0.0 (0.0) Refer to Map 2A.1. Intersection of Franklin Avenue and U.S. Hwy 97 (3rd Street) in Bend, OR. Drive south and remain on Hwy 97 (3rd Street) through town.
Bend is located just north and slightly west of Newberry Volcano, on a wide expense of basaltic pahoehoe lava flows (Figure 2.2) originating from fissure vents on the shield volcano’s lower north flank that were erupted no more than 780,000 years ago, based on their normal magnetic polarity (Sherrod, et al., 2004). These fluid lavas poured to the north beyond Redmond into the canyons of the Crooked and Deschutes Rivers, and form intracanyon basalts such as those observed at Cove Palisades State Park. The source vents for these lavas were buried by Holocene lava flows associated with the Northwest Rift Zone. The Newberry basalts have been differentiated in the Bend area into two units; the Basalt of Bend (Jensen, 2006) and the Basalt of the Badlands (Sherrod et al., 2004). The Basalt of Bend underlies much of Bend east of the Deschutes River and extends to the southern edge of Redmond. This is the same flow that extended down Newberry’s western flank, forcing a rerouting of the Deschutes River into its modern channel, and the same flow which contains the lava tube of Lava River Cave (visited on Field Trip 1C). The western lobe of the Basalt of the Badlands enters Bend from the northeast and is part of a larger field of lavas erupted from a chain of spatter cones on Newberry’s lower northeast slope. These spatter cones may be a shield volcano on Newberry’s lower flank, or more likely, they represent rootless vents overlying lava tubes draining from an upslope eruptive source higher on Newberry itself. Pilot Butte is the prominent cinder cone on Bend’s eastern edge (visited on Field Trip 1C). This small, middle late Pleistocene volcano and its associated andesitic lava flow have been dated at less than 780,000 years old based on its normal magnetic polarity, but must be older than the Basalt of Bend because it forms a kipuka surrounded by these younger lavas.
0.3 (0.3) Underpass beneath the Burlington Northern Santa Fe Railroad. Road cuts on either side of the road expose typical basalts from Newberry Volcano’s lower flanks (Sherrod et al., 2004). These basaltic lava flows have been offset by NW-SE trending faults along the Tumalo Fault Zone which extend through the Bend area (Jensen, 2006) and are highlighted in Field Trip 1C.
1.6 (1.3) The highway passes over the Central Oregon Canal which diverts water from the Deschutes River for irrigation.
2.9 (1.3) Hwy 97 (3rd Street) crosses a major northwest trending fault trace associated with the Tumalo Fault Zone. The fault block to the northeast has been dropped downward. Look carefully, another fault scarp of the Tumalo Fault Zone is clearly visible on Map 2A.1 north of Pilot Butte.
3.0 (0.1) Junction of U.S. Hwy 97 (3rd Street) and the Bend Parkway. Turn left (south) and remain on Hwy 97.
4.6 (0.9) Refer to Map 2A.2. Southbound off ramp to Baker Rd and Knott Rd, remain on Hwy 97. Almost immediately, the highway crosses the Arnold Canal and another northwest trending fault trace here, with the northeast block dropped downward.
5.1 (0.5) Northbound off ramp to Baker Rd and Knott Rd lies on the opposite side of the highway here. Field Trip 1C diverges from the route here.
5.9 (0.8) Good view of Lava Butte at 12:00; notice Lava Butte’s relatively small size and conical shape; geologist’s refer to this type of volcano as a cinder (or scoria) cone; U.S. Hwy 97 passes around the eastern side of this cinder cone. This volcano and its associated lava flows form part of a northwest trending linament of cones and flows on Newberry Volcano’s lower northwest flank (Figure 2A.1). They erupted along the Northwest Rift Zone (Jensen, 2006), a system of youthful normal faults that cut across Green Mountain northwest of Lava Butte and continues southeast to the East Lake Fissure above East Lake in Newberry Volcano’s caldera rim.
Figure 2A.1. A simplified geologic map of the Northwest Rift Zone of Newberry Volcano showing the distribution of fissure vents, cinder cones, and basaltic lava flows (modified from Jensen, 2006).
9.4 (3.5) Intermittent road cuts on both sides of the highway for about the next half mile reveal a thickening and then thinning blanket of dark tephra erupted from Lava Butte as the highway traverses across the NE trending axis of the cinder cone’s ash plume (Figure 2A.2). Charcoal obtained from the base of the deposit indicates the tephra erupted about 7,000 years ago (Jensen, 2006). A well-preserved section of Mazama ash occurs beneath the Lava Butte tephra. Several cylindrical columns of reworked Lava Butte tephra were excavated here into the top of the Mazama ash during highway expansion in 1988 and Jensen (2006) indicates that these features represent the molds of trees rooted in the underlying Mazama ash. The tephra buried the trees deeply enough to kill them, and as they subsequently rotted, volcanic cinders from the surrounding tephra filled the void to form the tree molds.
Figure 2A.2. A simplified geologic map showing the successive lava flows from Lava Butte, the Gas-Line flows, tephra plumes associated with the lava flow-producing eruptions, and locations where the lava flows blocked and altered the course of the ancestral Deschutes River (tephra data from Jensen, 2006).
9.8 (0.4) FS Rd 9710 lies on the opposite side of the highway here. Field Trip 1B merges with the route at this point.
10.0 (0.2) The basaltic aa lava flow margin from Lava Butte lies on the right side of the highway (Figure 2A.2); Lava Butte is in the background. The highway continues around the eastern margin of the flow for about the next mile.
10.4 (0.4) The Northwest Rift Zone of Newberry Volcano is expressed by a shallow fault scarp to the left side of the road here.
10.5 (0.1) Gas-Line lava flows occur to the left and Lava Butte flows are to the right (Figure 2A.2).
The Gas-Line flows are also dated at about 7,000 years, but underlie the Lava Butte flows. Jensen (2006) suggests eruptive activity along this portion of the Northwest Rift Zone began with vents near the Gas-Line flows and finally coalesced at a vent now under Lava Butte.
10.9 (0.4) Junction of U.S. Highway 97 and FS Rd 9702 to Lava Lands Visitor Center and Lava Butte (these sites are described in Field Trip 1A). Continue south on Hwy 97.
11.9 (1.0) The highway passes over Lava River Cave near here. Not to worry though, the lava tube’s roof is at least 50 feet thick.
12.0 (0.1) Road cuts for about the next 0.3 miles expose the basalts of the Lava River Cave Flow, believed to be the equivalent of the middle to late Pleistocene Basalt of Bend. The vent for this extensive flow occurred upslope from here, but is now buried by younger lavas.
13.4 (1.3) Refer to Map 2A.3. The highway passes over younger basaltic lava flows here, erupted from a chain of cinder cones to the left (east), the largest of which occurs at the site of the Camp Abbot cinder pit (visited just ahead).
14.1 (0.7) Refer to Map 2A.4. Sunriver Junction highway interchange. Exit 153 which provides access to FS Rd 40 and Field Trip 1A on the right (west) or FS Rd 9720 and Field Trip 1B or 1C to the left (east). Continue south on U.S. Hwy 97.
18.6 (4.5) Deep road cut here exposes typical basaltic lava flows from Newberry Volcano.
21.9 (3.3) Junction of U.S. Hwy 97, FS Rd 9735 (left), and the entrance road to La Pine State Recreation Area (right). FS Rd 9735 provides alternate access to young lava flows associated with the Northwest Rift Zone (visited on Field Trip 1B). La Pine State Recreation Area to the right lies about five miles west on the banks of the Deschutes River. Stream bank exposures there exhibit fine examples of La Pine Basin lacustrine and fluvial deposits. As Newberry Volcano has grown during the Pleistocene, lava flows on its western flank have gradually accumulated higher and expanded further westward. This process has forced the Deschutes River to the west and formed a dam to downstream transport of sediment, creating the La Pine Basin. This basin contains as much as 1300 feet of sediment at its center.
23.1 (1.2) Junction of U.S. Hwy 97 and FS Rd 21 (Newberry – Paulina Lake Rd; remain on Hwy 97. A left turn here would take you to Newberry Volcano and merge with Field Trip 3A.
As you continue south, Hwy 97 traverses sediments of the La Pine Basin covered by a veneer of Mazama Ash. The uppermost La Pine basin sediments are chiefly reworked material derived from a dark lapilli tuff blanketing the western slopes of Newberry Volcano.
