Road Log

0.0 (0.0)       Refer to Map 4B.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 related to late Pleistocene Suttle Lake 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 4.1).  A small graben forms the northern end of this trough which is crossed on 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 here 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 US 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 US Highway 20/Oregon Highway 126 (Santiam Highway) and W. Hood Avenue, the connector road for Oregon Highway 242 (McKenzie Highway).  Turn left (south) onto the Hwy 242 connector road.

0.7 (0.2)       Intersection of the Hwy 242 connector road (W. Hood Ave) and Oregon Hwy 242 (McKenzie Highway).  Turn right (west) onto Hwy 242.  Excellent views of the Three Sisters volcanoes to the southwest.

1.9 (1.2)       Junction of Hwy 242 and FS Rd 15.  Turn right (southwest) onto FS Rd 15; continuing straight on Hwy 242 diverges onto Field Trip 4A.

4.9 (3.0)       Junction of FS Rd 15 and FS Rd 1513 to the left (east); continue on FS Rd 15.  Shortly after this junction, the road begins climbing the lower eastern flank of Trout Creek Butte, a small Pleistocene shield volcano comprised mostly of basaltic andesite lavas.

6.6 (1.7)       Refer to Map 4B.2.  FS Rd 15 begins traversing morainal deposits at approximately this location related to the Suttle Lake advance of the Cabot Creek glaciation (Scott, 1977 and Bevis et al., 2011).   Note the till exposed in road cuts as you drive ahead.

This sediment marks the extent of glacial ice occupying the Whychus Creek drainage during the last glacial maximum (LGM) of the late Pleistocene.  This author has recently completed detailed mapping of the glacial deposits in the drainage (Bevis et al., 2011) and has found no evidence for older till of the late middle to late Pleistocene Jack Creek and Abbott Butte glaciations, in agreement with the reconnaissance work of Scott (1977).  Figure 4B.1 shows the limit of the LGM ice mass during the late Pleistocene Suttle Lake advance of the Cabot Creek glaciation.  Older deposits are not exposed in the Whychus Creek watershed because the more recent glaciation was more extensive; these older deposits were either buried by or incorporated into younger material as the Suttle Lake (LGM), Whychus Creek glacier advanced. The unusually extensive LGM ice mass was probably an artifact of a larger area of higher elevation for ice accumulation associated with growth of the Middle and South Sister Volcanoes.  Both volcanoes began erupting about 100-90,000 years ago and had fully developed by roughly 50,000 years ago, long after the Jack Creek glacial period had ended, but prior to the Cabot Creek glaciation.  Note that the glacial deposits seen here along FS Rd 15 were likely generated by an early advance of Suttle Lake ice, when the glacier was coalescent between the Trout Creek and Whychus Creek drainages; later Suttle Lake advances were slightly less extensive and a medial moraine separated glaciers in Whychus Creek and Trout Creek.  Figure 4B.2 shows a reconstruction of the maximum Suttle Lake glacial system occupying the Whychus Creek watershed during the LGM (Bevis et al., 2011).

Figure 4B.1 - Whychus Creek Basin Drift Units Figure 4B.1.  The aerial extent of glacial deposits in the Whychus Creek basin; note that deposits of the older late-middle Pleistocene Jack Creek glaciation are not exposed in the watershed and are presumably buried by those of the younger late Pleistocene Cabot Creek glaciation (Bevis et al., 2011).

Figure 4B.2 - Whychus Creek Basin LGM Ice Cap Figure 4B.2.  Reconstruction of the alpine glacial system in the McKenzie River basin during the LGM Suttle Lake advance of the late Pleistocene Cabot Creek glaciation (Bevis et al., 2011).

