turn left (northwest) onto County Rd 2016.

15.5 (1.3)     Drive Deschutes County Rd 2016 up onto a wide ridge formed of a lobe of older basaltic lava from Newberry Volcano, to a point where you approach the steeper north-facing slope on the ridge’s northwest side.  Park here, and walk to the highpoint to the left of the road at the edge of the slope.  Faulted Horse Ridge is in full view just to the north (Figure 3B.3).

Figure 3B.3 - Horst and Graben copyrighted

Figure 3B.3.  The northwest trending normal faults of the Brothers Fault Zone disrupting 7 million-year-old basalts of Horse Ridge.

Horse Ridge is comprised of a 7-million-year-old complex of basaltic lava flows and near-vent flow breccias that is cut by multiple northwest trending normal faults of the Brothers Fault Zone (Figure 3.3), some with upwards of 500 feet of vertical offset (Walker and Nolf, 1981).  These faults form a complex graben structure; notice the high ridges to the north and south with a valley in the center, formed of alternate up- and down-thrown fault blocks (Figure 3B.4).  Millican Valley lies to the east of Horse Ridge, occupying a basin central to the Dry River system that was dammed by lava flows and pyroclastic deposits from Newberry Volcano lapping onto the edge of Horse Ridge from the southwest.  The Dry River was forced to carve an outlet to the northwest through the saddle between Horse Ridge and the faulted terrain of Bear Creek Buttes to the north.  The ridgeline of Bear Creek Buttes in also dissected by numerous northwest trending normal faults, some of which have associated localized basaltic vents forming small cinder cones (Walker and Nolf, 1981).  Dry River’s outlet occurs as a deeply- notched canyon complete with meander bends, obviously the product of gradual entrenchment of a low-gradient stream associated with uplift of the greater Horse Ridge – Bear Creek Buttes ridge complex since the late Miocene.  Millican Valley contains a thick sequence of sedimentary fill of the Millican basin, including lacustrine deposits formed during its most recent occupation by the late Pleistocene pluvial Lake Millican.

Figure 3B.4 - Horst and Graben structure on Horse Ridge

Figure 3B.4.  A shaded-relief map displays an aerial view of Horse Ridge; bisecting normal faults (indicated by black lines) describe a classic complex graben structure formed of alternate up- and down-thrown fault blocks.

Turn around for good views of Pine Mountain to the southeast, China Hat and East Butte to the south, and Newberry Volcano and its cinder cone-dimpled eastern flank to the southwest.

Return to your vehicle, turn around, and head back to the junction of Deschutes County Rd 23, FS Rd 23, FS Rd 25.

17.1 (1.6)     Junction of Deschutes County Rd 23, FS Rd 23 to the left, and FS Rd 25 to the right.  Turn left (southeast) onto FS Rd 23.

FS Rd 23 quickly takes you across the eastern branch of Tepee Draw where a narrow finger of very fluid, pahoehoe basalt of the Pot Holes Lava Flow persisted in reaching just downcanyon from this point and now occupies the canyon floor.  The road follows the downslope margin of the Pot Holes Lava Flow for several miles and affords numerous excellent opportunities to visit the flow margin’s superbly preserved inflation features (Chitwood, 1994).  In the background, Newberry Volcano’s cinder cone-dappled eastern flanks are nicely displayed.  The Pine Mountain rhyolite dome complex lies to the left (northeast) of the road.

21.4 (4.3)     Refer to Map 3B.3.  The margin of the lava flow comes close to the road here, and remains close for about half a mile.  Pull to the right-hand edge of the road in a convenient location and park.  Walk over to the lava flow and carefully climb to its top to examine any of a number of beautiful inflation features (Figure 3B.5); this author has never seen better.  As you scramble around, recall our earlier discussion of the development of inflated pahoehoe lava flows at mile 8.5.  A word of caution though, the lava flow’s margin is steep and covered in loose, rough, basaltic rubble; watch your footing.

Figure 3B.5 - Inflated pahoehoe lava

Figure 3B.5.  The downslope  margin of the Pot Holes Lava Flow (A) displays many excellent examples of inflation blisters (B).

23.6 (2.2)     FS Rd 23 rounds a resistant ridge of rhyolitic rock protruding from the lower flank of Pine Mountain; note the dark gray rock in outcrop here which contrasts significantly from the black basalts just observed at mile 21.4 (Figure 3B.6).

Figure 3B.6 - Pine Mountain rhyolite outcrop

Figure 3B.6.  An outcrop of resistant rhyolite along the road is an eroded remnant of the Pine Mountain silicic dome complex, a 22 million year old lava dome surrounded by younger lava flows of the High Lava Plains.

25.8 (2.2)     The road leaves the margin of the Pot Holes Lava Flow at this point.

27.1 (1.3)     Refer to Map 3B.2.  The road begins crossing a lengthy, mostly treeless plain covered in a gradually thickening, then thinning veneer of siliceous tephra; this is the distal portion of the Newberry Pumice tephra plume (Figure 3A.6 and Figure 3B.7).

Figure 3B.7 - Distal Newberry Pumice

Figure 3B.7.  In this location near Pine Mountain, distal to the Big Obsidian eruption, the Newberry Pumice is much thinner and the size of its lapilli is considerably reduced (dime for scale in the inset photo).

28.9 (1.8)     Refer to Map 3B.5.  Junction of FS Rd 23 and FS Rd 2300-308.  FS Rd 23 soon leaves Pumice Flat; the Newberry Pumice is still over two feet thick here.

29.3 (0.4)     The road passes under a major power line corridor here, the power lines come from dams on the Columbia River and are headed for California.  The field trip route parallels the power lines for some distance.

29.9 (0.6)     Intersection of FS Rd 23, FS Rd 22 to the right, and FS Rd 2312 to the left.  Continue straight (southeast) on FS Rd 23.

Sand Springs, an artificially dug, perennially flowing spring, lies near this road intersection; Sand Springs Campground  (another of those eastside facilities with little to offer but a tent space) is immediately down FS Rd 22 to the right.  Tephra stratigraphy originally exposed in the excavation of the spring showed about 20 inches of Newberry Pumice resting on 36 inches of Mazama Ash (Jensen, 2006).  The calcareous cement of the buried soil at the base of the tephra may provide enough induration to create the perched water table here.

There is relatively little in the way of geological interest for about the next ten miles (the sparse ponderosa pine woodland are quite picturesque in the right light, but make viewing difficult), but this route does provide the quickest access to the Christmas Valley basin and passes by lavas of the Devils Garden Lava Flow (and its associated Derrick Cave lava tube and The Blowouts spatter cones), the Four Cones Lava Flow, and Crack-in-the-Ground.

32.8 (2.9)     Notice that the Newberry Pumice thins to almost nothing by this point; you have reached the outer edge of the tephra plume (Figure 3A.6).

Warning: several potentially confusing road junctions lay ahead, follow directions carefully, and be sure that you take the right road!

34.3 (1.5)     “Y” Junction of FS Rd 23 and FS Rd 2315 (to the left).  Veer right (south) to remain on FS Rd 23.

35.2 (0.9)     Junction of FS Rd 23 and FS Rd 2316 (to the right).  Remain to the left (south) on FS Rd 23.  (Don’t cross under the power lines).