23.7 (0.6) Refer to Map 2A.5. Crossing Paulina Prairie, the floodplain of Paulina Creek. There is a nice view left at 8:00 of Newberry Volcano and the highest point on its caldera rim, Paulina Peak. Newberry Volcano was named after Dr. John Strong Newberry, the first geologist to explore this area in 1855 while accompanying the Pacific Railroad surveys. Note Newberry Volcano’s broad, shield-like profile covered with dimples; these features are known to geologists as shield volcanoes and are often composed of stacked basaltic lava flows (although its anatomy and recent volcanic history, revealed in Field Trip 3A, is actually more complex). Its back is covered with smaller basaltic cinder cones and rhyolite domes, giving the dimpled appearance.
28.9 (5.2) Intersection of US Hwy 97 and 1st Street (Reed Rd) in La Pine, OR (the city limits). This location marks the starting and ending points of Field Trip 3A; continue through town.
La Pine sits approximately in the central axis of the La Pine Basin where the sedimentary fill is thickest (about 1300 feet thick), and is the geographic location after which the basin is named.
29.8 (0.9) Refer to Map 2A.6. Leaving La Pine, OR.
31.1 (1.3) Junction of U.S. Hwy 97 and Oregon Hwy 31. Continue south on Hwy 97. Hwy 31 provides access to the maar volcanoes and late Pleistocene pluvial lake basins of the greater Silver Lake – Fort Rock – Christmas Valley basin which are explored on Field Trip 3A and 3B.
The highway follows the flat expanse of Long Prairie at the southern end of the La Pine Basin. A chain of small Pleistocene shield volcanoes to the west (Wright Butte, Haner Butte, and Gilchrist Butte, north to south) obscures the view of the High Cascades here.
40.3 (9.2) Refer to Map 2A.7. The highway crests a small ridge here. Good views to the west of Diamond Peak at 3:00 and Mt. Thielsen at 1:00.
42.4 (2.1) Refer to Map 2A.8. The road cuts to the left of the highway in this area are in Mazama Ash, the air-fall pumice from the 7,700 year old cataclysmic eruption of Mt. Mazama which produced the Crater Lake caldera southwest of here.
44.1 (1.7) Refer to Map 2A.9. The long complex ridge now visible on the skyline to the southeast is Walker Rim. Walker Rim represents a series of northeast trending en echelon normal faults with displacement down to the northwest and paralleling the Cascade volcanic arc.
44.6 (0.5) More road cuts on the left expose Mazama Ash; a discerning eye will notice a thickening of the tephra (you are nearer its source).
45.1 (0.6) Passing through the small town of Gilchrist, OR.
46.3 (1.2) Junction of U.S. Hwy 97 and FS Rd 61 (Crescent Cutoff Rd). Turn right (east) onto FS Rd 61.
46.7 (0.4) The road climbs from the valley of the Little Deschutes River as it curves gently to the right; then offers an extended view of Odell Butte, a Pleistocene shield volcano (Figure 2.2), lying straight ahead at 12:00. Look for the fire tower at the butte’s summit; see you at the top!
50.9 (4.2) Refer to Map 2A.10. FS Rd 61 veers right at this road intersection, be sure to stay on it (it heads around the north side of Odell Butte).
52.5 (1.6) The road begins following the margin of a Holocene, basaltic andesite lava flow from Black Rock Butte to the right. Notice the steep-sided flow front and the rough, blocky nature of the flow surface, both features characteristic of aa flows (recall that pahoehoe lavas generally have smooth, ropy-textured surfaces and more gentle flow fronts because of slightly lower viscosity).
53.3 (0.8) Junction of Crescent Cutoff Rd and FS Rd 5815 (Odell Butte Rd). Turn left onto FS Rd 5815 and begin ascending toward the Odell Butte fire tower. As you drive, observe the road cuts on the left side of the road exposing Mazama Ash. Initially, the road is on the downwind side of air-fall deposition from the 7,700 year old Mt. Mazama eruption, but the ash thickens as you round the butte to the upwind side.
55.0 (1.7) A particularly thick exposure of Mazama Ash occurs in a road cut on the left.
58.5 (3.5) “Y” junction of FS Rd 5815 and 5815-500. FS Rd 5815 descends to the right and eventually joins Oregon Hwy 58. FS Rd 5815-500 ascends to the left toward Odell Butte’s summit and its fire tower. Take FS Rd 5815-500.
60.1 (1.6) FS Rd 5815-500 is blocked by a gate. Park here to the left of the gate and examine the exposure at the edge of the parking area. Mazama Ash overlies a soil developed in volcanic colluvium weathered from the slope above. The air-fall deposit is the product of the major Crater Lake caldera-forming eruption of Mount Mazama about 7,700 years ago. Loose air-fall pumice is scattered about on the slope; note how large some of the fragments are, and then consider that the volcanic source is relatively close by, just a few dozen miles to the southwest. Where preserved undisturbed, Mazama Ash serves as a significant stratigraphic marker in central Oregon as well as other parts of the Columbia River basin and Great Basin to the north, east, and south, where its physical and chemical character are readily identifiable. Determining the relationship of a late Quaternary deposit or geomorphic surface to Mazama ash is fundamentally important to determining its approximate age. The position of this site on a slope ensured rapid burial and preservation of the Mazama ash, although the overlying reworked ash and scoria clearly indicates that Mazama ash can be readily striped from steep, unstable slopes where erosive processes are active, so determining its relationship to deposits or surfaces is not always clear.
Now walk up the road the remaining distance to the Odell Butte fire tower, a little over one mile round-trip. Several road cuts on the way expose basaltic volcanic breccias related to the last stages of the Odell Butte eruptions. Normally the fire tower is manned during the summer months, and unless posted as closed, access is permitted onto the deck surrounding the fire tower’s observation platform (Figure 2A.3). The 360° views are awesome! On a clear day, every mountain, butte, bump, lake, pond, and stream can be seen. Newberry Volcano’s shield lies far to the northeast, while the normal fault block of Walker Rim lies somewhat nearer to the southeast. To the southwest, Mt Bailey next to Diamond Lake, and Mt Scott on Crater Lake caldera’s east rim in the background are nicely displayed. Particularly impressive though is glacially-carved Diamond Peak and glacial moraine-dammed Odell Lake and Crescent Lake almost due west of the fire tower (Figure 2A.4). A rewarding view to the north is provided by t he upper watershed of the Deschutes River surrounded by smaller, glacially scoured peaks along the Cascade Crest and larger stratovolcanoes of South Sister, Broken Top, and Mt Bachelor to the east (Figure 2A.5). Just to the northeast, the nearly symmetrical, unglaciated flanks of the Hamner Mountain shield volcano are prominently displayed (Figure 2A.5 and Map 2A.7); with dark, basaltic, Holocene lava flows from Black Rock Butte draped along its southern flank.
When you feel you have taken in the sights sufficiently, return to the parking area and drive back the way you came to the junction of FS Rd 5815 and Crescent Cutoff Rd.
Figure 2A.3. The Odell Butte fire tower.
Figure 2A.4. A view to the west from the Odell Butte fire-tower reveals glacially-carved Diamond Peak and glacial moraine-dammed Odell Lake (right) and Crescent Lake (left) in the upper watershed of the Deschutes River.
Figure 2A.5. The view northward from the Odell Butte fire-tower displays Davis Lake sandwiched between basaltic shield volcanoes to left (The Twins) and right (Hamner Mountain); The Twins lie on the Cascade Crest and were subjected to glacial sculpting during the late Pleistocene Cabot Creek glaciation.
66.9 (6.8) Back to the junction of FS Rd 5815 and Crescent Cutoff Rd. Turn left (west) onto Crescent Cutoff Rd. Black Rock Butte Lava Flow remains on the right for about a quarter mile.
68.7 (1.8) Junction of Crescent Cutoff Rd and FS Rd 46 (Cascade Lakes Highway). Turn right (north) onto FS Rd 46. Remaining on Crescent Cutoff Road begins Field Trip 2B.
70.2 (1.5) Refer to Map 2A.11. Road intersection with FS Rd 46; FS Rd 4672 to the left (west) and FS Rd 4680 to the right (east). Turn right (east) onto FS Rd 4680. This road leads to a small cinder cone, one source of the Black Rock Lava Flow.
72.5 (2.3) Junction of FS Rd 4680 and FS Rd 6210. Take FS Rd 6210 to the right (southeast).
