Field Trip Road Log

0.0 (0.0)       Refer to Map 4A.1.  Intersection of US Highway 20/Oregon Highway 126 (W. Cascade Avenue) and FS Rd 16 (S. Elm Street).  Follow Hwy 20/126 west through downtown Sisters, OR.

Sisters, OR is located on a veneer of alluvial outwash sands and gravels (Figure 4.2) related to late Pleistocene glaciation in watersheds draining the eastern slope of the Cascade Crest (Sherrod et. al., 2004).  This glacial outwash overlies older Pleistocene basalt and basaltic andesite lava flows, filling a trough formed by a faulted half-graben west of McKinney Butte (Figure 4.2).  A small graben forms the northern end of this trough which is described near the end of Field Trip 4D.  McKinney Butte itself lies east of town and is comprised of Pliocene andesite uplifted along the normal fault that bounds the eastern margin of the half-graben.  Faulting in this area is transitional between the northern end of the northwest trending Tumalo Fault Zone and the High Cascades graben at Green Ridge, a zone of en echelon extensional faulting named the Sisters Fault Zone (Sherrod et al., 2004).

0.2 (0.2)       Intersection of U.S. Highway 20/Oregon Highway 126 (W. Cascade Avenue) and Oregon Highway 242 (McKenzie Highway).  Hwy 242 is a one-way street here (do not enter); continue on Hwy 20/126 (now Santiam Highway).

0.5 (0.3)       Intersection of U.S. Highway 20/Oregon Highway 126 (Santiam Highway) and the connector road (West Hood Avenue) for Oregon Highway 242 (McKenzie Highway); turn left (south) onto West Hood Avenue.

0.7 (0.2)       Intersection of West Hood Avenue and Oregon Hwy 242 (McKenzie Highway); turn right (west) onto Hwy 242.  There are now excellent views of the Three Sisters volcanoes to the southwest.

1.9 (1.2)       Junction of Hwy 242 and FS Rd 15.  Remain on Hwy 242; a left (southwest) turn onto FS Rd 15 diverges onto Field Trip 4B.

2.5 (0.6)       Road cuts on the south side of the highway are Pleistocene basaltic lava flows from small shield volcanoes to the southwest.  For about the next two and a half miles the highway ascends an inclined surface over late Pleistocene alluvial outwash sands and gravels that are related to former glaciation in the Whychus Creek watershed and other drainages east of the Cascade Crest nearer McKenzie Pass.

4.6 (2.1)       Refer to Map 4A.2.  The entrance to Cold Spring Campground is on the right (north) side of the road.   In about two-tenths of a mile, Hwy 242 makes a left-hand curve near the site of Cold Spring.  The spring issues from the margin of a basaltic andesite lava flow, its outflow can be observed passing through Cold Spring campground.  The source of the basaltic andesite lava flow was a cinder cone near South Matthieu Lake about nine miles southwest of here.  The highway crosses this lava flow for the next mile, and then ascends the surface of an older basaltic flow for a similar distance.

7.0 (2.4)       Fourmile Butte, one of ten late Pleistocene cinder cones found between Black Butte and Black Crater that erupted chiefly mafic lavas (Taylor, 1968 and 1981), can be seen about half a mile north of this highway position (Map 4A.2).  Many lava flows issued from these cones, forming a lava field covering approximately 25 square miles.  The western margin of this lava field is covered by younger late Pleistocene glacial (till and outwash) deposits.  Relief on the youthful, basaltic andesite lava flow from Bluegrass Butte is readily observed on Map 4A.2 as it passes downslope between Fourmile and Graham Buttes.

7.9 (0.9)       Highway 242 makes a sharp right-hand turn here, and ascends over the crest of a distinctive right-lateral moraine formed during the late Pleistocene last glacial maximum about 22,000 to 18,000 years ago, the Suttle Lake advance of the Cabot Creek glaciation (Scott, 1977).  This moraine was left by one of several lobes that occupied drainages flowing eastward from the Cascade Crest (Figure 4.2) connected to a much larger ice cap blanketing the higher elevations of the Cascade Range upslope and to the west of this location.

The valley of Trout Creek to the south was similarly occupied by a Suttle Lake glacier (Bevis, et al., 2011), although its end moraine is not well preserved.  However, the eastern flank of Black Crater displays a distinctly-carved cirque and recessional moraines associated with this late Pleistocene glaciation readily apparent on Map 4A.2.

8.2 (0.3)       The highway curves back to the left.  Little Butte, the low mound just to the north, forms part of the same end moraine you just crossed (notice the curvilinear shape of the hummocky ridge on Map 4A.2 which wraps around from Black Crater to Bluegrass Butte).  On the left side of the road, outcrops of till are exposed; note the relatively thin, moderately well-developed soil formed on the surface of the till.  From here, the highway follows the inner flank of the right-lateral moraine for about three miles upslope.

8.6 (0.4)       Intersection of FS Rd 1030-800 on the north side of the highway.  Just west and upslope of this road lies the easternmost (downslope) extension of Holocene basaltic lava flows from Belknap Crater Volcano (Figure 4A.1).  The highway follows the outer (southern) margin of these flows upslope to the west for a little over three miles.

Figure 3.5A.1 - McKenzie Pass Lava Flows

Figure 4A.1.  A geologic map of the McKenzie Pass area delineating the major late Holocene basaltic and basaltic andesite vents and lava flows (modified from Taylor, 1968).

12.0 (3.4)     Refer to Map 4A.3.  Windy Point pullout is on the left (north) side of the road.  Park here for your first spectacular view of the Cascade Crest between McKenzie Pass and Santiam Pass (Figure 4.1), an area comprised of numerous youthful outpourings of mafic lavas (Figure 4A.2).  Windy Point is a glacially eroded promontory of basaltic andesite lavas and cinders on the northwest flank of Black Crater Volcano.  From the pullout, one can garner an excellent perspective of Mt. Washington Volcano in the background, with its broad shield-like flanks, surmounted by a glacially dissected summit cone and volcanic plug.  Nearer the pullout, late Holocene, aa lava flows from Belknap Crater and Yapoah Crater pour downslope to the east (Figure 4A.1); in the middleground lies Belknap Crater lavas; and in the foreground, the lava flow from Yapoah Crater.

Figure 3.5A.2 - View from Windy Point copyrighted

Figure 4A.2.  The view from Windy Point on Oregon Hwy 242 offers up a plethora of volcanic features, including Mt. Washington Volcano in the background, Belknap Crater and its associated lava flow in the middleground, and the Yapoah Crater Lava Flow in the foreground.

The highway now follows the outer margin of the Yapoah Crater Lava Flow upslope for about the next 3 miles (Figure 4A.1).  Examine Map 4A.3; Yapoah Crater Lava Flow’s rougher surface and greater relief makes its outer margin readily distinguishable from Belknap Crater lavas.  Outcrops of glacially rounded basaltic andesite from Black Crater and basalt from several smaller Pleistocene cinder cones can be seen along the south side of the highway over the same distance.

12.4 (0.4)     The parking area for the Black Crater Trail is on the left (south) side of the road here.  This trail is steep, but fairly short; it offers wonderful views of the Three Sisters stratovolcanoes and many other recently formed volcanic features in the McKenzie Pass area (see Black Crater Trail under Optional Hiking Trails at the end of this road log for a complete description of this hike).