36.8 (1.6)     Refer to Map 3B.6.  “Y” Junction of FS Rd 23 and FS Rd 2320.  FS Rd 23 veers off to the right at this point; stay left (south) on FS Rd 2320.  (Don’t cross under the power lines).

37.4 (0.6)     Junction of FS Rd 2320 and FS Rd 2325.  FS Rd 2320 heads to the left at this juncture; turn right (south) onto FS Rd 2325.

As you drive, look to the far right (about 4:00 to 5:00) for intermittent views of Quartz Mountain, a complex of faulted, Pleistocene silicic domes.

37.9 (0.5)     FS Rd 2325 passes to the west of the power line corridor here.  Fox Butte, the tallest cinder cone in eastern Oregon, lies directly to your rear.

40.3 (2.4)     The road passes under the power line corridor and remains on the power line service road (beneath the power lines) for now, heading due south.

41.5 (1.2)     Junction of FS Rd 2325 and FS Rd 2325-800; remain on FS Rd 2325 (south, under the power lines).

41.9 (0.4)     “Y” road junction; veer to the left (now to the east of the power lines) and down slope here to follow a dry gully formed along the outer perimeter of an old, highly eroded cinder cone.  The road remains close to, and sometimes beneath the power lines for more than a mile.

42.8 (0.9)     Deschutes National Forest Boundary.  Entering the Fort Rock District of the Bureau of Land Management.  FS Rd 2325 becomes BLM Rd 6179, a packed earth road.  Normally, this road is suitable for passenger cars, but be careful during wet conditions. The road soon begins passing the eastern margin of the Devils Garden Lava Flow, one of several Holocene basaltic lava fields lying on the northern margin of the Fort Rock-Christmas Valley-Silver Lake basin (Figure 3B.8).

Figure 3B.8 - Lava Fields of Fort Rock-Christmas Valley basin

Figure 3B.8.  Several large Holocene age mafic lava flows and their associated vents and cinder cones lying along the northern margin of the Fort Rock-Christmas Valley-Silver Lake basin.

The main vent area for the Devils Garden Lava Flow can easily be reached from this location by walking about a quarter mile along the fence line to the right (west).  The vent area is bordered by low spatter ramparts (Figure 3B.9); lavas flowing to the northwest were channeled through a well-developed lava gutter system, while flow to the southwest was through soon-to-be-visited Derrick Cave’s large lava tube system.  After your long, dusty drive, the walk here is especially worth it!

Figure 3B.9 - Devils Garden source vent copyrighted

Figure 3B.9.  The main vent of the Devils Garden Lava Flow bordered by low spatter ramparts (in the nearground) and drained by a well-developed lava gutter to the northwest (in the background).

43.1 (0.3)     BLM Rd 6179 passes beneath the power lines here.  Just down the road to your left is a good view of the main vent area for the Devils Garden Flow.  For about the next half mile, the road passes near a chain of small spatter cones aligned along a fissure.

43.5 (0.4)     The parking area for Derrick Cave is on the right (the old road that led directly to Derrick Cave is walled off by a low stone buttress).  Park here and before going to the cave, walk back up the road you just drove down a short distance to examine several well-preserved spatter cones (Figure 3.3B.10) composed of agglutinate spatter blobs (they resemble stacked cow pies).  These cones formed where molten lava spurted up through fractures in the congealed surface of the lava flow.  The pahoehoe lava flow surface in the area surrounding the cave preserves good examples of ropy-texture (Figure 3B.11), formed when the taffy-like, partially solidified surface of the flow is dragged from beneath by hotter, more fluid lava.  The flow surface also displays well-formed squeeze-ups (Figure 3B.11) where fractures in the congealed “crust” of the flow occasionally opened to allowed passage of fluid lava from the flow’s interior to ooze out. For a detailed description of lava tube formation and some examples of flow structures, see the description associated with Lava River Cave in Field Trip 1C.

Figure 3B.10 - Derrick Cave Spatter Cones copyrighted

Figure 3B.10.  Several aligned agglutinate spatter cones near Derrick Cave.

Figure 3B.11 - Lava Squeeze Ups in the Devils Garden Flow copyrighted

Figure 3B.11.  The surface of pahoehoe lavas near Derrick Cave preserve excellent examples of ropy-textures and squeeze-ups, observed side-by-side in this photograph.

Walk back to the parking area and follow the old road beyond the stone buttress to the dual entrances to Derrick Cave (Figure 3.3B.12).  After walking a short distance, the lava tube comes into view.  Derrick Cave is named after H.E. Derrick, a pioneer rancher in this area and the Fort Rock District Ranger in 1909.  Derrick Cave developed just southwest of the main vent for the Devils Garden Lava Flow and was an important conduit for channeling lava away from the vent area to the southwest.  Upslope from the cave entrances, you can see the open lava river channel, with some short, thin-roofed sections.  The northeast cave is a tube about 600 feet long with thin roofs, several collapses creating skylights, and short multi-leveled sections.  The main southwest cave, a tube about 1200 feet long, slopes downward with a deep, narrow passage and thick roof.  Two skylights provide some light in the upper portion of the cave, but a head lamp or lantern is very handy deeper into the cave.

Figure 3B.12 - Derrick Cave map

Figure 3B.12.  A map and cross-section of Derrick Cave (modified from Larson and Larson, 1987b).

The northeast cave is relatively small and not terribly exciting (it was made into a fall-out shelter during the 1950s).  The southwest cave contains some very fine examples of lava tube flow features; its entrance is enormous and certainly beckons to be explored (Figure 3B.13).  The skylights in Derrick Cave are roof collapse features formed long after the lava tube was evacuated for the last time (Figure 3B.14a); they provide interesting mood lighting to the cave and reveal such curiosities as multiple layers of lava peeling from the cave walls that indicate the plastering of lava to the cooler inner tube surface as successive lava flows poured through the tube (Figure 3B.14b), and lava “blisters” peeling from the cave ceiling, formed as still-congealing, taffy-like lava clung to the inner surface of the tube after the last lava flow passed through.  Deeper in the cave, further evidence of multiple lava flows occupying the tube can be observed where a smaller tube is inset within the main tube’s floor (Figure 3B.15).

After viewing this wonderful little cave, make your way back to your vehicle and continue south on BLM Rd 6179.

Figure 3B.13 - Derrick Cave entrance copyrighted

Figure 3B.13.  The southwest entrance to Derrick Cave; formed as a collapse of the lava tube roof.

Figure 3B.14 - Derrick Cave Skylight

Figure 3B.14.  Partial collapse of Derrick Cave’s roof produces not only the cave entrances, but skylights (A) which illuminate some of the cave interior, revealing lava levees pastered to the cave wall (B) that indicate reoccupation of the lava tube by multiple flows.

Figure 3B.15 - Lava tube within a tube copyrighted

Figure 3B.15.  A lava tube preserved on the floor of Derrick Cave provides evidence for multiple lava flow events that occupied the tube in the past.

44.1 (0.6)     The road passes through a gated fence.  This is open range: if the gate is closed, open it, drive through, then close it again.  If the gate is open, leave it open.  You will pass through several more gated fences over the next several miles.  Be courteous to local ranchers, and use the same procedure in each case.