72.8 (0.3) Drive into the obvious cinder quarry on the north flank of the cinder cone on your right and park wherever convenient. Explore the quarry area, but be sure to walk up the road along the outer edge of the cinder cone for a good view to the south of Odell Butte, the Black Rock Butte Lava Flow, and the Black Rock Lava Flow emanating from the southern base of this cinder cone. Much of the cinder cone here has been excavated for road fill, but a dense plug of reddish basalt remains near the center of the cone, probably marking the vent; and exposures in the cone’s flank at the eastern edge of the quarry reveal bedded cinders sloping outward and downward from the now missing summit. Beautiful examples of volcanic breccia, formed of an aggregate of welded reddish basaltic cinders, are found just west of the basalt plug (Figure 2A.6).
Return along FS Rd 4680 to its junction with FS Rd 46 (Cascade Lakes Highway).
Figure 2A.6. A volcanic breccia, formed of an aggregate of semi-welded, reddish, oxidized, basaltic cinders from a small cinder cone associated with the Black Rock Lava Flow; the inset shows a close up of the volcanic breccia (dime for scale).
75.4 (2.6) Junction of FS Rd 4680 and FS Rd 46. Turn right (north) on FS Rd 46.
78.4 (3.0) Begin passing through the area burned by the 2005, Davis Lake Fire. For about the next four miles, as you drive along the highway, notice that some areas appear much more thoroughly burned than others, a patchiness to fire intensity that is typical of forest fires. Although the area looks ravaged, keep in mind that fires are a natural part of forest ecosystems, and that they serve the very basic function of generating a forest diverse in species types and ages.
79.4 (1.0) A good view of The Twins to the left at 9:00, a Pleistocene shield volcano situated on the Cascade Crest. Some lava flows from this volcano reach all the way to this highway as you shall see shortly. Field Trip 2B affords an opportunity to hike to the dual summits of this volcano.
80.2 (0.8) FS Rd 46 rounds a gentle curve to the right and begins traversing the eastern side of Davis Lake (occasionally glimpsed through the trees to your left). Davis Lake formed when the valley of Odell Creek between the older Pleistocene Pine Butte shield volcano to the west and the Davis Mountain shield volcano to the east was damned by eruption of the late Holocene Davis Lake Lava Flow.
84.1 (3.9) Refer to Map 2A.12. The entrance road to Lava Flow Campground on the left. It is about one and a quarter miles to the campground, a good location from which you can view and explore the Davis Lake Lava Flow. This aa lava flow was erupted from a short chain of volcanic vents now buried by the cinder cones near the flow’s center. An alternative site at mile 86.9, which is described in detail, provides easier access to the lava flow and has superb views from the top of the flow toward the north. Continue following the highway, the lava flow margin is on your left.
86.9 (2.8) An abandoned road to the left offers a small parking area; pull in here. This location provides an excellent alternative access to the Davis Lake Lava Flow and an easier climb up its steep flow front (if you visited the lava flow at Lava Flow Campground at the north end of David Lake earlier, then you may not wish to stop here).
Park and walk along the abandoned road (now an ATV/jeep trail) for about 150 feet, then take the unused road to the left at a fork for another 150 feet. Turn left when you see a cleft between two flow lobes and walk to the margin of the Davis Lake Lava Flow. Climb the flow margin to the top of the flow on the right-hand lobe where several large Ponderosa pine have produced a good deal of organic debris, providing a safer footing. Climb carefully! The view north from the top of the flow is quite nice (Figure 2A.7). Notice the steep sided flow front and the rough, blocky nature of the flow surface, both features characteristic of aa flows. These basaltic lavas have a slightly higher silica content, and thus, more viscosity than pahoehoe. Their surfaces tend to cool faster, solidifying, breaking up, and being dragged along by the deeper, hotter, less viscous part of the flow. Consequently, aa lava flows do not flow as fast or as far as pahoehoe, and tend to stack up rather than spread out.
Figure 2A.7. View to the north from the northern margin of the Davis Lake Lava Flow. Wickiup Reservoir is in the foreground, with Mt Bachelor and Broken Top Volcanoes in the background (South Sister is hidden by the small shield volcano of Cultus Butte in the middleground).
87.2 (0.3) The highway crosses a small inlet of Wickiup Reservoir here. Look to the left across the water to the margin of the Davis Lake Lava Flow.
88.1 (0.9) Entrance road for the North Davis Creek Campground on the left. If you missed the abandoned road and parking area at mile 86.9 and wish to return for a look at the Davis Lake Lava Flow, this is a safe place to turn around.
91.8 (3.7) Youthful looking lava flows from The Twins shield volcano cross the highway here and are exposed in road cuts on both sides of the road. These lava flows are buried by late Pleistocene glacial maximum Suttle Lake moraines several miles upslope to the west, so the eruption that produced them occurred prior to about 24,000 to 18,000 years ago.
Drive ahead to a broad shoulder area on the right side of the road near the junction with FS Rd 42, then walk back to examine these flows. In this location, the road cuts display a well-preserved zonation to the lava flow profile, with a rubbly upper layer and a denser, more solid lower layer. The typical, thin, rubbly basal layer is not exposed here. This is common in aa lavas where the flow surface cools and solidifies, only to be broken up and dragged along by the hotter, taffy-like inner flow. The lavas are overlain by a mantle of loess (wind-blown sediment) and Mazama Ash. One particularly large cooling fracture in the lava flow top preserves a nice example of slope colluvium overlain by planar-bedded sediment and tephra (Figure 2A.8). Apparently, to my geologist’s eye, rubble from the flow surface first began filling the fracture, which was then overlain by wind-blown loess, quite likely picked up from nearby unvegetated glacial sediments during deglaciation, only to be capped by a thin layer of tephra from the eruption of Mt. Mazama, and then more fine slope-washed colluvium.
Figure 2A.8. A cooling fracture in The Twins Lava Flow is filled with stratified, post-glacial sediments representing volcanic rubble, loess, tephra (Mazama Ash), and slope colluvium in ascending order.
92.0 (0.2) Junction of FS Rd 46 (Cascade Lakes Highway) and FS Rd 42. Turn right (east) onto FS Rd 42.
The northern margin of The Twins lava flow parallels the road for about half a mile.
You will return to this junction shortly. For those interested in a shorter field trip, this is a convenient breakaway point. FS Rd 42 is entirely paved, and can easily be driven back to the Sunriver Junction on U.S. Hwy 97 (mile 14.1 of this trip), which can then be followed north back to Bend, Oregon.
95.3 (3.3) Browns Mountain Crossing of the Deschutes River. To the north, the Deschutes River flows through a narrow valley between Browns Mountain on the west, an old shield volcano, and the lava flows from the Wuksi Butte-Twin Lakes volcanic chain on the east (Figure 2A.9). To the northeast, the successive cinder cones of the Wuksi Butte-Twin Lakes volcanic chain are in view. Shukash Butte is the tallest of these.
Figure 2A.9. Map showing the cinder cones and maars of the Wuksi Butte-Twin Lakes volcanic chain; PB is Palanush Butte, SP is Shukash Butte, WB is Wuksi Butte, NTL is North Twin Lake, and STL is South Twin Lake (modified from Scott and Gardner, 1990).
96.5 (1.2) Junction of FS Rd 42 and FS Rd 4260 (Twin Lakes Rd). Turn right(south) onto FS Rd 4260.
96.7 (0.2) Entrance to North Twin Lake Campground on the left. It is a half mile round-trip drive to the boat launch parking area. Drive in for a quick look at North Twin Lake (Figure 2A.10). This nearly circular lake has filled a depression that originally formed as a volcanic maar, a crater and surrounding tuff ring generated by the explosive eruption resulting from magma rising toward the surface that came in contact with groundwater-saturated sediment. North Twin Lake’s vent is part of the Wuksi Butte-Twin Lakes volcanic chain (Figure 2A.9).
Figure 2A.10. The nearly circular south shore of North Twin Lake, one of several volcanic maars (tuff rings and associated explosion craters) of the Wuksi Butte-Twin Lakes volcanic chain.
97.2 (0.5) Return to the junction of the campground entrance road and FS Rd 4260. Turn left (south) onto FS Rd 4260.
99.4 (2.2) Pullout on the right. Park here, cross the road to your left, and hike cross-country through the trees, over a low ridge, and down to the shore of South Twin Lake.