Black Crater is a large late Pleistocene cinder cone formed over a slightly older shield volcano base (Taylor, 1968 and 1981) (Figure 4.2).  Its flanks are comprised of basaltic andesite lava flows; its summit is capped by near-vent cinders, lava blocks, and bombs.  Black Crater was erupted to the east of the main Cascade Crest and was not covered by the late Pleistocene ice cap; however, its flanks have been modified by glacial activity, probably during more than one glacial advance.  The “crater” in Black Crater is a cirque carved into the northeast slope of the volcano’s summit cone.

14.9 (2.5)     Intersection of Oregon Hwy 242 and FS Rd 900.  FS Rd 900 leads to the Pacific Crest Trail parking area and Lava Camp Lake Campground.  Drive about three tenths of a mile on FS Rd 900 and turn right into the trailhead parking area for the Pacific Crest Trail.  (Remaining on FS Rd 900 takes you to the campground).  From this location, hike the Pacific Crest Trail southward to Yapoah Crater, a late Pleistocene basaltic cinder cone on North Sister Volcano’s northwest flank, and one of the main sources for lavas seen in the McKenzie Pass area (see Yapoah Crater Trail under Optional Hiking Trails at the end of this road log for a complete description of this hike).

15.0 (0.1)     Highway 242 cuts through the east levee of the Yapoah Crater Lava Flow (Figure 4A.1).   A named “Flow” can be comprised of multiple flow units as seen here.  Several flow units can be observed in which thin layers of dense lava are separated by thicker layers of rubble formed at the base and top of a given flow.

15.3 (0.3)     The Dee Wright Observatory just east of McKenzie Pass is on the left (north) side of the road.

This observatory offers a wonderful 360° panorama of significant landmarks near and far (all of volcanic origin) and is a true highlight of this field trip.  The observatory is built on a blocky basaltic andesite lava flow erupted from Yapoah Crater (Figure 4A.1) during an early phase of Holocene volcanic activity in the area that lasted from about 2,900 to 2,600 years ago (Taylor, 1968 and 1981).  This flow lobe of Yapoah lava moved over the area toward the north and east, but its advance was halted by lava flows of Belknap Crater Volcano lying just to northwest which had only just recently been erupted themselves.  Arccuate pressure ridges developed on the Yapoah flow’s surface and subsequent lavas were diverted to the northeast of the observatory where the main lava flow channel can be observed today.  The Lava River Trail is a short, paved, interpretive trail through the main channel area of the Yapoah Crater Lava Flow that can be taken from the east end of the observatory parking area that displays many excellent mafic lava flow features (see Lava River Trail under Optional Hiking Trails at the end of this road log for a complete description of this hike).

Yapoah Crater marks only one of several significant late Holocene vents in the area near the Dee Wright Observatory.  Looking toward the south and southeast, you can observe a long ridge running slightly northeast, seemingly connecting North Sister Volcano to Black Crater Volcano (Figure 4A.1).  This glaciated ridge of late Pleistocene basaltic andesite lavas consists of multiple, small cinder cones formed along a five-mile-long chain (Taylor, 1968 and 1981).  These cones are comprised of cinders, scoria, and unusually large and abundant lava blocks and bombs.  Try to imagine the amazing array of lava fountains on the skyline generated by the volcanic eruptions thatbuilt this ridge.  Two large Holocene cinder cones are superimposed on the southern end of this ridgeline (Figure 4A.1); Yapoah Crater lies slightly to the left (east) and is nearer the observatory, while Collier Cone is more directly to the south nearer the base of North Sister.  Vents at each location produced voluminous outpourings of basaltic andesite lavas.  As observed here, the Yapoah Crater lavas moved down the northwest flank of the ridge in several lobes which coalesced at the observatory and flowed northeast downslope adjacent to the route of Oregon Hwy 242, ultimately traveling for more the eight miles from the vent.  The lavas from Collier Cone erupted about 1,500 years ago (Sherrod et al., 2004) and spread outward in two lobes, one that flowed northwest from the ridge to pool and stall out in a large area of relatively low gradient near the cone, the other, younger lobe poured off the ridge to the west and down the glaciated trough of the White Branch of the McKenzie River, reaching a distance over seven miles from the vent.

Four-in-One Cone (Figure 4A.1), a complex of coalesced basaltic cinder cones, lies below and to the west of the ridge, sandwiched between the lava flows from Yapoah Crater and Collier Cone.  This is a unique ridge cone, breached in four locations on its northwest, downslope-facing flank.  The cone complex erupted andesitic lavas about 1,900 years ago (Sherrod et al., 2004), generating two flow lobes that generally wrap around the northeast and southwest sides of a ridge punctuated by Condon Butte.  The southwestern lobe is in contact with Collier Cone’s more northerly lava flow.  Four-in-One Cone is part of a north-south alignment of nineteen vents.

Turn around and look to the northwest at the vast sea of dark lavas (Figure 4A.1).  In the foreground lies the summit cone of Little Belknap Crater, perched on the southeast flank of the massive shield of Belknap Crater Volcano, its buff-colored summit cone just upslope to the northwest (Figure 4A.3).  South Belknap Cone, a small, breached cinder cone lies surrounded by younger lavas mid-way up Belknap Crater’s southern flank.  This area was the nexus for a long, complex period of Holocene basaltic and basaltic andesite volcanism that erupted nearly continuously from at least 3,000 years ago to about to roughly 1,500 years ago, building a complex shield volcano on the Cascade Crest as much as 1700 feet thick and 1.3 cubic miles in volume (Taylor, 1968 and 1981).  Initially, multiple basaltic lavas flowed from Belknap Crater’s main vent area downslope nearly seven miles to the east (Figure 4A.1).  The age of these eruptions is unknown, but must be greater than Little Belknap lavas dated at about 3,000 years old (Sherrod et al., 2004), which are superimposed on the early Belknap flows.  Little Belknap Crater erupted over a major subsidiary vent on Belknap Crater’s southeast flank, extruding substantial overlapping flow lobes of basaltic andesite that resulted in the formation of a shield volcano.  Two obvious kipuka can be observed just northwest of here where younger lavas from Little Belknap Crater flowed around the former high ground of small, basaltic andesite volcanoes formed earlier on the Cascade Crest near McKenzie Pass (Figure 4A.4).  About 2,600 years ago, South Belknap Cone formed (Sherrod et al., 2004).  Later, a major pulse of volcanism produced basaltic andesite that issued from vents on the north and south flanks of Belknap Crater, near the base of the summit cone, mainly moving downslope to the west as much as twelve miles from their source (Figure 4A.1).  These flows even reached the glaciated valley of the upper McKenzie River, disrupting its drainage.  Smaller flow lobes dated at about 1,800 years and 1,500 years old (Sherrod et al., 2004), moved southwest into Lake Valley on the west side of Scott Mountain, surrounding South Belknap Cone in the process.  Belknap Crater’s summit contains three main vent craters, and the northeast face of the larger, southern cone shows signs of an incipient glacially-carved cirque.