44.2 (0.1)     BLM Rd 6179 passes several spatter cones to your right erupted along the same fissure observed at Derrick Cave; two of the cones are exceptionally large and are called The Blowouts (Figure 3B.16).  Park near the large boulder between the cones and explore.  This makes a nice primitive camping area; bring water and pack out your trash!  The north cone is about 150 feet high and 400 feet in diameter; the south cone is slightly smaller.  These cones formed as a late phase vent eruption that feed lava into the Devils Garden Lava Flow and are likely located along the same fracture system as the main vent further north.  The north cone is breached on its western flank by a lava gutter where pieces of the spatter rampart built around the cone were torn loose and jammed into the channel.  From the eastern spatter rim, you can look north and west toward Newberry Volcano’s southern flank, dimpled by numerous cinder cones and older silicic domes.  Be sure to examine the many fine examples of agglutinate spatter making up the cone’s spatter ramparts (Figure 3B.17).

Figure 3B.16 - View of The Blowouts copyrighted

Figure 3B.16.  The Blowouts, large spatter cones formed in association with the Devils Garden Lava Flow, are found just south of Derrick Cave (the northern spatter cone can be seen through the gap in the southern spatter cone in the foreground).

Figure 3B.17 - Agglutinate Spatter copyrighted

Figure 3B.17.  The spatter cones known as The Blowouts are formed of cow-pat-like blobs of congealed basalt called agglutinate spatter (quarter for scale).

When you have finished your explorations of the spatter cones, return to your vehicle.  Avoid the road here that travels south along the margin of the Devils Garden Lava Field and rejoin the better traveled BLM Rd 6179 just to your east.

44.6 (0.4)     The road once more crosses the path of the power line corridor.

46.0 (1.4)     Refer to Map 3B.7.  Junction with an unnamed road merging from the left that goes past Ludi Butte cinder cone; continue south on BLM Rd 6179.

46.9 (0.9)     Junction with an unnamed road merging from the right; continue south on BLM Rd 6179.  The road to the right travels along the margin of the Devils Garden Lava Field back to The Blowouts.  The small valley that it occupies lies between Homestead Butte to the north and Hogback Butte to the south.  The Devils Garden Lava Flow poured into the upper end of this valley from the northwest.

47.6 (0.7)     An old homestead on the right; still an active ranch.  The road seemingly passes through the rancher’s front yard, so drive slowly and avoid being obtrusive.  Shortly, you will pass the last gated fence, be sure to leave it open or closed, just as it was.  The road is now on back on crushed cinders.

A careful examination of Map 3B.7 reveals that BLM Rd 6179 crosses an older lava flow lobe from Ludi Butte here.  Other lobes from Ludi Butte poured to the east and southeast and are readily discernible from the map.

49.3 (1.7)     Junction of BLM Rd 6179, BLM Rd 6159 to the left, and Lake County Rd 5-12 (Derrick Cave Rd) straight ahead.  Continue south on Lake County Rd 5-12.

50.4 (1.1)     Junction of Lake County Rd 5-12 (Derrick Cave Rd) and Rd 5-12-C.  Lake County Rd 5-12 becomes paved; continue south.  Lake County Rd 5-12-C heads to the left to the BLM’s Fort Rock Station.

51.1 (0.7)     The road comes very near to the inflated flow margin of the Devils Garden Lava Flow at this point as it passes beneath the BPA power lines.

51.7 (0.6)     Once again (and for the last time) the road approaches very close to the margin of the Devils Garden Lava Flow (Figure 3B.8).  This is a great spot away from the power lines to park on the road shoulder, and walk over to explore the lava flow.  The Devils Garden pahoehoe lava flows erupted in late Holocene time and consequently many of their original flow features are still preserved.  Wonderful examples of ropy-textured pahoehoe lava abound.  The flow margin is inflated through a process by which the rapidly congealing flow front is lifted by the infilling of still fluid lava from upslope (Chitwood, 1994).  This area superbly illustrates the hummocky topography that forms on an inflated flow surface as surface lava cools, solidifies and contracts, is blistered upward by fluid lava from underneath, and/or subsides as still fluid lava is evacuated from beneath the congealed surface as it breaks out at the flow front and finds a new avenue forward.  Individual hummocks are known as tumuli.

52.1 (0.4)     Junction of Lake County Rd 5-12 (Derrick Cave Rd) and 5-12-B (Sink Rd).  Rd 5-12-B provides access to the Four Cones Lava Field and Crack-in-the-Ground.  Remain on Lake County Rd 5-12 for now; you’ll return to this junction shortly.  The silicic dome of Cougar Mountain is visible to the southwest and the tuff rings of Table Mountain are visible to the southeast.

53.4 (1.3)     The entrance to a small gravel pit is on the left.  Park on the road shoulder here and walk into the gravel pit to examine its contents.  Notice that this is truly a gravel pit (not a cinder pit) comprised of stratified sand and rounded pebbles and cobbles (Figure 3B.18).  Now walk back to your vehicle and observe that you and the gravel pit are on a low rise or bench between Table Mountain to the east and Cougar Mountain to the west situated at 4480 feet (Map 3B.7).  You have been looking at shoreline sediments associated with the highstand of the pluvial lake that once occupied the Fort Rock-Christmas Valley basin.  Imagine, waves lapped against this beach as little as 12,000 years ago, and all of the basin south of you was filled with water to this level.

Figure 3B.18 - Pluvial Lake Deposits

Figure 3B.18.  Gravelly deposits comprising a beach ridge associated with a late Pleistocene stillstand of pluvial Fort Rock-Christmas Valley Lake; (A) shows a gravel pit excavated into the beach deposits and (B) provides a close up view of the stratified, gravelly nature of the material making up the beach ridge.

This is a good location to briefly describe the formation of pluvial lakes and the pluvial lake history of the northwestern Great Basin.  The Great Basin can be defined on the basis of two geological characteristics; 1) it is a region of block-faulted mountain ranges and intervening valleys, referred to as horsts and grabens, formed by plate tectonic motions that induced the stretching, thinning, and upward buckling of  a vast portion of the North American plate from central Utah to eastern California, and southern Oregon and Idaho to northern Mexico; and 2) it is a region of internally-drained topographic basins that have no outlet to surrounding oceans.  During the Earth’s most recent Ice Age, beginning in the late Pliocene about 2.5 million years ago, this region experienced multiple periods of cooling and reduced evaporation brought on by worldwide glaciations and the growth of the continental ice sheets; and more locally, the growth of mountain glaciers.  At each of these times, the mid-latitude jet stream flowing across North America also shifted south of the Cordilleran and Laurentide ice sheets that expanded to cover much of Canada and the northern United States, bringing enhanced moisture to a large part of the Great Basin.  The combined effects of reduced evaporation and enhanced moisture, plus a little added glacial meltwater, resulted in many of the enclosed basins within the Great Basin to partially fill with water and form pluvial lakes.

In the northwestern Great Basin of south-central and southeastern Oregon, several large pluvial lakes cyclically developed throughout Pleistocene time; the much-reduced remnants of some of these lakes remain today.  The compound Fort Rock-Christmas Valley-Silver Lake basin, on the northern edge of which you now stand, is the northwesternmost internally drained basin in the Great Basin.  The basin is about 64 km long and 40 km wide and was occupied by pluvial Fort Rock- Christmas Valley Lake, probably from late Pliocene to late Pleistocene time (Freidel and McDowell, 1992). Evidence for the former existence of this lake is recorded in well-preserved shoreline features, including wave-cut terraces, sandy to gravelly beaches and spits (such as the beach you are standing on), and deeper-water, muddy sediments.  A higher lake level corresponding to isolated shoreline remnants found at an elevation of 4540 feet occupied the more hydrologically complex Fort Rock -Christmas Valley-Silver Lake basin (Figure 3B.19) at least once during the late Pleistocene (Allison, 1979), although no evidence for this highest-stand lake occurs along this part of the field trip route.