South Twin Lake occupies a tuff ring at the southern end of the 7-km-long, north-trending Wuksi Butte-Twin Lakes chain of vents (Figure 2A.9). This volcanic chain is offset about 8 km west of the Mount Bachelor volcanic chain in an en-echelon pattern. Northern vents in the Wuksi Butte-Twin Lakes chain are characterized by scoria cones surrounded by broad aprons of basaltic lava flows, with hyaloclastites forming only a small percentage of the erupted material (Scott, et al., 1989; Scott and Gardner, 1990). However, at the southern end of the chain, volcanism was spectacularly influenced by the explosive eruption of magma into groundwater-saturated sediment at the western edge of the La Pine basin. Southern vents produced only hydrovolcanic deposits and are marked by several tuff rings as much as six tenths of a mile in diameter. Some of the tuff rings are water-filled, forming volcanic maars, such as those occupied by North and South Twin Lakes. These tuff rings have steep inner walls and gentle outer slopes composed of interbedded, mafic, finer-grained air-fall and coarser-grained base surge deposits, produced by numerous explosions as basaltic magma interacted with groundwater.
Now return to the parking area and walk down to the shoreline of Wickiup Reservoir to your right. The reservoir occupies a narrow valley where Davis Creek, flowing in from the southwest, originally joined the Deschutes River which bends around to the northeast here. Shoreline exposures at this site (best viewed at low water level in the fall) are about 600 feet from the shore of South Twin Lake and reveal several meters of partially indurated tuff. The tuff displays planar-parallel, wavy, and low-angle cross beds, showing transport outward from the South Twin crater by basal surges (Figure 2A.11). Mafic volcanic rock fragments and pumice lapilli are scattered throughout the tuff (Figure 2A.12). The tuff overlies stream-deposited sand and pebble gravel, in turn overlying deformed fine-grained sediment and diatomite of La Pine basin fill.
Figure 2A.11. Partially indurated tuff from the eruption that produced the South Twin Lake maar volcano as exposed along the shore of Wickiup Reservoir.
Figure 2A.12. The partially indurated tuff from South Twin maar volcano contains mafic volcanic rock fragments (A) and pumice lapilli (B) within a finer, silicic, ashy matrix; quarter for scale.
The age of the Wuksi Butte-Twin Lakes volcanic chain is not well constrained, although geologic evidence suggests that activity here may have occurred contemporaneously with late Pleistocene early eruptions in the Mount Bachelor chain (Scott, et al., 1989; Scott and Gardner, 1990). Lava flows at the north end of the chain, east of Crane Prairie Reservoir, are overlain by loess and ash in which a well-developed soil formed prior to burial by Mazama ash. The lava flows overlie, and are in part overlain by, outwash of the Suttle Lake advance of the Cabot Creek glaciation, while tuffs at the southern end of the chain overlie this outwash.
Return to your car and drive down the road to Gull Point Campground.
100.0 (0.6) Entrance to Gull Point Campground on the right. This is a wonderful place to camp and is a convenient location for a multi-night stop on this field trip. If you choose to camp here, the shoreline of Wickiup Reservoir can be accessed from several campsites or from the boat launching area. A short hike north up the shoreline will bring you to the same exposures of the South Twin Lake Tuff described above.
In any case, turn around and return to the junction of FS Rd 4260 and FS Rd 42.
103.0 (3.0) Junction of FS Rd 4260 and FS Rd 42. Turn left (west) onto FS Rd 42.
107.5 (4.5) Junction of FS Rd 42 and 46. Turn right (north) on FS Rd 46 (Cascade Lakes Highway).
107.6 (0.1) The road cuts on the left are in lava flows from The Twins (discussed earlier).
108.9 (1.3) Entering Crane Prairie (much of it now forested by Lodgepole pine). Good views ahead of Cultus Mountain at 11:00, Middle and South Sister at 12:00, Broken Top at 1:00, and Mount Bachelor at 2:00.
Road cuts on the right side of the highway ahead expose Mazama ash over a well-developed soil formed in sand and pebble gravel overlying well-bedded silt and sand. Pronounced soil development suggests that these deposits are older than the Suttle Lake glacial advance, although their age is unknown.
113.8 (4.9) Refer to Map 2A.13. Junction of FS Rd 46 (Cascade Lakes Hwy) and FS Rd 4635. Turn left (west) onto FS Rd 4635 which provides access to Cultus and Little Cultus Lakes; Cultus Mountain, a shield volcano whose northern and southern flanks are oversteepened by glacial erosion, lies between the two lakes at 12:00. Both lakes are dammed by terminal moraines of the late Pleistocene glacial maximum. Bevis and Moreland (2013) correlate these moraines with the Suttle Lake advance of the Cabot Creek glaciation (Scott, 1977). Figure 2A.13 displays the extent of the Suttle Lake advance in the entire upper Deschutes River and Tumalo Creek watersheds which includes the Cultus Lake area; while Figure 2A.14 offers a plausible reconstruction of the Suttle Lake glacier system itself (Bevis and Moreland, 2013).
The road traverses Suttle Lake age outwash here.
Figure 2A.13. The extent of the late Pleistocene Suttle Lake advance of the Cabot Creek glaciation in the upper Deschutes River and Tumalo Creek watersheds.
Figure 2A.14. A reconstruction of the Suttle Lake glacier system in the greater upper Deschutes River-Tumalo Creek drainages.
114.6 (0.8) Intersection of FS Rd 4635 and 4630. Continue on FS Rd 4635; the road immediately begins to climb onto the terminal moraine of the late Pleistocene glacial maximum Suttle Lake advance that encircles Cultus Lake. Multiple road cuts for the next three quarters of a mile expose till deposited as moraines.
115.7 (1.1) Parking for the Cultus Lake Day Use Area is on the right. Park here and walk down to the shoreline of Cultus Lake. From the shoreline, looking west, you can see Cultus Mountain at 11:00, and several relatively subdued, eroded shield volcanoes perched along the Cascade Crest, including Irish Mountain at 12:00, Little Roundtop Mountain at 1:00, and Packsaddle Mountain at 2:00.
After taking in the view and perhaps a swim and lunch, return to the intersection of FS Rd 4635 and FS Rd 46. Alternatively, the Cultus Lake Campground just down the road is another great place to camp and a convenient location for a multi-night stop on this field trip.
117.6 (1.9) Junction of FS Rd 4635 and FS Rd 46 (Cascade Lakes highway). Turn left (north) onto FS Rd 46.
117.8 (0.2) Good views of Mt. Bachelor at 11:00, Sheridan Mountain at 12:00, and Lookout Mountain at 2:00. Mt. Bachelor stratovolcano and Sheridan Mountain shield volcano are the largest members of the Mt. Bachelor volcanic chain (Figure 2.2). Lookout Mountain shield volcano is somewhat older and not related to the others.
119.5 (1.7) The southeastern margin of the dacitic lava flow associated with Bench Mark Butte first comes into view on the left side of the road.
120.0 (0.5) The road cuts on the left here, and for about the next one and a half miles, expose the dacite of Bench Mark Butte (Map 2A.13). This butte is an isolated occurrence of silicic volcanism in an area dominated by mafic eruptions. The dacitic lava flows were viscous when erupted, stacking up to 120 meters in thickness, forming a flat-topped, steep-sided butte covering several square kilometers. In outcrop, the dacite is fine-grained to glassy, containing inclusions of more mafic rock (Figure 2A.15). Till and outwash of the Suttle Lake glaciation overlap its northern and western flanks (Figure 2A.13); this, combined with its well-preserved morphology, suggests that the eruptions which constructed Bench Mark Butte are only slightly older than the Suttle Lake advance of the late Pleistocene Cabot Creek glaciation.
Figure 2A.15. A roadside exposure of the Benchmark Butte Lava Flow; the concoidal fracturing is indicative its fine grained to glassy, dacitic composition.
121.6 (1.6) Several road cuts on the left side of the highway for the next several miles occur in a belt of right lateral and terminal moraines of the Cabot Creek glaciation in this area (Figure 2A.13 and Figure 2A.16). These moraines mark the outermost position and gradual retreat of the ice lobe occupying the upper Deschutes River valley during the late Pleistocene Suttle Lake (LGM) glacial advance 24,000 to 18,000 years ago.
Figure 2A.16. Till deposited as an end moraine during the maximum advance of the late Pleistocene Suttle Lake (LGM) glacial advance; note the relatively thin, moderately well developed soil capping the deposit.