Figure 3.5A.3 - View north from McKenzie Pass copyrighted

Figure 4A.3.  The view north from the Dee Wright Observatory at McKenzie Pass; from left to right is Mt. Belknap, Little Belknap, Mt. Washington, (a distant Mt. Jefferson), and Black Butte.  The Yapoah Crater Lava Flow enters from the right and merges with lavas from Little Belknap Crater just right of center.

Figure 3.5A.4 - Kipuka at McKenzie Pass copyrighted

Figure 4A.4.  Kipuka at McKenzie Pass, formed where younger lavas from Little Belknap Crater flowed around the former high ground of two small, basaltic andesite volcanic cones.  Belknap Crater and Little Belknap Crater (the source of this lava) lay upslope.

Cascade Range.  Black Crater’s basaltic andesite cinder cone is the only major peak rising to the east of the observatory.

The dual volcanic edifices of the magnificent North and Middle Sisters (Figure 4A.5) lie to the south, beyond Collier Cone.  North Sister is the older of the two, a basaltic andesite and andesite composite volcano constructed on a broad shield volcano base beginning as much as 300,000 years ago (Taylor, 1968, 1981, and 1990).  Its flanks have been considerably eroded by ice masses of two glaciations and expose the central volcanic plug, dike systems, and near-vent volcanic breccias.  Middle Sister is a composite volcano comprised chiefly of basalt porphyry lava flows and lesser amounts of basaltic andesite, andesite, dacite, and rhyodacite lavas and pyroclastic deposits.  Its formation is believed to coincide with South Sister which began erupting about 93,000 years ago (Hill, 1992).  Collier Glacier lies sandwiched in a north-facing valley between North and Middle Sister Volcanoes, Oregon’s largest surviving glacier (Figure 4A.5).  Slightly to the southwest, Scott Mtn. can be seen in the near distance, a small Pleistocene summit cone perched on a broad shield volcano base.  Condon Butte lies a bit further south and somewhat closer (Figure 4A.1).  This subdued peak is a late Pleistocene cinder cone formed on the back of a glaciated ridge of basaltic andesite.  The cone is fairly large, but its eruption did not produce associated lavas.

Satisfied with the scenery and your newly acquired knowledge of its origins?  Return to your vehicle and continue the tour route, there is more to absorb.

Figure 3.5A.5 - North & South Sister copyrighted

Figure 4A.5.  North and Middle Sister Volcanoes as seen from near the Dee Wright Observatory at McKenzie Pass.  Collier Cone is visible on the ridge immediately in front of North Sister, and Collier Glacier, Oregon’s largest, lies sandwiched between the volcanoes.

15.5 (0.2)     McKenzie Pass; the Pacific Crest Trail crosses Highway 242 at this location.  You are at the heart of the High Cascades graben (Figure 4.2 and Figure 4.3).

As you drive, notice that fewer trees grow on the older lavas from Little Belknap Crater as compared to the younger Yapoah Cone.  This seems counter intuitive, but is primarily related to the greater coherency of the surface of the Little Belknap Lava Flow.  On the other hand, the surface of the Yapoah Cone Flow is rough and blocky, with ample spaces for the collection of finer particles to serve as a substrate for tree roots.  The colonization of trees is more closely governed by climate and substrate, than by the age of the lava flows.

15.8 (0.3)     The trailhead parking area to access the Pacific Crest Trail north of the highway is on the right-hand side of the road.  The summit of Belknap Crater and its smaller partner Little Belknap Crater can be reached by following the Pacific Crest Trail northward from this location; a marvelous day hike (see Belknap Crater and Little Belknap Crater Trail under Optional Hiking Trails at the end of this road log for a complete description of this hike).

16.1 (0.3)     Here, the highway passes over a narrow tongue of lava that poured southwestward from Little Belknap Crater (Figure 4A.1).

17.4 (1.3)     Refer to Map 4A.4.  Glacial striations on pre-Belknap-eruption bedrock surfaces just south of the highway near Craig Lake indicate a northwesterly flow of ice down an ancestral valley that is now completely buried by growth of the Holocene Belknap Crater shield volcano complex.

19.0 (1.6)     Pull your vehicle safely onto the road shoulder here and explore the margin of the lava flow from South Belknap Cone (Figure 4A.1), erupted about 1,800 years ago.  This is a good opportunity to examine the nature of the lava flow surfaces associated with Belknap Volcano.

Carefully climb onto the flow surface.  Notice how the surface here is fairly coherent, forming large, fractured slabs that occasionally exhibit ropy textures much like pahoehoe lava.  The rock composition itself is characterized by clusters of fine plagioclase, olivine, and clinopyroxene crystals.  This location also offers a great view of the southwest flank of Belknap Crater’s shield-like profile.  Twin Craters cinder cone which erupted about 2,600 years ago is visible to the northwest.

20.3 (1.3)     A small pullout on the right-hand side of the road here serves as a trailhead for hiking the trail to Hand Lake, blocked on its northeastern end by lavas from Twin Craters (Figure 4A.1).  The highway follows the depression of Lake Valley, formed between the older volcanic rock of Scott Mountain on the west, and the younger lava flows from Twin Craters and South Belknap Cone to the northeast, as well as Sims Butte to the south (Figure 4.1).

21.3 (1.0)     Refer to Map 4A.5.  Junction of OR Hwy 242 and FS Rd 2400-260.  This side road takes you to the Scott Mountain Trailhead parking area and Scott Lake Campground, an out and back drive of about two and half miles.  The hiking trail to the summit of Scott Mountain winds past several lakes and provides excellent views of the youthful mafic volcanism of the High Cascades from the east side, as well as views into the upper McKenzie River valley and the older, highly dissected Western Cascades from its southwest side (see Scott Mountain Trail under Optional Hiking Trails at the end of this road log for a complete description of this hike).

21.5 (0.2)     The highway returns briefly to Map 4A.4.  A good view of the Three Sisters stratovolcanoes lies to the southeast.  North Sister is the oldest, a basaltic andesite composite volcano constructed on an older basaltic shield volcano base.  Middle Sister is younger and South Sister is the youngest; both are composite volcanoes with much greater compositional variation and both lack a shield volcano base.

21.8 (0.3)     FS Rd 2400-250 is on the left, the entrance road to the Obsidian Trailhead and former Frog Camp Campground.  This is the trailhead to be used for the Obsidian and Scott Trails Loop which takes you past several significant Holocene cinder cones and their associated lava flows, as well as affording opportunities to visit the Obsidian Cliffs and Obsidian Falls, Collier Glacier (Oregon’s largest surviving glacier), and the summit of Middle Sister Volcano (see Obsidian and Scott Trails Loop under Optional Hiking Trails at the end of this road log for a complete description of this hike).

22.3 (0.5)     Refer to Map 4A.5.  Hwy 242 makes a right-hand curve here.  Sims Butte lies about half a mile to the southeast, an 800-foot-high basaltic andesite cinder cone, breached by lava flows on its western side (Figure 4A.1).  Sims Butte erupted between the end of late Pleistocene glaciation about 12,000 years ago and the eruption of Mt. Mazama about 7,700 years ago, based on the evidence that its lava and cinders overlie unweathered glacial deposits and in turn are blanketed by Mazama Ash (Taylor, 1968 and 1981).  Lava flows from Sims Butte poured westward down the glaciated trough of the White Branch of the McKenzie River for over nine miles and were later partially buried by lavas from Collier Cone (Figure 4A.1).