Figure 3B.19 - Fort Rock-Christmas Valley Pluvial Lake

Figure 3B.19.  A map showing the extent of the late Pleistocene highstand of the compound Fort Rock-Silver Lake-Christmas Valley-Fossil Lake pluvial lake.

Cross the road to its right-hand side before returning to your vehicle, and climb to a high point on the beach ridge.  Look to the west at the southern edge of Cougar Mountain, a 4.4 million year old rhyolite dome.  It should be plain that the mountain has a distinct notch eroded into its flank; this is a wave-cut cliff and terrace, the vertical and horizontal surfaces of the notch respectively, formed at the same elevation as this beach.  Caves found along this wave-cut bench were occupied by early Native Americans almost 12,000 years ago, based on radiocarbon dates obtained from charcoal (11,950 years BP) and a tule fiber sandal (8,500 years BP).

Now look to the east at the southern edge of Table Mountain.  Its lower flank nearer the power lines exhibits the same wave-cut feature as Cougar Mountain.  Indirectly, Table Mountain provides further evidence that this basin was once water-filled.  It is a volcanic maar comprised of two overlapping tuff rings (Jensen, 2006), formed by explosive, hydrovolcanic eruptions as rising magma came in contact with groundwater or lake water.  The northern ring of Table Mountain is an elongate, flat-topped ridge 250 feet high and roughly a 1000 feet in diameter.  The southern ring is lower and has about half the diameter.  Both rings are cored by basalt representing lava lakes trapped within the tuff ring craters.  Lava spilled over the north rim of the northern tuff ring and down the outer slope.  Lava flows between the two rings and on the south flank of the southern ring were fed by dikes and sills breaching through the tuff rings that are now exposed along the southern and eastern slopes of Table Mountain.  Eruptions of basaltic magma such as the one at Table Mountain occurred along faults that trend diagonally across the basin and adjacent uplands, forming numerous maar volcanoes of this type within and on the margins of the former lake basin (Heiken, 1971; Heiken et al., 1981; Peterson and Groh, 1961 and 1963).  Many cinder cones and associated lava flows formed concurrently on the uplands and subsequently within the confines of the former lake after it had desiccated (Peterson and Groh, 1963).

Maar volcanoes and tuff rings, of which you will see several more on this field trip, are the result of violent hydrovolcanic explosions that originate when magma rising toward the surface encounters either material saturated by ground water alone, or in the case of the Fort Rock basin maars, sediments directly underlying a body of water.  A maar volcano is generally a relatively shallow, flat-floored explosion crater whose walls are composed predominantly of a ring of tuffaceous material (Heiken, 1971; Peterson and Groh, 1961 and 1963).  The tuff rings generally consist of fragments of the surrounding country rock and minor amounts of magmatic material which is often overlain or interbedded with layers of volcanic ash and breccia.  The pyroclastic material often forms gentle to steeply dipping layers up section within the tuff ring and may show steeply dipping layers which drape back into the crater.  These latter layers may form near the end of the eruption cycle.

In addition to well-preserved shoreline features such as those seen here, the Fort Rock-Christmas Valley basin is underlain by a thick accumulation of lacustrine sediments, and localized interbedded tuffs and alluvium derived from the surrounding uplands (Heiken et al., 1981).  The base of the lacustrine sediments contains thick diatomite layers which are being extracted and processed for industrial and domestic absorbants (Colbath and Steele, 1982).

Return to your vehicle and proceed back to the junction of Lake County Rd 5-12 and 5-12-B.

If you desire to shorten the trip at this point, you can instead continue on Lake County Rd 5-12 (southwest) to Fort Rock, linking up with the end of Field Trip 3B and mile 78.6 of Field Trip 3A.

54.7 (1.3)     Junction of Lake County Rd 5-12 (Derrick Cave Rd) and 5-12-B (Sink Rd).  Turn right (east) onto Rd 5-12-B toward the Four Cones Lava Field and Crack-in-the-Ground.  Much of this road parallels the approximate high stand of the pluvial Fort Rock-Christmas Valley-Silver Lake basin at 4540 feet.

58.4 (3.7)     Lake County Rd 5-12-B crosses a normal fault trace running NW-SE up this small valley, one of several faults you’ll pass along this segment of the field trip route.

These faults, while being fairly unimpressive visually, are significant because they represent the northernmost extension of Basin and Range normal faulting.  You’ll soon see plenty more, some of which are quite impressive.

59.3 (0.9)     Make a brief stop here.  The road crosses another poorly expressed fault trace at this location, see if you can’t pick it out.  The fault trends NW-SE, but curves slightly eastward to the right of the road, and slightly northward, left of the road.  Now look to the northwest to prominent Twin Buttes, a cinder cone nearly chopped in half by this very same fault (Map 3B.7).  North of the cinder cone, the fault trace is expressed by an actual fault scarp.

As you continue driving, notice the dark sea of basaltic lavas just north of the road.  This is another major late Holocene outpouring of lavas in the area called the Lava Mountain Lava Field (Figure 3B.8).

59.9 (0.6)     Lake County Rd 5-12-B crosses another buried fault here; the fault is better expressed further to the south.

60.3 (0.4)     Cross yet another poorly expressed normal fault at this location.

61.2 (0.9)     Refer to Map 3B.8.  Junction of Lake County Rd 5-12-B and the Green Mountain Rd.  Turn right (south) onto the Green Mountain Rd.  Basaltic pahoehoe lavas of the Lava Mountain Lava Field come very close to County Rd 5-12-B just down the road from this junction; it is worth your while continuing a short distance it visit these lavas before heading south on the Green Mountain Rd.

64.0 (2.8)     The summit of Green Mountain and the Green Mountain lookout tower.  On a clear morning or early evening, the view from here is spectacular.  Most stunning are the surrounding dark lava fields which lie in such sharp contrast to the area’s sagebrush and juniper woodlands (Figure 3B.8).  Four Craters Lava Field is the nearest, lying just a few miles to the southeast, with its four prominent cinder cones aligned along a NW-SE trending fault trace.  Green Mountain itself is the northern scoriaceous summit cone (its twin is East Green Mountain plainly visible a stone’s throw to the southeast) of a small, older Pleistocene shield volcano; its basaltic lava-flow- covered flanks long since weathered and covered in a blanket of tephras, soil, and vegetation.

64.1 (0.1)     Green Mountain Campground; primitive, but you can’t beat the solitude or the starry nights.

64.2 (0.1)     A spur road here leads to the summit of East Green Mountain, the southern twin of Green Mountain’s summit cone.

64.7 (0.5)     After descending steeply from the summit cone, the road curves to the east, passing through several grassy meadows on its way toward the western edge of the Four Craters Lava Field.  The route of the field trip switches back and forth between Map 3.3B.8 and Map 3.3B.9 here.