124.1 (2.5) Refer to Map 2A.14. The meadow to the right (east) formed where sediment was trapped behind recessional moraines of the Suttle Lake advance to the left (west) and lava flows from Sheridan Mountain to the right (east). The earliest lava flows from Sheridan Mountain intertongue with Suttle Lake glacial deposits, indicating that an ice lobe still occupied the upper Deschutes River valley as volcanic activity began building the Mt. Bachelor volcanic chain. A radiocarbon date on organic matter obtained from near the base of a sediment core taken from the center of the meadow has an age of about 12,200 years B.P., providing a minimum age for deglaciation of the valley and extrusion of the lava flows in this area.
125.0 (0.9) Intersection of FS Rd 46 (Cascade Lakes Hwy) and FS Rd 46-500 to Lava Lake, Little Lava Lake, two campgrounds, and a small, privately run resort. Turn in here and drive to both lakes (approximately four miles round-trip). Superb views of South Sister and the Mt. Bachelor volcanic chain can be had at either lake (Figure 2A.17). The lakes are appropriately named, each is bordered on its eastern edge by young lava flows from the Mt. Bachelor volcanic chain. Little Lava Lake (or Lava Lake during especially high water during spring snow melt) provides the headwaters of the Deschutes River.
Figure 2A.17. View to the northeast of the Mt. Bachelor volcanic chain from the western shore of Little Lava Lake; Mt. Bachelor is to the north, Kwohl Butte is in the middle, and Sheridan Mountain is to the south.
From your lake-side vantage point, take some time to consider the formation of the Mt. Bachelor volcanic chain (Figure 2A.18), some of the youngest eruptive activity in the central Oregon Cascades. Volcanic eruptions along the Mt. Bachelor chain of vents did not occur as one continuous event, but instead occurred as discrete pulses of overlapping lava flows from localized segments of the chain over time. Gardener (1989a and 1989b) identified five distinct periods of volcanic activity beginning during the late Pleistocene and continuing through much of the Holocene: 1) early porphyritic basaltic-andesite lava flows from vents associated with Sheridan Mountain shield volcano and basaltic lava flows from Red Crater and Katsuk Butte; 2) a second pulse of lava flows from vents on the east side of Sheridan Mountain and two early basaltic lava flows associated with the Siah chain of cinder cones; 3) later basaltic volcanism from multiple vents coalesced to form the Siah chain of cinder cones and associated flows; 4) basaltic lava flows from the Kwohl Butte and Mt. Bachelor shield volcanoes; and 5) basaltic lava flows on the north flank of Mt. Bachelor culminating in the construction of Egan Cone.
Return to your vehicle, perhaps after a swim, and drive back to FS Rd 46.
Figure 2A.18. Map showing the Mt. Bachelor volcanic chain; since the late Pleistocene this group of vents has constructed a complex of small shield volcanoes comprised of Mt. Bachelor, Kwohl Butte and Sheridan Mountain as well as several cinder cones and their accompanying lava flows (modified from Scott and Gardner, 1990).
129.1 (4.1) Intersection of FS Rd 46-500 and FS Rd 46 (Cascade Lakes Hwy). A road cut on the left side of FS Rd 46 at this intersection provides a good opportunity to examine an exposure of Suttle Lake age till from a recessional moraine. Pull off carefully at the intersection and cross the road to the exposure. Notice the yellow-brown soil developed on the gray, unweathered till at depth. Till is a poorly sorted, poorly stratified sediment composed of subangular to subrounded boulders and cobbles in a fine- grained matrix of sand, silt, and clay that has been deposited directly from melting glacial ice.
132.9 (3.8) A road cut to the left of the highway exposes till of Suttle Lake age overlain successively by about 50 cm of scoria from Red Crater, Mazama Ash, and a thin layer of pumice lapilli from the 2,000 year old eruption from the Rock Mesa vent on South Sister. The minimal weathering of Suttle Lake till at this site suggests that the eruption of Red Crater occurred soon after this area was deglaciated in the late Pleistocene.
133.0 (0.1) Entrance road to Beach Day Use Area on the right. Turn in here and park near the south shore of Elk Lake. From here, the shoreline offers a superb view of South Sister northward up the axis of Elk Lake, a view especially nice at sunset (Figure 2A.19). To the east of the beach, young lava flows from Red Crater come right down to the lake margin. Elk Lake, and Hosmer Lake to the east, have no outlets, groundwater seeps toward the Lava Lakes through glacial deposits under lava flows from Mt. Bachelor and Red Crater that otherwise block surface drainage.
After enjoying the view (the swimming here is good too) return to the junction with FS Rd 46.
Figure 2A.19. View to the north of the South Sister’s massive volcanic edifice from the southern shore of Elk Lake; Devils Hill is the highpoint just to the right, while Broken Top Volcano peeks above the trees to the far right.
133.3 (0.3) Junction of Beach Day Use Area road and FS Rd 46 (Cascade Lakes Highway). Turn right (north) onto FS Rd 46.
134.0 (0.7) Pullout on the right side of the highway. Park here, this location affords excellent views across Elk Lake to the major volcanoes of the Mt. Bachelor volcanic chain. Mt. Bachelor is to the north, Kwohl Butte is in the middle, and Sheridan Mountain is to the south. Red Crater lies further south in the foreground, the largest and northernmost of a chain of latest Pleistocene cinder cones stretching 2.5 km southward.
Return to your vehicle and continue north on FS Rd 46.
135.4 (1.4) A good view of Broken Top Volcano at 12:00.
137.4 (2.0) Refer to Map 2A.15. The highway climbs onto the margin of the postglacial, basaltic-andesite lava flow from Le Conte Crater. Mazama ash overlies the Le Conte Crater lava flow, which may be similar in age to early Holocene vents and flows on the north flank of Mount Bachelor. The flow can be observed along the road for about the next one and a half miles. Road cuts show the thickening and coarsening blanket of tephras erupted from late Holocene silicic vents at Rock Mesa and Devils Hill.
137.8 (0.4) Good view of Devils Hill, a glaciated Pleistocene rhyolite dome and the unrelated namesake of the Devils Hill volcanic chain, ahead at 12:00.
138.2 (0.4) Glimpses of Katsuk Butte and Talapus Butte through the Lodgepole pines at 3:00. These late Pleistocene buttes are cinder cones perched on a steep-sided plateau of basaltic lava flows overlying hydrovolcanic deposits.
139.6 (1.4) Devils Lake is on the left, dammed by the early Holocene lava flow from Le Conte Crater cinder cone. Devils Hill is ahead at 12:00. Park in the large turnout on the right side of the road. Carefully cross the highway to the road cuts on the left to examine the hydrovolcanic deposits from the initial volcanic eruptions that constructed Talapus and Katsuk Buttes, and the tephras from the Rock Mesa and Devils Hill silicic vents (Figure 2A.20).
Figure 2A.20. Road cuts near Devils Lake expose volcaniclastic materials related to several recent volcanic eruptive events, from base to top these include: 1) hyaloclastite of Talapus and Katsuk Buttes; 2) Mazama Ash; 3) tephra from the Rock Mesa eruption; and 4) tephra from eruptions of the Devil’s Hill volcanic chain.
Continue walking east along the edge of the highway for about two tenths of a mile. The road cut on the right side of the road at the base of Talapus Butte exposes more hyaloclastite deposits. On the left, road cuts expose the southernmost rhyolite lava flow erupted about 2,000 years ago from the Devils Hill volcanic chain. The lava flow overlies tephra from the Devils Hill explosive eruption phase, which in turn overlies tephra from the Rock Mesa explosive eruption phase, then Mazama ash, then hyaloclastite of Talapus and Katsuk Buttes. Road cuts just up the highway to the east display till of the late Pleistocene Suttle Lake advance under the hydrovolcanic material.