22.7 (0.4)     The highway curves left here and crosses the western lobe of the Sims Butte Lava Flow (Figure 4A.1).

23.1 (0.4)     There is a pullout on the left side of the highway here; to the south is the high-standing margin of the younger Collier Cone Lava Flow (Figure 4A.1).  The highway begins a series of tight switchbacks just beyond the pullout, descending into the White Branch’s classically U-shaped, glaciated valley.  A major outlet glacier of the High Cascades ice cap occupied this valley during the last glacial maximum of the late Pleistocene.  Numerous road cuts expose the weathered top of lavas from Sims Butte along the descent.

26.9 (3.8)      Alder Campground is on the right at this location.  The trailhead parking area for Linton Lake lies to the left. The lake formed where lavas from Collier Cone (Figure 4A.1) ponded and partially back-filled into the mouth of Linton Creek and dammed its drainage to the west (Taylor, 1968 and 1981).  Currently, there is no surface flow from the lake, instead water seeps through the porous lavas and underlying glacial deposits downslope.  The popular hike to Linton Lake is only about four miles round-trip, and with some scrambling off-trail when you first reach the lake, it provides good opportunities to explore the Collier Cone Lava Flow over several nested lava levees and the central lava gutter.

28.5 (1.6)     Parking areas occur on both sides of the highway here for the Proxy Falls Trailhead (see Proxy Falls Trail under Optional Hiking Trails at the end of this road log for a complete description of this hike).  The trail to pretty upper and lower Proxy Falls is a short, one and a half mile loop hike traversing the lower end of basaltic-andesite aa lavas from Collier Cone (Figure 4A.1).  A pleasant, family-oriented hike; who doesn’t enjoy a good waterfall now and then?

From here to the junction of Oregon Highway 242 and 126, the drive offers occasional views of the steep walls of White Branch’s canyon.  These slopes expose a sequence of basaltic lava flows as much as 1500 feet thick erupted between 3.9 and 2.1 million years ago.  They form the base of the High Cascades platform (Taylor, 1981 and Priest et al., 1983), essentially a series of overlapping and intermingled shield volcanoes (Figure 4.2).  Many of the flows are thin and separated by brick-red, baked soil horizons developed on sand and gravel; compositionally, they are relatively coarse-grained, containing aggregates of olivine and augite in a matrix of fine, tabular plagioclase feldspar.  In the lower reaches of the canyon, older volcanic and volcaniclastic rocks of the Tertiary Outerson Formation of the Western Cascades geologic province become exposed below a disconformity.  Road cuts and glimpses through the trees near the road reveal the rubbled surface of the late Pleistocene to early Holocene Sims Butte Lava Flow.  White Branch Creek and Lost Creek appear at springs to flow short stretches, only to disappear into sinks again, within the Sims Butte lavas.

29.3 (0.8)     A sharp right-hand bend in the road here lies very close to the lowest elevation in the valley that was reached by lavas from Collier Cone (Figure 4A.1).  The road remains on the Sims Butte lava flow.

35.5 (6.2)     Refer to Map 4A.6.  The road passes from Sims Butte lavas to glacial outwash at about this locality (the actual position is obscured by vegetation).  The terminus of the Sims Butte Lava Flow occurs to the right shortly beyond this point (Figure 4A.1).

37.4 (1.9)     Junction of Oregon Hwy 242 and Oregon Hwy 126.  Turn right (northeast) onto  Hwy 126; a left (west) turn here would instead begin Field Trip 4C.  The hamlet of McKenzie Bridge lies about two miles down the highway to the left.  If you are in need of gas, food, restrooms or otherwise, this is the only location where those amenities are available on this field trip outside of Sisters, Oregon.  There are also nearby Forest Service Campgrounds if you are in need of a more lengthy rest; this location does provide a convenient layover for this field trip.  McKenzie Bridge also houses a Willamette National Forest Office with interpretive displays, and publications available on local human and natural history.

Foley Ridge lies south of the junction and extends about five miles downvalley.  Taylor (1981) and Priest et al. (1983) indicate that it is comprised of intracanyon basaltic lava flows of early High Cascades origin (Figure 4.2). That is, the lavas poured down a pre-existing drainage, presumably a tributary of the ancestral McKenzie River, only to become a resistant ridge today as new streams cut through older, more weathered, and less resistant material around the margins of the younger basalts.  Foley Ridge forms a classic example of what geologists refer to as inverted topography (Figure 1A.10 in FIELD GUIDE TO THE BEND AREA).  Intracanyon lava flows also occur as erosional remnants perched on the slopes of the McKenzie River valley for about nine miles downstream.

Road cuts to the east and west of this junction provide excellent exposures of glacial till associated with the terminal moraine deposited during the late Pleistocene Suttle Lake advance of the Cabot Creek glaciation (Figure 4A.6).  Research by this author (Bevis et al., 2008) indicates that the White Branch glacier was one of several glacial lobes that extended down the western flank of the High Cascades platform, part of the larger ice cap that covered much of the upper watershed of the McKenzie River (Figure 4A.7).  A similar lobe extended down the U-shaped valleys of the upper McKenzie River, the South Fork of the McKenzie River, and Horse Creek, just on the other side of Foley Ridge.  Older glacial deposits are plastered on the inner slopes of the McKenzie River valley at least as far as Blue Ridge Reservoir (Bevis et al., 2008) (Figure 4A.6).  Evidence of older glaciations is also preserved in the valley of the South Fork of the McKenzie River.  These glacial episodes may represent one or more middle to late Pleistocene glaciations of the High Cascades, possibly correlative with the Jack Creek and Abbott Butte glaciations of Scott (1977).

Figure 4A.6 - McKenzie River Basin Drift Units

Figure 4A.6.  Aerial extent of glacial deposits in the McKenzie River watershed associated with the late Pleistocene Cabot Creek glaciation and late-middle Pleistocene Jack Creek glaciation (Bevis et al., 2008).

Figure 4A.7 - McKenzie River Basin LGM Ice Cap

Figure 4A.7.  Reconstruction of the alpine glacial system in the McKenzie River basin during the late Pleistocene Cabot Creek glaciation (Bevis et al., 2008).

38.4 (1.0)     A road to the left here leads to Belknap Hot Springs and Resort.  This is the commercially-developed equivalent of three hot springs in the area aligned along north trending normal faults that make up the western boundary of the High Cascades graben (Figure 4.2 and Figure 4.3).  Sharp bends in the valleys of the upper McKenzie River (as visibly displayed on Figure 4.1), Horse Creek, and the South Fork of the McKenzie, as well as offsets in the volcanic and volcaniclastic rocks of the Western Cascades geologic province, reveal the presence of these faults; although their actual locations are often disguised by overlying, younger High Cascades lavas and glacial deposits (Taylor, 1981; Priest, 1983; Priest et al., 1983; Priest, 1990; and Taylor, 1990).  These valleys (and faults) generally mark the western edge of the High Cascades geologic province in this area.