65.5 (0.8)     The road enters a third successive meadow, offering an impressive view due east of the Four Craters Lava Field with its northernmost and largest cinder cone.

66.6 (1.1)     Refer to Map 3.3B.9.  Junction of Green Mountain Rd and Crack-in-the-Ground Rd.  You’ll turn right (south) onto Crack-in-the-Ground Rd here.  The small meadow located here was likely a sag pond formed on the downthrown block of the normal fault you just crossed before reaching this road junction, now filled in with sediment washed in from the surrounding slopes and overgrown with vegetation.  The fault can be traced to the southeast for about three-tenths of a mile, approximately underlying the juniper-meadow edge.

Before continuing onward, pull your vehicle safely to the side of the road and get out to explore the nearby Four Craters Lava Field; this is an excellent opportunity to see an aa lava flow and a cinder cone up close.  Watch your step when navigating the surface of this aa flow.  For a superb view of the expanse of these lavas and the alignment of their associated cinder cones, climb the near side of the cinder cone directly in front of you.  The cinder cone here is the largest cone in the field, and the most accessible.  The cinder cones formed along a fissure eruption, each marking a location where copious fountains of gas-rich basaltic magma was ejected to build a conical mound of scoria.  This cinder cone lies over the fissure vent at the most upslope position, so lavas generally poured down and outward from here (dual breaches indicate where lavas poured to the south and east from its lower flanks).  Looking to the northwest, Green Mountain lies in the near distance.  Careful observation will reveal the scarp of the fault you crossed just before parking which can be easily traced northwestward up the eastern flank of Green Mountain (Figure 3B.20).

      After your jaunt around this area, return to your vehicle and continue on the trip.

Figure 3B.20 - View of Green Mountain copyrighted

Figure 3B.20.  View to the northwest from the summit of the northernmost of four cinder cones aligned NW-SE along the Four Craters Lava Flow; Green Mountain lies to the right in the distance, a fault trace can be observed in the foreground below Green Mountain extending from the road intersection to the photo’s edge.

66.9 (0.3)     The road recrosses the fault trace here onto the upthrown block.  The fault itself disappears under the youthful lavas of the Four Craters Lava Field to the south.  As you ascend the low ridge ahead, look to the east between 9:00 and 11:00 for good views of the second and third of the Four Craters cinder cones.  Note their asymmetry, in part related to accumulation of scoria on the downwind flank of the cone, and in part related to breaching of cones on their western sides.

68.6 (1.7)     The low escarpment to your immediate right is a normal fault; you are driving on the eastern, downthrown block.  This is the northern extension of the same fault that forms Crack-in-the-Ground; the road crosses the fault scarp to the western upthrown block just ahead.

68.7 (0.1)     After crossing the fault, look directly ahead to where you can see that a lobe of the Four Craters Lava Field spilled across the fault trace.  Close examination of the lava flow shows that it has been offset slightly by movement on the fault, suggesting that this Basin and Range fault is still active (Weldon et al., 1992).

69.8 (1.1)     The trailhead parking area for Crack-in-the-Ground is on the left.  Park here, the trail across the road provides access to the Crack-in-the-Ground and to Four Craters Lava Flow.  There are several fairly good primitive camping areas along Crack-in-the-Ground Rd, both north and south of the trailhead.

One of the area’s unique geological features, Crack-in-the-Ground is a northwest trending tension fracture in Early Holocene pahoehoe basaltic lava flows from Green Mountain and is easily observed on Map 3.3B.9.  The Crack is approximately 5 km long and shows a maximum vertical displacement of about 7 meters.  It forms the normal-faulted western boundary of a small 10 km long and 5 km wide graben that is partially filled with younger aa lava flows from the Four Craters Lava Field (Figure 3.3B.8), cinder cones aligned on a short, NW-SE trending fissure on the eastern side of the graben.  Peterson and Groh (1964) suggest that evacuation of magma during eruptions from Four Craters caused localized sinking in the vicinity of Crack-in-the-Ground.  A reexamination by Weldon et al. (1992) indicates that faulting has been active since the Four Craters eruption, and that the fault itself is probably part of a much longer structure that hasn’t ruptured the unconsolidated materials in the area, but has caused the more brittle basalts to fracture.

The walk to the trail registry along an old road bed provides the most convenient access to the Crack-in-the-Ground and is about half a mile round-trip.  Once at the registry, however, you can walk northwest and/or southeast along the western rim of the Crack, or within the Crack itself.  From the rim southeast of the registry, there are good views to the north and east across the graben to the Four Craters cinder cones and Four Craters Lava Flow (Figure 3.3B.21).  If you have little time, a quick walk down the slope to your left from the registry provides good views into the Crack in either direction; examine the fracture to the north and offset along the fault is readily apparent (Figure 3B.22).  Walking within the tension fracture in either direction will require some scrambling over fallen rubble now and then, once in, there are only a few places where you can exit.  This author recommends traversing the Crack as far to the south as you would like, but remember, it’s a scramble through the sagebrush on the way back.  Walking inside the fracture is a unique experience; it affords interesting views of the columnar jointed basalts of the Green Mountain flows (Figure 3.3B.23).  Many of the joints acted as conduits for downward migration of water which over time has created numerous vertically oriented, contorted weathering pits.

After exploring the Crack-in-the-Ground, retrace your steps from the trail registry to your vehicle, drive south.

Figure 3B.21 - Four Craters & Crack in the Ground copyrighted

Figure 3B.21.  A view looking northeast from the west rim of Crack-in-the-Ground’s tension fracture; the fault scarp runs diagonally from the lower right to the upper left of the photo.  Notice that the landscape appears sunken in the middleground, this is the area originally believed to have collapsed as the magma chamber that produced the Four Craters Flow emptied.  The four cinder cones associated with the flow form the larger humps on the skyline, while the Four Craters Flow occupies much of the depression between the cones and Crack-in-the-Ground.

Figure 3B.22 - Crack in the Ground copyrighted

Figure 3B.22.  A short walk past the trail registry brings you to this view of Crack-in-the-Ground (looking northwest); note the down-to-the-northeast offset of the Crack, indicating a normal fault.

Figure 3B.23 - View inside Crack in the Ground copyrighted

Figure 3B.23.  A view from inside Crack-in-the-Ground.

72.3 (2.5)     The road descends a steep scarp here formed in columnar jointed basalt.  The abruptness of the scarp suggests erosion; it probably formed as a wave-cut cliff on the margin of a lower-level stillstand of the pluvial Fort Rock-Christmas Valley Lake (Figure 3B.19).

77.0 (4.7)     Refer to Map 3B.10.  The junction of Crack-in-the-Ground Rd and Lake County Rd 5-14 (Christmas Valley-Wagontire Rd).  Turn right (west) onto Lake County Rd 5-14.  Alternatively, from this junction, it is 7.1 miles east to the junction with Fossil Lake Rd which can be followed north and east to the Fossil Lake Sand Dunes (about 14 miles) or the Lost Forest (about 16 miles) areas; interesting places to visit, but not official destinations described in this guidebook.

78.0 (1.0)     Junction of Lake County Rd 5-14 (Christmas Valley-Wagontire Rd) and Lake County Rd 5-14-F (Old Lake Rd).  Turn left (southwest) onto Lake County Rd 5-14-F.   This happens to be the center of the community of Christmas Valley, OR, and the only location for miles that offers gas, a limited selection of groceries, and bathrooms.