Talapus and Katsuk Buttes are cinder cones that sit on a gently southward-sloping platform composed of multiple, thin, basaltic lava flows that terminates along much of its margin in high, steep slopes. Flow surfaces are unglaciated and display well-preserved blocky and ropy surfaces, tumuli, and other flow-top features that indicate youthfulness. At the northern end of the platform, exposures display lava flows overlying unusual hydrovolcanic deposits (called hyaloclastites and viewed in the road cut here), which in turn overlie glacial deposits of late Pleistocene Suttle Lake age. Scott et al. (1989) and Scott and Gardner (1990) interpret this sequence of deposits as resulting from a series of volcanic eruptions that: 1) initially occurred in a lake melted into the receding edge of the Suttle Lake ice lobe occupying the uppermost Deschutes River drainage; 2) subsequently occurred in a drained lake basin which excluded water from the vent and allowed normal Strombolian eruptions to ensue; and 3) eventually allowed the construction scoria cones while lava flows issued from between and at their bases, filling the remaining depression between the rising volcanic platform and glacier margins, and “skying out” as steep platform margins against a now vanished ice buttress. The age of this volcanic activity is not precisely known, although paleomagnetic data from platform rim lavas is very similar to lava flows from Red Crater and the Sheridan Mountain shield volcano further south in the Mount Bachelor volcanic chain, suggesting these eruptions were coeval. This paleomagnetic data, combined with a similarity of stratigraphic relationships between the volcanic products of these eruptions and Suttle Lake advance glacial deposits, clearly suggests volcanism at or shortly following the late Pleistocene glacial maximum.
Two silicic late Holocene tephra units overlie soil formed on Mazama ash and the hydrovolcanic deposits from Talapus and Katsuk Buttes. These tephras cover a wedge-shaped area centered just south of South Sister and Broken Top volcanoes that covers several hundred square kilometers with individual lobes spreading to the east and south (Figure 2A.21), and are best exposed in the road cuts described here. The lower tephra consists of a thin basal layer of grayish lapilli and coarse ash and a slightly thicker upper layer of brownish-gray ash with a conspicuous brick-red marker bed about 1 cm thick. The upper tephra is much thicker and coarser-grained than the lower unit. Where it isn’t eroded, it is more than half a meter thick, consisting of pumice bombs up to 30 cm in diameter and lithic fragments averaging 20 cm in diameter, in a coarse ash and lapilli matrix. Much of this tephra is glassy, gray to black rhyolite, with large clasts exhibiting bread-crusted surfaces. Slight weathering and reworking of the lower tephra suggests they were erupted only a few hundred years apart.
Figure 2A.21. Map showing the vent sources, lava domes and flows, and tephra distribution of the late Holocene Rock Mesa and Devil’s Hill silicic volcanism (modified from Scott and Gardner, 1990).
The lower tephra with its distinctive red marker bed occurs as a lobe that thickens and coarsens northwestward to an inferred graben, several small rhyolite domes, and 400-meter-long series of vents, all clustered about 1.2 km northeast of the summit of the Rock Mesa rhyodacite flow (Figure 2A.21). Tephra produced from the vent buried beneath the rhyolitic lava flows of Rock Mesa spreads in a separate arm to the south. This tephra shares a nearly identical composition to the lower tephra described above, although lithic fragments contained in the tephra of this arm are dominantly basalt and basaltic andesite, rather than andesite, dacite, and rhyolite. The thick, coarse upper tephra occurs as a distinct, more compact lobe that originated from now buried vents associated with the Devil’s Hill volcanic chain (Figure 2A.21). Devil’s Hill itself is an older, Pleistocene rhyolite dome sitting just west of the volcanic chain’s southern end. Fragments of rhyolite from this dome are incorporated into the upper tephra. The pumiceous nature of the Rock Mesa and Devil’s Hill tephras, their pattern of dispersal, and their associated pyroclastic-flow and surge deposits indicate Plinian style eruptions (Scott, 1987), according the classification of Walker (1973 and 1981).
The dual nature of tephra deposits and lava flows indicates that these silicic volcanic eruptions flanking South Sister’s southern face occurred in two phases. Eruptions began with an explosive phase, producing tephra from several aligned vents fed by dikes or segments of a single dike emanating from the same magma source (Scott, 1987). Eventually, feeder dikes were locally enlarged along the dike lineaments, enhancing magma supply to these locations, which became the principal conduits during the latter lava-extrusive phase that followed tephra eruptions. Presumably, the size of the feeder dike and volume of magma it produced is expressed by the surface dimensions of the individual rhyolite lava domes and flows.
Return to your vehicle; then drive east on FS Rd 46 toward Bend, Oregon.
140.0 (0.4) Pullout on the left here allows a closer examination of the southernmost rhyolitic lava flow of the Devils Hill volcanic chain.
141.3 (1.3) Refer to Map 2A.16. The entrance road for the Green Lakes Trailhead parking area is on the left and a pullout for Sparks Lake is on the right. First turn into the pullout for Sparks Lake. An information sign indicates that the lake was named after ‘Lige’ Sparks, a pioneer stockman. On August 31, 1855, geologist Dr. John Strong Newberry passed by here accompanied by members of the Pacific Railroad survey party. The lake is only a maximum of 8 feet deep and readily shrinks and swells with minor climatic fluctuations.
The views to the east of Mt. Bachelor from the meadow at the north end of Sparks Lake are superlative (Figure 2A.22). Mount Bachelor is a large shield volcano constructed along the Mount Bachelor volcanic chain beginning in the late Pleistocene. The generally mafic eruptions of magma associated with the chain produced copious lava flows and lesser amounts of pyroclastic debris, building shield volcanoes and cinder cones covering an area over 250 km2, and stretching 25 km north-south from the informally named Egan cone on the north flank of Mount Bachelor to southeast of Lookout Mountain. Scott (1990) used several lines of evidence reported in Scott, et al. (1989) and Scott and Gardner (1990) and which will be discussed in this guidebook to show that eruptions related to the Mount Bachelor volcanic chain began after the late Pleistocene glacial maximum (about 22,000 to 18,000 years B.P.), but before full ice retreat from the area and the termination of glacial conditions in the High Cascades. The earliest glaciation that carved the cirque into the northeast flank of Mount Bachelor has been correlated with the latest Pleistocene Canyon Creek glacial advance (Scott, et al., 1989; Scott, 1990; and Scott and Gardner, 1990), indicating that volcanic eruptions building Mount Bachelor were chiefly completed by that time and suggesting that Mount Bachelor’s massive shield was essentially built in 10,000 years or less.
Figure 2A.22. View to the east of the Mt. Bachelor shield volcano from the meadow at the north end of Sparks Lake.
Notice the prominent vent about midway up the north flank skyline, the source for the apron of lava flows pouring down toward Sparks Lake meadow that were erupted during the later stages of development of Mt. Bachelor volcano (Figure 2A.18). Pine Marten Lodge lies just south of the vent; several ski trails emanate from the lodge diverging downslope across the flows. Looking north and northwest across the meadow, the gray, very rough, rhyolitic lava flows of the Devil’s Hill volcanic chain can be observed in the foreground along the southern flanks of South Sister volcano erupted between 2,300 and 2,000 years B.P. (Scott, 1987). Devils Hill itself lies just behind the fresh-looking flows. The Devil’s Hill volcanic chain occurs as a series of aligned smaller flows starting due north of the highway and continuing upslope to a much larger flow clinging to the middle and lower slopes of the southeast flank of South Sister. This 10-km-long lineament continues out of view on the northeast flank of South Sister as a 1.2-km-long series of small domes near Carver Lake. The cinder cones and volcanic steep-edged platform of Talapus and Katsuk Buttes are to the west and southwest.
Now drive across the highway to the Green Lakes Trailhead parking area. No trip to the Cascades Lakes would be complete without a hike to the Green Lakes, arguably the premier alpine lakes of the region. Park here to access two hiking options; both are described as multi-day backpacking trips. The first is a circumnavigation of Broken Top Volcano, highlighting the
Green Lakes and Broken Tops’ intensely sculpted flanks which exhibit an amazing display of a stratovolcano’s internal stratigraphy and preserve an incredible record of Holocene neo-glaciation. The second is a trek to Moraine Lake, which takes in an excursion to the summit of South Sister Volcano, and includes a possible day-hike to Le Conte Crater cinder cone and the Rock Mesa rhyodacitic lava dome and flows (see the Broken Top Loop Trail and Moraine Lake Trail under the Optional Hiking Trails section at the end of this road log for complete descriptions of these hikes). Lengthy day-hikes to the Green Lakes or Moraine Lake could also be undertaken from this trailhead.
141.8 (0.5) Return to USFS Rd 46 (Cascade Lakes Highway). Turn left (east) onto USFS Rd 46.
142.2 (0.4) Highway road cuts on the right just west of Soda Creek expose basaltic lava flows originating from Cayuse Crater on the southwest flank of Broken Top. Cayuse Crater is a postglacial cinder cone built of red scoria and breached on its southwest side by lava flows, erupted between 12,000 and 9,500 years B.P. (Scott, al et., 1989; Scott and Gardner, 1990).