As you drive north, the ridge to your left (west) is capped by early High Cascades basaltic andesite and basalt dated to as young as 5 million years, which overlie weathered slopes comprised of older Tertiary volcanics of the Outerson Formation (Figure 4.2).  On the right (east), the Outerson Formation is only exposed at the base of the slopes, with a thicker pile of younger, Pliocene and Pleistocene High Cascades volcanic rocks above, including intracanyon lava flows dated to as old as 4 million years, which pass over the fault where the McKenzie River has breached the escarpment.  This suggests that active normal faulting and formation of the High Cascades graben occurred between about 5 and 4 million years ago (Taylor, 1981; Priest, 1983; Priest et al., 1983; Priest, 1990; and Taylor, 1990).

39.7 (1.3)     Junction of Oregon Hwy 126 and FS Rd 2649 (Scott Creek Rd) to the right.  A relativelyshort drive of about four miles round trip eastward and upslope on FS Rd 2649 traverses the unconformable boundary between the Western Cascades and High Cascades volcanic rocks (Taylor, 1981).  Below the boundary, road cuts reveal nonporphyritic platy basalts, porphyritic basalts, basalt breccias, and tuffaceous interbeds (Figure 4A.8) of the Tertiary Outerson Formation dipping gently to the south and cut by E-W normal faults.  Above the boundary are palagonitic tuffs, basaltic dikes and lava flows, and basaltic pillow lavas, overlain by columnar jointed, intracanyon basalts (Figure 4A.9); a thick sequence of the Pliocene to Pleistocene early High Cascades volcanics.  The presence of pillow basalts within these volcanic rocks suggests that the ancestral McKenzie drainage was frequently dammed by lava flows to form small lake basins at the base of the dissected Western Cascades escarpment.

Figure 3.5A.6 - Pyroclastic Deposits copyrighted

Figure 4A.8.  Tuffaceous interbeds of the Tertiary Outerson Formation; note the coarse rhyolitic rock fragments dispersed throughout (the large one in the lower portion of the photo deforms the ash layer beneath it indicating near simultaneous deposition).

Figure 3.5A.7 - Intracanyon Lava Flows copyrighted

Figure 4A.9.  Columnar jointed, Pliocene to Pleistocene intracanyon lava flows.

On the upper McKenzie River valley for about the next ten miles, road cuts along Hwy 126 and cliff exposures on adjacent canyon walls reveal the variously faulted volcanic and volcaniclastic rocks of the Western Cascades geologic province overlain by early High Cascades volcanics.

42.4 (2.7)     Junction of Oregon Hwy 126 and FS Rd 2654 (Deer Creek Rd) to the left.  A short drive down FS Rd 2654 and across a bridge over the McKenzie River brings you to a parking area on the right.  Take the path leading down to the river from the parking area to visit a free, publicly accessible hot springs.  Be forewarned, exposures here are not necessarily of the earthly variety!

44.4 (2.0)     Refer to Map 4A.7.  Junction of Oregon Hwy 126 and FS Rd 2657 (Olallie Rd) to the right.

This road follows the left-hand margin of a youthful looking lava flow from Scott Mountain.  Shortly, the road enters a U-shaped glaciated valley, the conduit for another ice lobe from thelate Pleistocene ice cap covering the Cascade Crest; the Scott Mountain lavas become buried by glacial moraines.  Higher up on the flanks of Scott Mountain, lava flows are glacially scoured or entirely removed by glacial erosion, the summit of Scott Mountain itself having formed the prow of the ship around which the late Pleistocene ice cap flowed (Figure 4A.7).

45.7 (1.3)     The Trailbridge Reservoir parking area is on the left-hand side of the road.  Pull in here to examine the road cut to the right of the highway.  Watch for traffic and falling rocks!

This is an exposure of basaltic andesite ash-flow tuff, faulted in several places and forming blocks tilted down to the northeast (Taylor, 1968 and 1981).  The volcanic rock unit is about 5 million years old.  It is generally mafic in composition with large crystals of olivine, pyroxene, and plagioclase feldspar, exhibits vertical zonation of rock fragments which decrease in abundance upward, lacks sorting with regard to size or density of rock fragments, has a micro-vesicular top and base, and its elongate rock fragments show a distinctively preferred orientation parallel to the base; all of these characteristics suggesting its origin as a hot, pyroclastic flow.  The unit can be traced discontinuously for about seven miles north to south.

46.3 (0.6)     Junction of Oregon Hwy 126 and FS Rd 2600-730 (Trailbridge Reservoir Rd).  A left (northwest) turn onto FS Rd 2600-730 and a one mile round-trip drive will take you to a trailhead for the McKenzie River Trail and provide access to Tamolitch Falls and Pool.  Immediately after crossing the bridge over the McKenzie River, turn right (northeast) onto FS Rd 2600-655.  As you cross the bridge, glance left to the hydroelectric power station on the right bank of the McKenzie River (Figure 4A.10).  This station generates electricity by water siphoned from the river several miles upstream at Carmen Reservoir, brought by tunnel to Smith Reservoir on Browder Creek through the ridge up which FS Rd 2600-655 passes, and by tunnel again through the same ridge to the power station. Siphoning of McKenzie River water for hydroelectric power creates a dry stretch of the river (and the dry Tamolitch Falls) which now only fills with water in the spring after exceptionally wet winters.  Several miles below Carmen Reservoir water gushes from springs in the river bed that issue from the plunge-pool (Tamolitch Pool) at the base of dry Tamolitch Falls.

Figure 3.5A.8 - Hydropower Plant copyrighted

Figure 4A.10.  A small hydroelectric power plant on the upper McKenzie River near Trailbridge Reservoir.  Siphoning of water upstream to generate electricity here produces a stretch of the river bed that commonly dries out except during exceptionally wet springs, resulting to Tamolitch Pool (once Tamolitch Falls).

Follow FS Rd 2600-655 about three tenths of a mile to the first switchback in the road and a small parking area for the McKenzie River Trail.  This trail provides access to Tamolitch Falls and Pool as it skirts the western side of the McKenzie River, traversing old growth temperate rain forest and a Holocene intracanyon aa lava flow from Belknap Crater (see Tamolitch Falls (Pool) Trail under Optional Hiking Trails at the end of this road log for a complete description of this hike).

Return to the main highway after completing this wonderful little hike.

47.4 (1.1)     The highway begins crossing Kink Creek valley on artificial fill as it ascends a long grade up onto the crest of late Pleistocene, High Cascade, intracanyon basaltic lava flows in the upper McKenzie River valley.  These lava flows entered the McKenzie valley prior to the development of the Belknap Crater shield volcano complex when streams on the western slopes of Mount Washington over to Black Crater coalesced to drain into the upper McKenzie at this location (Taylor, 1968 and 1981).  Road cuts beyond the valley fill section of the highway expose these lavas.

50.0 (2.6)     Refer to Map 4A.8.  Hwy 126 crosses onto 1,500-year-old basaltic lava flows from the north base of Belknap Crater Volcano (over eight miles upslope to the east) (Figure 4A.11).  A double cascade of flows poured down steep terrain to the east and into the McKenzie River valley, eventually reaching a couple of miles downriver (the aa lavas that can be observed on the Tamolitch Falls Trail).

Figure 3.5A.9fix - Santiam Pass Lava Flows

Figure 4A.11.  Geologic map of the Upper McKenzie River valley and Santiam Junction area delineating the major Holocene basaltic and basaltic andesite vents and lava flows (modified from Taylor, 1968).