This juncture offers another opportunity to shorten the length of Field Trip 3B.   From here, you can continue on Lake County Rd 5-14 west to it junction with Lake County Rd 5-10 (Fort Rock Rd), linking up with Field Trip 3B at mile 110.7.

81.0 (3.0)     Lake County Rd take a sharp bend to the right (west) here.  The spur road heading straight climbs a succession of shoreline benches of pluvial Fort Rock-Christmas Valley Lake (Figure 3B.19) on the north flank of the highly eroded volcanic Green Hills.

81.8 (0.8)     The road climbs toward the crest of northwest-trending Sevenmile Ridge, cored by bedrock, but plastered with shoreline sandy gravel deposits; this ridge once formed a tombolo connecting rocky islands along the ridge to the main shoreline to the southeast.  The spur road to the right here takes you to an old gravel pit excavated into these pluvial lake sediments.

83.0 (1.2)     The road crosses an earthen dam at the location, confining a small reservoir.  A pullout on your right provides a good spot to view the western part of the Fort Rock-Christmas Valley basin.

83.9 (0.9)     Here, you are on the far side of the tombolo at the edge of a shoreline bench marking a temporary stillstand in the overall recession of pluvial Fort Rock-Christmas Valley Lake.  Looking to the south, one can see other shoreline features etched into the slope at higher positions (the highstand position of the lake occurs at 4540 feet, near the break in slope); and looking west, more shoreline benches exist at lower levels.

88.8 (4.9)     Refer to Map 3B.11.  A ranch road to the right; pull safely to the side of the road here.

A distinctive normal fault scarp lies to the left of the road here; another glimpse of Basin and Range extensional faulting.  To the west lies a superb view of the Table Rock maar.  Table Rock (the tallest promontory) is a tuff cone (recall Flat Top visited on Field Trip 3A), surrounded by a complex of tuff rings that all make up a maar volcano erupted on the edge of a low ridge (the southern end of the Connelly Hills) separating the Fort Rock-Christmas Valley subbasin from the Silver Lake subbasin.  The flanks of the tuff ring complex are eroded by wave action to form two distinct notches from the late Pleistocene pluvial lake that occupied much of the compound basin as little as 12,000 years ago (Figure 3B.19 and Figure 3B.24).  These notches form a pair of wave-cut cliffs and terraces that indicate significant stillstands of water (periods of time when the water level did not change) associated with pluvial Fort Rock-Christmas Valley-Silver Lake.

Figure 3B.24 - Table Rock and shorelines copyrighted

Figure 3B.24.  A pair of wave-cut cliffs and terraces are cut into the southern flank of the Table Rock maar complex, each was formed during a significant stillstand of the late Pleistocene pluvial lake that occupied the FortRock-Christmas Valley-Silver Lake basin.

The bed of now dry Thorn Lake lies at the foot of the leftmost ridge (Map 3B.11); look for other small, semi-circular, dry lake depressions scattered in this part of Christmas Valley.

90.5 (1.7)     Lake County Rd 5-14-F (Old Lake Rd) drops off a normal fault scarp here, clearly visible south of the road.  The conical, juniper covered hills along the fault scarp are small cinder cones, formed where magma locally rose to the surface along the fault.  The two prominent notches cut into the southern flank of the Table Rock maar are well displayed from here.

92.8 (2.3)     Refer to Map 3B.12.  BLM Christmas Valley National Backcountry Byway information kiosk on the right.  Pull in here and park.  After reviewing the interpretive information on the Native American and American history of Christmas Valley, walk over to the intersection of Lake County Rd 5-14-F and Oregon Highway 31.  Cross the highway, and then look south down the axis of Silver Lake Valley.

The Silver Lake basin, and the bounding uplifted blocks of Egli Rim to the east and Winter Rim to the west, form a normal-faulted set of horst-graben-horst (Figure 3B.25).  The block-faulted Silver Lake basin is a ubiquitous feature characteristic of the basin and range topography of the Great Basin extending to the south and east from here.  This is the northernmost basin and range, the only portion of which you will see in this guidebook.  The structures here have formed by ongoing late Teritary and Quaternary extension, upwarp, thinning, and fracture of the Earth’s crust and lithosphere, related to interactions between the North American, the Farallon (Juan de Fuca), and Pacific plates.

Figure 3B.25 - Silver Lake basin copyrighted

Figure 3B.25.  Silver Lake basin, bounded by normal faults along Egli Rim (to the right) and Winter Rim (to the left), forms a classic Basin and Range graben structure, albeit a small one.

The low pass to the east between the Silver Lake basin and the Fort Rock-Christmas Valley basin is comprised of the Pleistocene Table Rock Maar Complex to the north and a northward-tilted fault block of Picture Rock Basalt to the south forming Egli Rim.  These basalts are similar in age and composition to the 15 million year old Steens Basalt covering much of southeastern Oregon, and exposed along the prominent cliffs on the east side of the Silver Lake basin (Heiken et al., 1981).  This pass was submerged under the hydrologically complex pluvial lake that occupied much of the Fort Rock-Silver Lake-Christmas Valley-Fossil Lake basin to a depth of 4540 feet (Figure 3B.19) at least once, and likely several times during the middle and late Pleistocene (Allison, 1979).

92.9 (0.1)     Junction of Lake County Road 5-14-F (Old Lake Rd) and Oregon Highway 31.  Turn right (northwest) onto Hwy 31.  A sand dune ridge burying gravelly lake floor deposits parallels the road to the right for about the next three-quarters of a mile.  This ridge formed when Silver Lake desiccated and prevailing wind blew the finer sediment from the former lake surface, stacking it up on the downwind side of the basin against Table Rock and the Connelly Hills.

93.8 (0.9)     Refer to Map 3B.13.  Now the sand dune ridge parallels the highway to the left side of the road for about the next one and a half miles.

96.1 (2.3)     Refer to Map 3B.14.  Intersection of Oregon Highway 31 and Lake County Rd 5-14 (Christmas Valley-Wagontire Rd).  Turn right (north) onto Lake County Rd 5-14.  After making the turn, pull off onto the road shoulder, park, and walk back across Hwy 31 for another great view of the entire Silver Lake graben.  Piecing Map 3B.12 and Map 3B.13 side-by-side here provides an alternative perspective where the fault-bounded basin is easily observed.

Before returning to your vehicle, take a long look at the Table Rock Maar Complex immediately to the east.  It forms an elongate NNW-SSE trending oval at the divide between the Silver Lake subbasin and main Fork Rock Lake basin (Heiken et al., 1981).  The complex consists of the Table Rock tuff cone, two large tuff rings, and six smaller tuff rings and associated vents which overlie approximately 220 meters of lacustrine sediments, sand and gravel from the adjacent Connelly Hills, and interbedded tuffs (Figure 3B.26).  Recall from earlier discussion that maar volcanoes and tuff rings result from violent hydrovolcanic explosions that originate when magma rising toward the surface encounters either material saturated by ground water or sediments directly underlying a body of water.  A maar volcano is often comprised of a relatively shallow, flat-floored explosion crater whose walls are composed predominantly of a ring of tuffaceous material (Heiken, 1971; Peterson and Groh, 1961 and 1963).  The tuff rings usually consist of fragments of the surrounding country rock and minor amounts of magmatic material which is often overlain or interbedded with layers of volcanic ash and breccia.  The pyroclastic material often forms gentle to steeply outwardly dipping layers up section within the tuff ring and may show steeply dipping layers which drape back into the crater.  Table Rock is capped by resistant basalts that once formed the lava lake filling the center of the tuff cone; erosion has subsequently remove much of the tuffaceous material on the cone’s flanks, similar to the Flat Top tuff cone visited on Field Trip 3A.