Immediately beyond the Cayuse Crater lava flows, the highway crosses Soda Creek. On October 7th, 1966, a flood and debris flow torrent was released down Soda Creek by the catastrophic failure of a small, moraine-dammed lake at the foot of Crook Glacier on the east side of Broken Top (Nolf, 1969). Debris was piled on the highway and covered about 40 percent of the meadow north of Sparks Lake. Another similar event occurred on White Branch Creek from a lake below Collier Glacier on the northwest flank of the Middle Sister. These lakes, and others like them in the High Cascades, are dammed behind unstable, Holocene, neoglacial morainal debris.
The lava flow east of Soda Creek on the right side of the highway is one of several erupted from Egan Cone, a Holocene cinder cone on the lower north slope of Mt. Bachelor and the youngest vent in the Mt. Bachelor volcanic chain. These flows blocked Soda Creek further south to form Sparks Lake.
142.3 (0.1) The entrance to Soda Creek campground is on the right. Exceptional views of Broken Top, South Sister, and Mount Bachelor are worth the short side trip to the boat launch area at Sparks Lake. Take the road leading to Soda Creek campground and continue beyond to the lake. These strato-volcanoes display a continuum from oldest and most eroded by glaciers to youngest, and least eroded.
144.1 (1.8) To the left is the margin of basaltic andesite lava flows from Egan cone. To the right, unweathered to slightly weathered till deposited by the late Pleistocene Suttle Lake ice advance is overlain by basaltic scoria from the northernmost vents associated with the Mount Bachelor volcanic chain, suggesting volcanism associated with Mt. Bachelor closely followed deglaciation. This material is in turn overlain by Mazama ash, followed by tephra from Rock Mesa and Devil’s Hill.
144.6 (0.5) Junction of FS Rd 46 and FS Rd 46-370 (Todd Lake Rd). Todd Lake sits in an elongated glacially scoured trough and offers a nice view of Broken Top volcano. Drive to the Todd Lake Day Use Area, and hike about half a mile round-trip to the shore of the lake and back.
145.3 (0.7) To the left, road cuts expose glaciated outcrops of basaltic andesite lava from a cone of the Tumalo Mountain volcanic center overlain by patches of Egan cone scoria, Mazama ash, and Rock Mesa and Devil’s Hill tephras. To the right at 1:00 is a small, isolated, glaciated cinder cone; Tumalo Mountain is the shield volcano ahead at 11:00.
145.8 (0.5) Here, you begin crossing Dutchman Flat, a closed basin formed by lava flows at its southern end from Mount Bachelor shield volcano that is now filled with reworked pyroclastic material from adjacent slopes. As you cross Dutchman Flat, look to the north and west (between 7:00 and 10:00) for great views of Broken Top and South Sister strato-volcanoes, plus Devils Hill and the young rhyolite lavas of the Devils Hill volcanic chain in the foreground (Figure 2A.23).
Figure 2A.23. View to the north and west across Dutchman Flat to Broken Top and South Sister Volcanoes to the right and center. Devil’s Hill, an older rhyolite dome, lies to the left with the southernmost rhyolitic flow of the Devil’s Hill volcanic chain at its feet.
146.3 (0.5) Junction of FS Rd 46 and the entrance road to the West Village of the Mount Bachelor Ski Area. Continue on FS Rd 46.
An excavation in the southwest corner of the West Village parking area once exposed about 1.5 meters of Mazama Ash which rested on unweathered to slightly weathered scoria correlated with nearby Egan Cone, suggesting that the tephra eruptions from Egan Cone are only slightly older than Mazama ash, and thus, Egan Cone forms the youngest vent of the Mount Bachelor volcanic chain (Scott, et al., 1989; Scott, 1990; and Scott and Gardner, 1990). The Mazama Ash preserved here is composed of two distinct zones: 1) a lower zone of fine- to medium-grained, grayish-white, laminated ash containing abundant, dark iron- and magnesium-bearing minerals and lithic fragments; and 2) a thicker yellowish-white upper zone of coarser-grained, ash grading upward into fine lapilli. These features are typical of Mazama ash spread northeastward from Crater Lake.
This entrance also provides access to Egan Lodge in the West Village parking area if you plan to ride the chairlift up Mt. Bachelor. Egan Lodge provides access to the Pine Marten Chairlift to Pine Marten Lodge at mid-mountain (7,775 feet) and a potential hike to the summit (9,065 feet); the Summit Chairlift to the top of Mt. Bachelor is no longer available for rides. Note that as of 2012, the chair lift was opened July 6th through September 2nd from 10:00am to 8:00pm on Friday, Saturday, and Sunday (5:00 to 8:00 pm for dinner guests only). Lift tickets cost $13.00 (Seniors 65+), $16.00 (13-64) and $10.00 (6-12), and FREE (5 and under); evening dinner tickets are slightly cheaper. Be sure to check Mount Bachelor Ski Resort’s website at www.mtbachelor.com for the latest information.
Personally, this author suggests that you take the hiking trail from the Sunrise Lodge parking area (ahead), which traverses less disturbed terrain, actually gets you to the mountain’s summit, is less crowded, and is free.
146.5 (0.2) Refer to Map 2A.17. Dutchman Flat Snow Park is on the left. This parking area also serves as the trailhead parking for the Tumalo Mountain Trail (see the Tumalo Mountain Trail under the Optional Hiking Trails section at the end of this road log for a complete description of this hike). The views from Tumalo Mountain’s summit are equally spectacular to those of Mt. Bachelor; and if you are inclined toward greater solitude and would rather not spend the money for the Pine Marten chairlift at Mt. Bachelor Ski Area’s West Village, then take this four mile round-trip hike instead. This author certainly recommends the trip.
Tumalo Mountain is a small shield volcano or large cinder cone primarily composed of basaltic andesite, with lesser amounts of basalt (Figure 2.2). The summit area is comprised of an assorted mix of basaltic pyroclastic cinders, scoria, and lava blocks and bombs. All of the volcanic rocks making up the mountain have normal magnetic polarity indicating that the volcano erupted within the most recent period of normal remnant magnetism in the Pleistocene and is less than 700,000 years old. The degree of glacial sculpting of its northeast flank suggests that it may have undergone several periods of glaciation and is likely to be of middle Pleistocene in age.
146.9 (0.4) The entrance to Sunrise Lodge and Chairlift of the Mt. Bachelor Ski Resort is on the right. Mt. Bachelor’s northern slopes, scarred by the northeast facing cirque, are visible from here at 3:00. For approximately the next mile, lava flows to the right from Mt. Bachelor’s summit cone and shield are partially buried by outwash of late-glacial and neoglacial age derived from glaciers occupying the cirque on its northern flank. Lava flows from Tumalo Mountain predating the late Pleistocene glacial maximum are exposed to the left. Thick deposits of Mazama Ash and underlying scoria erupted from the Mt. Bachelor volcanic chain are exposed in some road cuts. Parking at this entrance is no longer permitted by Mount Bachelor Ski Resort ‘s management. Park at the Dutchman Flat Snow Park just back down the road. For a detailed description of the hike, see the Mt. Bachelor Trail under the Optional Hiking Trails section at the end of this road log.
148.3 (1.4) An excellent view to the south and west between 3:00 and 5:00 of the Mt. Bachelor volcanic chain, including the Mt. Bachelor, Kwohl Butte, and Sheridan Mountain shield volcanoes. Glacial deposits at this location of Suttle Lake (LGM) age are completely buried by the volcanic products associated with the Mount Bachelor chain. Glacial erosion features on Tumalo Mountain and the plateau east of Broken Top, combined with the position of moraines to the east of here and at the northern edge of Crane Prairie, suggest that the ice margin of the late Pleistocene Suttle Lake glacier crossed the highway very near this location and headed in a southwesterly direction to connect with the moraines on the northwest side of Crane Prairie (Scott, et al., 1989; Scott and Gardner, 1990).
149.3 (1.0) Refer to Map 2A.18. Exposures of bouldery outwash associated with the late Pleistocene Cabot Creek glaciation are on the left side of the road. These deposits lie just south of moraines marking the margin of an ice lobe that wrapped around the east side of Tumalo Mountain (Figure 2A.13).
149.5 (0.2) Junction of FS Rd 46 (Cascade Lakes Hwy) and FS Rd 45 (south to Sun River). Remain on FS Rd 46. Tumalo Mountain is at 8:00. Its north-eastern flank is distinctly carved by glaciation. Glacial deposits with substantially greater weathering than the late Pleistocene Suttle Lake moraines mentioned earlier lie just south of the highway, suggesting the erosion of Tumalo Mountain may be related to more than one glacial period and therefore, it is considerably older than Mount Bachelor.