50.7 (0.7)     Several road cuts and cliffs on the right-hand side of the highway expose multiple, thin basaltic-andesite lava flows dated at about 1.2 million years (Figure 2), probably part of a large early Pleistocene High Cascade shield volcano whose central plug and flanking lavas can be seen in cross-section on cliffs across the McKenzie valley to the west (Taylor, 1968 and 1981).

51.8 (1.1)     There is a parking area on the left side of the highway for Sahalie Falls at this location.  Pull in and take the trail to the overlooks of both upper Sahalie Falls (Figure 4A.12) and lower Koosah Falls.  The round-trip distance for this short excursion is about one mile.  These falls have formed where youthful, resistant intracanyon lava flows have choked the McKenzie River valley.  Two basaltic andesite lava flows issued from the Sand Mountain chain of cinder cones about three miles upslope to the east and flowed into the valley about 3,000 years ago (Figure 4A.11); one flow blocked the valley at Koosah Falls, and the second at Sahalie Falls.

Figure 3.5A.10 - Koosah Falls copyrighted

Figure 4A.12.  Sahalie Falls (above) and its lower twin, Koosah Falls (not pictured), formed where resistant, late Holocene intracanyon lava flows from Sand Mountain choked the McKenzie River valley.

The field trip route now traverses a large area of youthful lava flows and cinder cones for about the next ten miles (Figure 4.2 and Figure 4A.11).  These features are variously associated with the Sand Mountain, Nash and Little Nash, and Lost Lake volcanic chains described herein.

52.3 (0.5)     Junction of Oregon Hwy 126 and FS Rd 2600-770 to the right.  FS Rd 2600-770 provides access to Clear Lake and Clear Lake Campground (a drive of just over two miles round-trip), another excellent potential stop-over on this field trip.  Clear Lake is nearly one and a half miles long and reaches 120 feet deep in places.  It was formed behind a lava dam that blocked the McKenzie River and serves as the source of that river.  The lava flow was erupted from a small cinder cone south of the Sand Mountain volcanic chain roughly 3,000 years ago (Figure 4A.11).  The rising waters of the lake inundated a forest and numerous snags are still rooted on the bed of the lake.  The aa lavas of the flow can be explored along the southeast shore of the lake near the campground.

The route briefly flips back to the northeast corner of Map 4A.7 before entering Map 4A.9.

54.0 (1.7)     Refer to Map 4A.9.  The outlet channel for Fish Lake can be observed on the left.  Fish Lake was formed when basaltic lavas issued from early, now buried, vents of the Sand Mountain chain of cinder cones and flowed across Hackleman Creek about 3,800 years ago (Taylor, 1968 and 1981).  The highway passes adjacent to these lavas for about the next half mile (Figure 4A.11).  Lava flow surfaces preserve many examples of ropy texture and some good tree molds.  Map 4A.8 and 4A.10 prominently display the aligned cinder cones of the Sand Mountain chain and related cones further northeast.

54.5 (0.5)     A road on the left (west) here goes to the Fish Lake interpretive area.  Notice that there is now no longer a lake here.  Fish Lake is apparently an early victim of climate change, as is the nearby Lava Lake (just northwest of the OR Hwy 126/US Hwy 20 junction).  As late as the early 20th century, it once contained water year-round, but by the later 1900’s, it contained water only seasonally after wet winters.  It has remained dry, except during exceptional spring snowmelt, since the mid-1990’s.

Just up the road from here, Hwy 126 passes from the thin, ropy-textured, basaltic lavas issued from the Sand Mountain volcanic chain to a younger, thicker, blockier, basaltic andesite aa lava flow from Nash Crater that flooded the uppermost McKenzie Valley about 3,000 years ago (Figure 4A.11).

55.9 (1.4)     Refer to Map 10A.  Oregon Hwy 126 merges with US Hwy 20 here.  Turn right (east) onto Hwy 20/126.  The junction area shows evidence of invasion by several lava flows (Figure 4A.11), the oldest is a 3,800-year-old flow erupted from a group of cinder cones between Nash Crater and Sand Mountain, a slightly younger flow issued from a now buried vent near Nash Crater which forms the lavas surrounding the junction itself, and the youngest is the 3,000-year-old Nash Crater aa lava flow encountered just south of the junction (Taylor, 1968 and 1981).

56.9 (1.0)     Sawyers Ice Cave, a short lava tube of no more than a few hundred feet, lies just off the right-hand side of the road at this location (Figure 4A.13).  The tube occurs in the lavas from Nash Crater.  If you wish to visit the lava tube, park carefully on the road shoulder where large boulders now block an old parking area; then walk uphill to the southeast several hundred feet to the cave entrance.

Figure 3.5A.11 - Sawyers Ice Cave copyrighted

Figure 4A.13.  Sawyers Ice Cave formed in lavas from Nash Crater.

57.3 (0.4)     The highway begins crossing an open area comprised of youthful lava flows; the one to the right from Nash Crater is slightly older and blockier than the one to the left which is from Little Nash Crater (Figure 4A.11).

58.2 (0.9)     FS Rd 2600-821 on the left (north) here goes to an active quarry on Little Nash Crater.  This small cinder cone was breached on its western flank during the later stages of its eruption and produced lavas that flowed outward, fan-like, nearly two miles downslope from the base of the breach (Figure 4A.11).  A short side trip into the quarry allows observation of the internal volcanic stratigraphy of a cinder cone (Figure 4A.14).  The quarry area is littered with volcanic bombs and fragments of scoria, and quarry walls reveal bedded cinders.  Watch for trucks and heavy equipment; if the quarry is being worked, do not enter.

Figure 3.5A.12 - Little Nash Crater copyrighted

Figure 4A.14.  Tephra stratigraphy exposed in the cinder quarry at Little Nash Crater.  Note the finer layers of ash interbedded with thicker, coarser layers of scoriaceous cinders, as well as slump faulting presumably related to the initial building of the late Holocene mafic cinder cone.

59.1 (0.9)     Santiam Junction; the junction of US Highway 20/OR Highway 126 and Oregon Highway 22.   Turn right (east) and remain on Hwy 20/126.

This area has a relatively complex late Quaternary geologic history (Taylor, 1968 and 1981).  The major ridges to the north and south are lateral moraines deposited by retreating Late Pleistocene glaciers about 18,000 years ago.  Early Holocene lava flows issued from vents between Nash Crater and Sand Mountain and flowed over the junction area from the south (Figure 4A.11).  These flows were overridden by the Lava Lake Flow from Nash Crater.  Subsequently, the Lost Lake Cones ahead formed in the Late Holocene and lava flows covered the area from the east.  After a period of quiescence, younger, slightly more siliceous basalt, the Fish Lake Flow from vents on the south and northwest sides of Nash Crater, buried portions of the area once more.  During this later period of volcanic activity, Little Nash Crater and its related basaltic flows were formed.

A well-preserved, late Pleistocene, recessional, right-lateral moraine of Suttle Lake age lies just northwest of the Santiam Junction area (Map 4A.10).  A road cut on OR Highway 22, a detour of about 3.5 miles round-trip from this intersection, exposes a moderately weathered till that is overlain by pyroclastic material comprised of a thin layer of Mazama Ash, fine mafic ash from Nash Crater’s eruption, and a couplet of coarse rock fragments and mafic lapilli from the eruption of Little Nash Crater.