Figure 3B.26 - Table Rock Maar Complex

Figure 3B.26.  A simplified geologic map of the Table Rock Maar Complex (modified from Heiken et al., 1981).

A well-defined wave-cut cliff formed at 4540 feet on the west flank of Table Rock lies just east of your location.  This marks the highstand of the late Pleistocene pluvial lake occupying the compound Fort Rock-Christmas Valley-Silver Lake basin (Allison, 1979).

98.0 (1.9)     A nicely expressed NW-SE trending normal fault scarp (displacement down to the west) lies to the left of the road here at 10:00.

99.9 (1.9)     Junction of Lake County Rd 5-14 (Christmas Valley-Wagontire Rd) with Lake County Rd 5-14-B (Table Mountain Rd).  Turn right (east) onto this road which provides access to the interior of the Table Rock Maar Complex.

It is 4.3 miles one-way to the summit of Table Rock’s tuff cone.  The road is in reasonably good shape because Table Rock’s summit houses telecommunications equipment that must be serviced periodically; however, the last mile is considerably rougher than the rest.  If you choose to take this side trip, the road gradually tightens in a clockwise spiral upwards, with the outer rim of the northern, larger tuff ring to the left of your position, and the tuff cone to your right.  Incidentally, this little jaunt offers several primitive camping options on BLM land, and the view of the Silver Lake basin and Basin and Range normal faulting is superb (Figure 3B.27).

Figure 3B.27 - View from Table Rock copyrighted

Figure 3B.27.  The view southward from the summit of Table Rock provides an excellent aerial perspective on the normal faulting in the Silver Lake basin area.

100.7 (0.8)   “Y” junction between Lake County Rd 5-14-B and the service road to the summit of Table Rock.  Turn right (southeast) onto the service road.

102.1 (1.4)  Refer to Map 3B.11.  Park along the outer shoulder of the road in a safe location near here.  Walk over to the rim of the tuff ring and climb to the top of the ridge using the large draw immediately to your east, but watch your step on the crumbly tuff.  It’s about a mile round-trip from the road to the crest of the tuff ring.  Notice that the layers of tuff are tilted toward the interior of the tuff ring as you climb, but the layers are tilted outward once you reach the rim’s apex. Examine closely the composition of the tuff.  It is coarse and angular fragments of basaltic rock in a fine ashy matrix.

Return to your vehicle at your leisure.

103.2 (1.1)   When you reach this point, you are at the first switchback on you upward trek.  This switchback has a wide enough shoulder room to accommodate a couple of parked vehicles.  The last section of road is quite steep and rocky, so you may wish to park here and hike the last mile to the top.  As you drive or hike, note that the rock composition changes from bedded tuff to a thick, dense, fine-grained basalt.  The basalt is the remains of the solidified lava plug and lake that filled the interior of the Table Rock tuff cone, now forming a resistant cap on its upper flanks and summit (similar to Flat Top observed on Field Trip 3A).  Eventually, you’ll reach Table Rock’s summit area.  Great views abound from here: to the south, a spectacular vista of the Silver Lake graben and its adjacent horsts, and associated normal faulting; and to the north, the Fort Rock – Christmas Valley basin.

After soaking up the fine scenery, return to the intersection with Lake County Rd 5-14.

106.5 (3.3)   Refer to Map 3B.14.  Junction of Lake County Rd 5-14-B (Table Mountain Rd) with Lake County Rd 5-14 (Wagontire-Christmas Valley Rd).  Turn right (north) onto Lake County Rd 5-14.

107.7 (1.2)   There is an unmarked dirt road to the right at this point.  Turn in here, drive to the large flat sandy area at the bottom of the draw, and park.  A short hike will bring you to a spectacular outcrop where tuffaceous breccias erupted from one of the smaller tuff rings at the northern end of the Table Rock Maar Complex (Figure 3B.26) are juxtaposed over older lacustrine sediments from a pluvial lake occupying the Fort Rock-Christmas Valley-Silver Lake basin (Heiken et al., 1981).  This location is truly indicative of the relationship between the hydrovolcanic eruption and the saturated sediment that caused it, and should not be missed.

Hike along the old jeep road (now an ATV trail) up the draw to your left (east); the round-trip distance is about half a mile.  Look for a cliff face resembling Swiss cheese left of the road and head directly up slope to its base.  The cliff-face outcrop exposes crudely bedded hyaloclastic tuff-breccia overlying a sequence of interbedded coarse, pumice-bearing, tuffaceous and/or lithic, arenitic to arkosic sandstones, and diatomaceous mudstones (Figure 3B.28).   The basaltic tuff-breccia consists of angular fragments of basalt in an oxidized, ashy matrix. The sandstones contain subangular to rounded pebbles of mafic to intermediate igneous rock derived from the Connely Hills and subangular to subrounded pumice lapilli related to the hydrovolcanic eruption.  Figure 3B.29 provides a close up view of the tuff, while Figure 3B.30 shows an outcrop of lacustrine sandstone comprised of a thinly bedded unit overlain by a massive layer of pumice-lapilli-bearing sandstone.  The sediments are deformed by the overlying weight of the tuff breccias; deformation increases upward in the sequence and at the top sediments are often injected into the overlying tuff as sandstone and mudstone dikes.  A more detailed description of the lacustrine sequence can be found in Heiken et al. (1981).

Figure 3B.28 Fort Rock Lacustrine Deposits copyrighted

Figure 3B.28.  A sequence of hyaloclastic tuff-breccia overlying interbedded coarse, pumice-bearing, tuffaceous and/or lithic, arenitic to arkosic sandstones, and diatomaceous mudstones exposed on the eroded northwest flank of one of the smaller tuff rings at the northwest end of the Table Rock Maar Complex.

Figure 3B.29 - Table Rock Volcanic Breccia copyrighted

Figure 3B.29.  A closeup views of the basaltic tuff-breccia of which the tuff ring is made (dime for scale).

Figure 3B.30 - Lacustrine Sandstones

Figure 3B.30.  An outcrop of lacustrine deposits (A) lying beneath the Table Rock Tuff is comprised of massive, pumice-lapilli- and lithic-bearing sandstone (B) overlying thinly bedded, arenitic to arkosic sandstone (C); dime for scale.

107.9 (0.2)   Return to Lake County Rd 5-14 and turn right (north).

109.7 (1.8)   Dirt roads to the right and left here.  Look to the northwest at about 10:00 to see a nice wave-cut cliff and terrace formed at approximately 1540 feet (Map 3B.14), the highstand of the compound pluvial lake that occupied the Fort Rock-Christmas Valley-Silver Lake basin one or more times during the late Pleistocene.

110.7 (1.0)   Junction of and Lake County Rd 5-14 (Christmas Valley-Wagontire Rd) and Lake County Rd 5-10 (Fort Rock Rd).  Continue straight ahead (north) on Lake County Rd 5-10.