FS Rd 45 was recently improved in 2007 which created many new road cuts. A short, 5.4 mile round-trip excursion down FS Rd 45 will take you to a beautifully exposed cross-section through a young basaltic aa lava flow from Mt. Bachelor that displays the rough, bouldery flow top and underlying dense, homogeneous inner flow.
150.8 (1.3) FS Rd 46 curves gently upward and around the flank of an old Pleistocene cinder cone. A large pullout occurs to the right at the top of the curving grade. Park here for a last, good look at Mt Bachelor (Figure 2A.24) and Tumalo Mountain just to the north. Note the small cirque sculpted into Mt Bachelor’s northeast flank, and then compare this glacially-carved bowl with that of Tumalo Mountain’s much more deeply scarred northeast face. Mt. Bachelor is the product of volcanic activity that began near the end of the last glacial maximum and ended roughly 10,000 years later (Scott, 1990). Its cirque was sculpted by only the latest Pleistocene Canyon Creek glacial advance and possibly two neoglacial advances in the Holocene. Tumalo Mountain’s age is not well constrained, but given the degree of glacial scouring of this shield volcano’s cirque, it must be substantially older.
Figure 2A.24. View to the southwest to Mt. Bachelor from a vantage point on FS Rd 46 (Casacde Lakes Highway). Note the shallow cirque carved into the mountain’s northeast flank.
153.9 (3.1) Junction of FS Rd 46 (Cascade Lakes Hwy) and FS 4615 (and the entrance to Virginia Meissner Snow Park) on the left. A right-lateral moraine associated with the late Pleistocene Suttle Lake (Scott, 1977) glacial maximum ice lobe wraps around the north and west side of the flat-topped ridge immediately west of FS Rd 4615. The highway begins following a narrow outwash channel originating from the ice margin where it approaches but never crosses the road uphill to the west. Lack of stream flow today is probably due to the high permeability of the outwash and fractured volcanic rock beneath. Most of the rounded hills on both sides of the road are cinder cones that are much older than either nearby Tumalo Mountain or Mount Bachelor. Several hills on the north side of the highway are rhyolite domes associated with the silicic Tumalo volcanic center (Taylor, 1978; Hill and Taylor, 1989).
157.9 (4.0) Refer to Map 2A.3. Good views to the southeast for the next mile of shield-shaped Newberry Volcano, a compositionally diverse, Quaternary volcanic center (MacLeod, et al., 1981; MacLeod, et al., 1995). The volcano is volumetrically among the largest volcanoes in the lower 48 states. The volcano is composed of silicic ash-flow tuffs, silicic domes and lava flows, and numerous basaltic cinder cones, fissure vents and related mafic lava flows.
160.3 (2.4) Intersection of FS Rd 41 and FS Rd 46 (Cascade Lakes Highway); continue on FS Rd 46 toward Bend, OR. Field Trip 2A merges with Field Trip 1A here.
161.9 (1.6) Refer to Map 2A.19. The highway crosses the trace of a northwest trending fault here, part of the Tumalo Fault Zone, with displacement down to the northeast.
163.0 (1.1) Refer to Map 2A.1. The highway crosses another NW-SE trending fault trace here, with displacement down to the northeast, and then descends through a small valley; the ridges on either side of the road were formed by topographic inversion (Figure 1A.10) (Jensen, 2006). In this area, silicic pyroclastic rock units erupted from the Tumalo volcanic center (Taylor, 1978; Hill and Taylor, 1989; and Hill and Scott, 1990) were laid down as sheets on a rolling tableland. Streams flowing from the topographic highlands to the west cut valleys into these pyroclastic rocks. Subsequent lava flows from the High Cascades then poured down these valleys from the west, destroyed the drainage systems, and formed elongated bodies of resistant basaltic rock. Later, the tributary streams of the older, now buried drainages still entering from the sides incise new channels into the less resistant pyroclastics along the margins of the elongate lava flows. As these younger streams cut their own valleys, the lava flows which once occupied the topographic lows within the older drainages are now preserved as basalt-capped ridges between modern stream valleys.
164.1 (1.1) Bend city limits. Century Drive becomes South Century Drive.
165.0 (0.9) Intersection of South Century Drive and Mt. Washington Drive. Continue north through the round-about on South Century Drive.
165.2 (0.2) Intersection of South Century Drive and SW Chandler Avenue. Continue north through the round-about on South Century Drive.
165.7 (0.5) Intersection of South Century Drive and Simpson Avenue. Continue north through the round-about on South Century Drive. South Century Drive becomes NW 14th Street.
166.3 (0.6) Intersection of NW 14th Street and NW Galveston Avenue. Turn right (east) onto NW Galveston Avenue.
166.6 (0.3) Cross the Deschutes River here. NW Galveston Avenue becomes NW Tumalo Avenue after crossing the river.
166.7 (0.1) Junction of NW Tumalo Avenue and NW Riverside Boulevard. Turn left (north) onto NW Riverside Boulevard. NW Riverside Boulevard curves gently to the east along the river and passes into downtown Bend.
167.2 (0.5) Traffic light. Intersection of NW Riverside Boulevard and NW Wall Street. Continue east through this and one more traffic light; NW Riverside Boulevard becomes Franklin Avenue after the first light.
167.6 (0.4) Overpass for the Burlington Northern-Santa Fe Railroad. Road cuts here are in the Basalt of Bend (Sherrod et al., 2004).
167.8 (0.2) Intersection of Franklin Avenue and Oregon Highway 97 (3rd Street). This ends Field Trip 2A.
Road Route Maps
Map 2A.1. Color shaded-relief map of the Bend 7.5” Quadrangle containing segments of Field Trip 1A-F and Field Trip 2A.
Map 2A.2. Color shaded-relief map of the Lava Butte 7.5” Quadrangle containing segments of Field Trip 1A-C and Field Trip 2A.
Map 2A.3. Color shaded-relief map of the Benham Falls 7.5” Quadrangle containing segments of Field Trip 1A and Field Trip 2A.
Map 2A.4. Color shaded-relief map of the Anns Butte 7.5” Quadrangle containing segments of Field Trip 2A and Field Trip 3A.
Map 2A.5. Color shaded-relief map of the Findley Butte 7.5” Quadrangle containing segments of Field Trip 2A and Field Trip 3A.
Map 2A.6. Color shaded-relief map of the La Pine 7.5” Quadrangle containing segments of Field Trip 2A and Field Trip 3A.
Map 2A.7. Color shaded-relief map of the Masten Butte 7.5” Quadrangle containing segments of Field Trip 2A and Field Trip 3A.
Map 2A.8. Color shaded-relief map of the Cryder Butte 7.5” Quadrangle containing a segment of Field Trip 2A.
Map 2A.9. Color shaded-relief map of the Crescent 7.5” Quadrangle containing a segment of Field Trip 2A.
Map 2A.10. Color shaded-relief map of the Odell Butte 7.5” Quadrangle containing a segment of Field Trip 2A, as well as the first and last segments of Field Trip 2B.
Map 2A.11. Color shaded-relief map of the Hamner Butte 7.5” Quadrangle containing a segment of Field Trip 2A.
Map 2A.12. Color shaded-relief map of the Davis Mountain 7.5” Quadrangle containing a segment of Field Trip 2A.
Map 2A.13. Color shaded-relief map of the Crane Prairie Reservoir 7.5” Quadrangle containing a segment of Field Trip 2A.
Map 2A.14. Color shaded-relief map of the Elk Lake 7.5” Quadrangle containing a segment of Field Trip 2A.
Map 2A.15. Color shaded-relief map of the South Sister 7.5” Quadrangle containing a segment of Field Trip 2A.
Map 2A.16. Color shaded-relief map of the Broken Top 7.5” Quadrangle containing segments of Field Trip 2A and Field Trip 4B.
Map 2A.17. Color shaded-relief map of the Mount Bachelor 7.5” Quadrangle containing a segment of Field Trip 2A.
Map 2A.18. Color shaded-relief map of the Wanoga Butte 7.5” Quadrangle containing a segment of Field Trip 2A.
Map 2A.19. Color shaded-relief map of the Shevlin Park 7.5” Quadrangle containing a segment of Field Trip 1E and 1D, as well as Field Trip 2A.