59.8 (0.7)     The highway makes a right-angle bend here and then curves gently left around the largest, central cinder cone of the Lost Lake volcanic chain (Taylor, 1968 and 1981).  In this area    area, three cinder cones were constructed across the now buried Lost Creek glacial trough (Figure 4A.11), two larger cones north of the highway, and another smaller cone south of the highway behind the glaciated ridge extending west from Potato Hill at 2:00.  The volcanic ridge formed from the eruption of the two northerly cones dammed Lost Creek to form Lost Lake.

60.6 (0.8)     FS Rd 2000-835 to the left leads to Lost Lake Campground.  Lost Lake is first seen to the left side of the highway shortly beyond this junction.

61.7 (1.1)     The basaltic andesite lava flows exposed in the road cut on the left form the base of Pleistocene Maxwell Butte, a relatively small, well-glaciated shield volcano.  The flows here exhibit a complex pattern of columnar jointing related to the filling of a paleochannel cut into the original topography of older flows by younger lavas.  When lava flows cool and congeal, they often develop columnar jointing perpendicular to the surface over which they have flowed.  Here, the younger flow occupying the former channel has developed a uniquely radial columnar jointing pattern perpendicular to the paleochannel’s curved surface (Figure 4A.15).

Figure 3.5A.13 - Columnar Basalt copyrighted

Figure 4A.15.  A lava flow filling a paleochannel cut into older lavas that exhibits a unique radial columnar jointing pattern formed perpendicular to the former channel’s surface.

62.5 (0.8)     Refer to Map 4A.11.  Highway 20/126 makes a gentle, nearly 180°, right-hand curve here and begins to climb as it passes around the base of Hogg Rock, a glaciated dome of mafic to intermediate volcanic igneous rock, a portion of the early High Cascades (Figure 4.2).  Notice the platy jointing in the basaltic andesite exposed in the extensive road cut.  This jointing pattern tends to form parallel to cooling surfaces.  It is often enclosed within columnar jointed blocks outlined by black, glassy rinds that form perpendicular to the cooling surfaces.

Highway 20/126 briefly returns to Map 4A.10.  As you round the base of Hogg Rock, now curving broadly left, glance to the south to view Hoodoo Butte and Hayrick Butte.  Hoodoo Butte is a late Pleistocene basaltic cinder cone with a small summit crater, but Hayrick Butte is much older, at least several hundred thousand years old based on its degree of glacial sculpting (Taylor, 1968 and 1981).  Hayrick Butte is composed of platy-jointed basaltic andesite similar to Hogg Rock.  Hoodoo Butte was partially shielded from the full affects of late Pleistocene glaciation by its youth and the intensely glaciated ridge of Hayrick Butte.

Examine Hogg Rock and Hayrick Butte on Map 4A.11.  Their shapes are very similar; and their essentially identical composition, coupled with their alignment, suggest that both flat-topped volcanic features formed nearly simultaneously, erupting from the same magma source, along the same north-south fracture.

63.7 (1.2)     Junction of U.S. Highway 20 and FS Rd 2690.  A right-hand turn here onto FS Rd 2690 will eventually take you to the trailhead to Sand Mountain and gorgeous views of the northwest side of Mt. Washington and Belknap Crater, the upper McKenzie River valley, and the Sand Mountain chain of cinder cones itself.  From there, you can readily observe young lavas from the Sand Mountain area and Belknap Crater further south that flowed into the drainage of the upper McKenzie.

To reach the trailhead, drive FS Rd 2690 southward, around the eastern side of Hayrick Butte for about 3.2 miles.  At the “Y” junction with FS Rd 2690-810, bear right onto FS Rd 2690-810.  Watch out from OHV enthusiasts!  This road is suitable for non-four-wheel-drive vehicles in dry weather, but don’t risk it if rain threatens, immediately after rains, or when snowmelt is ongoing in the spring.  Drive west for 2.9 until you reach the eastern flank of Sand Mountain at a road junction and a “Road Closed to Motorized Vehicles” sign.  Park here, and walk the remaining distance to the Sand Mountain fire tower lookout (see Sand Mountain Trail under Optional Hiking Trails at the end of this road log for a complete description of this hike).  If the stairway entrance to the tower is signed “closed”, please be sure to respect the privacy of the lookout attendant and do not enter.

This drive affords a great opportunity to view Hayrick Butte and Hoodoo Butte without the hazard of driving around a curve on a busy highway (Figure 4A.16).  Compare the two buttes; note Hayrick Butte’s unusual flat top and steep flanks; its shape betrays its origin.  In all likelihood, the butte formed when a silicic lava dome was erupted onto the surface beneath an ice cap.  Its steep walls were propped up by the surrounding ice, and its flat top formed the surface of an inverted “lava lake”.  Flowing glacial ice later sculpted and accentuated its features.  On the other hand, Hoodoo Butte is younger and is a product of mafic volcanism, accumulated as a typical conical-shaped cinder cone, albeit somewhat modified by glaciation.

Figure 3.5A.14 - Hoodoo Butte copyrighted

Figure 4A.16.  Hayrick Butte (right), its unusual shape the product of its origin as a silicic dome erupted at the base of a Pleistocene ice cap; in contrast, Hoodoo Butte (left) is a younger, less glacially influenced, mafic cinder cone.

64.6 (0.9)     Santiam Pass and the northern margin of a large glaciated area of Pleistocene basaltic lava flows spreading east and west from the crest of the High Cascade divide from here to Mt. Washington.

67.5 (2.9)     The entrance of the parking area for Corbett Snow Park is on the right.  The parking area has an interesting Forest Service interpretive kiosk describing the 90,000+ acre B & B Complex forest fire that occurred in this area in the late summer of 2003.

The affects of this fire have been evident from the highway since passing Hogg Rock.  The forests west of the Cascade Crest typically burn at intervals of 100 to 125 years and produce high intensity surface to crown fires where most trees are killed and the forest start anew.  The forests east of the Cascade Crest are in the rainshadow of the High Cascades and are drier, typically burning at intervals of 5 to 25 years.  These are low intensity fires where few mature trees are killed.  They serve to clear out undergrowth and dead trees, creating a more park-like forest.  Fire suppression and inappropriate timber harvesting practices in our National Forests over the previous century combined to result in the buildup of considerable undergrowth and dead wood, setting up a tinderbox scenario throughout the West, such as the one observed here.  Fortunately, forest managers have learned how better to manage these forests as natural ecosystems, helping to reduce the potential of devastating fires in the future.

68.5 (1.0)     To the right are the headwaters of Lake Creek valley.  Blue Lake occupies the upper end of the valley (Map 4A.11), formed in a Holocene volcanic crater surrounded by a rim of volcanic cinders and rock fragments that erupted about 3,500 years ago (Taylor, 1968 and 1981).  This volcanic feature is known as a maar volcano; no lava made its way to the surface, but eruptions must have been considerably violent, suggested by the crater blasted through bedrock and the profusion of rock fragments scattered in all directions.  Explosive eruptions of this type are usually associated with migration of magma upward into contact with overlying, water-saturated material.