111.6 (0.9)   Lake County Rd 5-10 makes the first of four right-angle bends here (to the west and north); each is spaced at one-mile intervals.

113.6 (2.0)   Refer to Map 3B.15.  This right-angle bend to the left lies at the southwestern edge of a large deflation basin (elongated northwest-southeast) marked by several smaller lake-basin depressions.  This basin has been periodically filled with water and subsequently desiccated and temporarily vegetationless in the late Holocene, indicated by the impressive deflation basin and dune field formed along its northeast margin.

114.6 (1.0)   Look straight ahead at 12:00 before making this right-angle bend to the right.  The bowl-shaped valley cut into the northeast facing flank of the Connelly Hills preserves a superb suite of wave-cut cliffs and shoreline ridges at 4540, 4525, 4475, 4455, 4425, 4410,  4395, 4360, and 4330 feet that mark the highstand and recessional stillstands of pluvial Fort Rock Lake (Figure 3B.19).

117.5 (2.9)   Intersection of Lake County Rd 5-10 (Fort Rock Rd) and Lake County Rd 5-10-C.  Turn left (west) onto Lake County Rd 5-10-C.

The field trip route veers off the paved roads here for a brief look at more shoreline features related to the compound late Pleistocene pluvial lake that occupied the Fort Rock-Christmas Valley-Silver Lake basin.

This short section of the field trip route can be skipped, especially if you’re weary of driving on washboarded gravel roads.  Continue straight (north) on Lake County Rd 5-10 (Fort Rock Rd) to its junction with Lake County Rd 5-12 (Derrick Cave Rd).  Remain on Lake County Rd 5-10 by turning sharply left (west) toward Fort Rock and a link up with the field trip at its terminus in Fort Rock.

118.0 (0.5)   The road rises and falls slightly over the next three-tenths of a mile as it crosses the northern extremity of a spit extending northeast from the Connelly Hills at an elevation of 4330 feet; gravel pits to either side of the road in this area are excavated in the sand and gravel of this low ridge.  This part of the spit marks one of the last recessional stillstands of the late Pleistocene pluvial Fort Rock-Christmas Valley Lake, although it was apparently active throughout desiccation of the pluvial lake.

121.1 (3.1)   Refer to Map 3B.16.  The road gradually approaches the northernmost spur of the Connelly Hills.   Pass a farm entrance road on the right.  Pluvial lake shoreline benches are nicely expressed along the base of the slope facing you to the southwest.

121.8 (0.7)   The road passes just north of the Connelly Hills, now due south at 9:00.

123.3 (1.5)   “T” junction of Lake County Rd 5-10-C with Lake County Rd 5-13.  A small dry lakebed called Morehouse Lake lies directly ahead.  Turn left (south) on Lake County Rd 5-13 for a brief look at some impressive shoreline features tucked away in this valley west of the Connelly Hills.  These are best viewed in morning or early evening sunlight.

124.8 (1.5)   The road begins rising here over several subtle benches (surfaces at roughly 4360, 4410, and 4450 feet are easy to pick out), each a former beach formed during a stillstand (a temporary stabilization of water level) as the Fort Rock-Christmas Valley pluvial lake gradually receded from its maximum position of 4540 feet (Figure 3B.19).

126.0 (1.2)   The road climbs to a beach ridge formed of sand and gravel at this location; this is a prominent spit formed between 4470 and 4430 feet.  Notice that the ridge extends in a semicircle, climbing higher to the east and dipping slightly to the west.  The pass through the Connelly Hills south of you actually lies at a lower elevation and would have been under water when this spit formed.

126.3 (0.3)   After cresting the ridge, the road dips toward a small basin.  A gravel pit lies to the immediate   right, excavated in the sand and gravel deposits of the ridge.  Pull into the gravel pit, park and explore.  Plenty of exposures display the stratified sand and gravel containing rounded pebbles that characterize these beach deposits (Figure 3B.31).

      After your examination of these deposits, return north, back the way you came.

Figure 3B.31 - Sediment associated with Fort Rock Lake spit copyrighted

Figure 3B.31.  Gravelly deposits of a former highstand of pluvial Fort Rock-Christmas Valley Lake.

129.3 (3.0)   Return to the junction of Lake County Rd 5-13 and Lake County Rd 5-10C, but this time, continue straight ahead (north).

132.6 (3.3)   Intersection of Lake County Rd 5-13 with Lake County Rd 5-10 (Fort Rock Rd) in Fort Rock, OR.  This ends Field Trip 3B.

Road Route Maps

Map 3B.1 - China Hat

Map 3B.1.  Color shaded-relief map of the China Hat 7.5” Quadrangle containing segments of Field Trip 3A and 3B.

Map 3B.2 - Firestone Butte

Map 3B.2.  Color shaded-relief map of the Firestone Butte 7.5” Quadrangle containing a segment of Field Trip 3B.

Map 3B.3 - Pine Mountain

Map 3B.3.  Color shaded-relief map of the Pine Mountain 7.5” Quadrangle containing a segment of Field Trip 3B.

Map 3B.4 - Evans Well

Map 3B.4.  Color shaded-relief map of the Evans Well 7.5” Quadrangle containing a segment of Field Trip 3B.

Map 3B.5 - Plot Butte

Map 3B.5.  Color shaded-relief map of the Plot Butte 7.5” Quadrangle containing a segment of Field Trip 3B.

Map 3B.6 - Fox Butte

Map 3B.6.  Color shaded-relief map of the Fox Butte 7.5” Quadrangle containing a segment of Field Trip 3B.

Map 3B.7 - Hogback Butte

Map 3B.7.  Color shaded-relief map of the Hogback Butte 7.5” Quadrangle containing a segment of Field Trip 3B.

Map 3B.8 - Jacks Place

Map 3B.8.  Color shaded-relief map of the Jack’s Place 7.5” Quadrangle containing a segment of Field Trip 3B.

Map 3B.9 - Crack in the Ground

Map 3B.9.  Color shaded-relief map of the Crack-in-the-Ground 7.5” Quadrangle containing a segment of Field Trip 3B.

Map 3B.10 - Christmas Valley

Map 3B.10.  Color shaded-relief map of the Christmas Valley 7.5” Quadrangle containing a segment of Field Trip 3B.

Map 3B.11 - Thorn Lake

Map 3B.11.  Color shaded-relief map of the Thorn Lake 7.5” Quadrangle containing a segment of Field Trip 3B.

Map 3B.12 - Egli Rim

Map 3B.12.  Color shaded-relief map of the Egli Rim 7.5” Quadrangle containing a segment of Field Trip 3B.

Map 3B.13 - Duncan Reservoir

Map 3B.13.  Color shaded-relief map of the Duncan Reservoir 7.5” Quadrangle containing a segment of Field Trip 3B.

Map 3B.14 - Tuff Butte

Map 3B.14.  Color shaded-relief map of the Tuff Butte 7.5” Quadrangle containing a segment of Field Trip 3B.

Map 3B.15 - Schaub Lake

Map 3B.15.  Color shaded-relief map of the Schaub Lake 7.5” Quadrangle containing a segment of Field Trip 3B.

Map 3B.16 - Fort Rock

Map 3B.16.  Color shaded-relief map of the Fort Rock 7.5” Quadrangle containing segments of Field Trip 3A and 3B.