68.9 (0.4)     For about the next three quarters of a mile, the road cuts on the left side of the highway expose basaltic andesite lava flows of the High Cascade platform, probably part of the shield volcano on which Three Fingered Jack now rests (Taylor, 1968 and 1981).  These basalts are reversely magnetized, indicating an age greater than 700,000 years.  The upper end of the left-lateral late Pleistocene Suttle Lake moraine caps the ridge; exposures of till can be observed overlying the volcanic rock in several road cuts.

69.3 (0.4)     A scenic pullout on the right side of the road here affords a good view of the glaciated upper drainage of Lake Creek valley.  Mt. Washington Volcano’s highly dissected shield to the southwest.  Cache Mountain is in the middleground to the south.  This prominent ridge is a remnant of the early Pleistocene High Cascades basaltic andesite platform (Figure 4.2).  It formed a divide between ice lobes emanating from the ice cap covering the Cascade Crest to the west, but the eastern portion of the ridge was itself not glaciated.  In the foreground to the southeast, Suttle Lake occupies a moraine-dammed basin in the Lake Creek valley.  Notice the long right-lateral moraine sweeping down from Cache Mountain that encircles the southeast side of the lake (combine Map 4A.11 and Map 4A.12).  This moraine forms part of a terminal moraine complex deposited by the Suttle Lake ice lobe, a set of nested moraines that forms the “type locality” of the late Pleistocene Suttle Lake advance of the Cabot Creek glaciation (Scott, 1977).  The moraines are covered by fine ash from the Holocene Sand Mountain eruptions and cinders from the eruption of the Blue Lake maar volcano to the west (Taylor, 1968 and 1981).

71.4 (2.1)     Refer to Map 4A.12.  Junction of US Highway 20/OR Highway 126 and FS Rd 2070; continue on Hwy 20/126.  A right turn onto FS Rd 2070 provides access to Suttle Lake and Scout Lake, a small kettle lake residing on the hummocky morainal topography surrounding Suttle Lake.  Several fine Forest Service Campgrounds are available here.  Shortly after this junction, several road cuts on the right-hand side of the highway over the next mile reveal till of the late Pleistocene terminal moraines enclosing the lower end of the Suttle Lake basin.

72.3 (0.9)     Junction of U.S. Highway 20/OR Highway 126 and FS Rd 12 (Camp Sherman and Metolius River Rd).  Remain on Highway 20/126.  A left turn here diverges onto Field Trip 4D, a route that highlights the glacial geologic history of the upper Metolius River watershed, and the location where Scott (1977) developed his chronology of glaciation for the Oregon Cascades.  The field trip route leads clockwise along the glaciated eastern flank of Three Fingered Jack, through the upper watershed of the Metolius River, over Green Ridge, around the northern and eastern flanks of Black Butte, eventually rejoining US Highway 20.  The morainal deposits of two Pleistocene glaciations, hiking options along the eastern slope of the Cascade Crest, the headwater springs of the Metolius River, and the Green Ridge escarpment offer numerous points of interest along the way.

As you drive, look for intermittent views of the Pleistocene Black Butte cinder cone to your left.  You may also notice the distinctive change in vegetation.  As you descended from Santiam Pass, you crossed into the rainshadow of the Cascade Range.  Orographic lifting of warm, moist air masses blowing eastward from the Pacific causes plentiful precipitation on the upwind side of the Cascades, but progressive drying of these air masses by the time they reach the leeward side of the mountains is responsible for much reduced precipitation east of the range crest, hence the name “rainshadow”.

76.9 (4.6)     Junction of U.S. Highway 20/Oregon Highway 126 and the entrance road to Black Butte Ranch resort.  Remain on highway 20/126.  The eruption of Black Butte cinder cone disrupted the former drainage of the upper Metolius River forming extensive wetlands on the upslope side of the cinder cone in this area.  Unfortunately, the wetlands have been extensively modified by the development of the Black Butte Ranch.

78.4 (1.5)     Junction of U.S. Highway 20/Oregon Highway 126 and FS Rd 11.  Remain on highway 20/126;  this road junction is the terminal point of Field Trip 4D.  Just ahead, Hwy 20/126 crosses Indian Ford Creek (usually only containing water in spring).  Indian Ford Creek formerly drained the now nonexistent Black Butte Swamp, once located at the southwest foot of Black Butte.  Draining the swamp and redirecting water courses for Black Butte Ranch has resulted in lowering of the water table in this area.

From here to Sisters, OR, the highway traverses a relatively flat alluvial plain consisting of a thick pile of distal outwash sands and gravels carried in by meltwater streams from the eastern footslope of the glaciated High Cascades, resting on early Pleistocene lava flows of the Deschutes Formation.

83.3 (4.9)     Junction of U.S. Highway 20/Oregon Highway 126 and the connector road to Oregon Highway 242 (W. Hood Ave).  Continue on Hwy 20/126 into Sisters, OR.

83.6 (0.3)     Junction of U.S. Highway 20/Oregon Highway 126 and Oregon Highway 242.  Proceed eas into Sisters, OR.

83.8 (0.2)     Junction of U.S. Highway 20/Oregon Highway 126 (W. Cascade Ave) and FS Rd 16 (Elm Street).  This is the end of Field Trip 4A.

Field Trip Road Maps

Map 4A.1 - Sisters web version

Map 4A.1.  Color shaded-relief map of the Sisters 7.5” Quadrangle containing segments of Field Trip 4A, 4B, and 4D.

Map 4A.2 - Black Crater

Map 4A.2.  Color shaded-relief map of the Black Crater 7.5” Quadrangle containing a segment of Field Trip 4A.

Map 4A.3 - Mt Washington

Map 4A.3.  Color shaded-relief map of the Mount Washington 7.5” Quadrangle containing a segment of Field Trip 4A.

Map 4A.4 - North Sister

Map 4A.4.  Color shaded-relief map of the North Sister 7.5” Quadrangle containing segments of Field Trip 4A.

Map 4A.5 - Linton Lake

Map 4A.5.  Color shaded-relief map of the Linton Lake 7.5” Quadrangle containing segments of Field Trip 4A.

Map 4A.6 - Belknap Springs web version

Map 4A.6.  Color shaded-relief map of the Belknap Springs 7.5” Quadrangle containing a segment of Field Trip 4A, as well as segments of Field Trip 4C.

Map 4A.7 - Tamolitch Falls

Map 4A.7.  Color shaded-relief map of the Tamolitch Falls 7.5” Quadrangle containing a segment of Field Trip 4A.

Map 4A.8 - Clear Lake

Map 4A.8.  Color shaded-relief map of the Clear Lake 7.5” Quadrangle containing a segment of Field Trip 4A.

Map 4A.9 - Echo Mountain

Map 4A.9.  Color shaded-relief map of the Echo Mountain 7.5” Quadrangle containing a segment of Field Trip 4A.

Map 4A.10 - Santiam Pass

Map 4A.10.  Color shaded-relief map of the Santiam Junction 7.5” Quadrangle containing a segment of Field Trip 4A.

Map 4A.11 - Three Fingered Jack

Map 4A.11.  Color shaded-relief map of the Three Fingered Jack 7.5” Quadrangle containing segments of Field Trip 4A and 4D.

Map 4A.12 - Black Butte web version

Map 4A.12.  Color shaded-relief map of the Black Butte 7.5” Quadrangle containing segments of Field Trip 4A and 4D.