7.2 (0.6)       Refer to Map 4B.3.  Junction of FS Rd 15 and FS Rd 1514.  Turn left (south) onto FS Rd 1514.  If you wish to hike to Camp Lake and access the eastern slopes of Middle and South Sister Volcanoes, simply remain on FS Rd 15 for another 5.4 miles until you reach the Pole Creek Trailhead (see the Camp Lake Trail under Optional Hiking Trails at the end of this road log for a complete description of this hike).  The hike into Camp Lake is fairly uneventful (some might say rather hot and dusty); however, once you reach the lake, wow!  This is probably one of the most alpine-like designated camping areas in the Oregon Cascades, and the unpredictable weather can make that impression stick.  On the other hand, Camp Lake is nestled into recently glaciated terrain between the massive stratovolcanoes of South and Middle Sister, an awe-inspiring location to say the least.  From the lake, strenuous, but not particularly challenging climbing routes provide access to the summits of both volcanoes; this, and the stunning scenery are reasons enough to journey there and linger for awhile.

FS Rd 1514 initially traverses undulating terrain comprised of till and outwash wedged between bedrock outcrops, but in a little over a mile, it begins crossing several nested moraines marking a significant advance of Suttle Lake (LGM) ice (Figure 4B.1).  “Nesting” of moraines implies a dynamic ice margin in which the glacier fluctuates repeatedly between periods of more or less stability.  The longer a glacier remains fixed in a given location, the more debris will accumulate at its margin to form larger end moraines.

8.5 (1.3)       Refer to Map 4B.2.  Junction of FS Rd 1514 and FS Rd 1513 on your left (northeast). Remain on FS Rd 1514; the road quickly crosses Pole Creek and begins to gently bend to the right around the nose of a “textbook” end moraine of the Suttle Lake glacial advance.

9.4 (0.9)       Refer to Map 4B.3.  Here, FS Rd 1514 cuts through the crest of the innermost nested, “recessional” moraine of the Suttle Lake advance on Whychus Creek (Figure 4B.3).  As you drive along the west side of the valley, notice the steep slopes to your right and the flat terrain to your left.  Here, the level valley floor lies within the U-shaped glacial trough carved by successive Pleistocene Whychus Creek glaciers; the valley’s flatness is a feature related to its filling by outwash during glacial retreat, a classic example of a valley train.

Figure 4B.3 - Till on Whychus Creek copyrighted Figure 4B.3.  Road cut exposing till of the Suttle Lake glaciation on Whychus Creek; note that the soil capping this recessional moraine is only moderately developed, a common aspect of the short period of weathering since deglaciation.

10.3 (0.9)     The sign posted on the right warns of a potential glacial-outburst flood from Carver Lake (Figure 4B.4).  Several small lakes are perched on the flanks of the Three Sisters and Broken Top volcanoes that are dammed by Little Ice Age moraines.  Meltwater from rapid glacial retreat in recent decades, probably related to global warming, has filled these lakes to capacity.  As a result, enhanced downcutting of outlet streams through the moraines rimming the lakes could cause their destabilization and send a deluge of water and debris down valley onto unsuspecting campers.

Figure 4B.4 - Glacial outburst flood sign copyrighted

Figure 4B.4.  A sign on FS Rd 1514 in the valley of Whychus Creek warning of potential flash-flooding from failure of the moraine damming Carver Lake in South Sister Volcano’s northeastern flank.

Carver Lake makes for an excellent day-hiking option from Camp Lake, but you must hoof it to Camp Lake first (see Camp Lake Trail under Optional Hiking Trails at the end of this road log for a complete description of this hike).

11.5 (1.2)     Crossing Park Creek, a major tributary of Whychus Creek (Park Creek’s South Fork heads at Carver Lake).  Several nice outcrops of glacial till associated with the Suttle Lake glacial advance occur along the right-hand side of the road ahead.

12.0 (0.5)     After curving around a rib of glacially scoured bedrock, the road crosses Snow Creek, a major tributary of Whychus Creek.  You are now driving on the east side of the stream valley.

13.1 (1.1)     Junction of FS Rd 1514 and FS Rd 1516 on your right (east).  Continue on FS Rd 1514; ahead, the road skirts around the nose of two right-lateral moraines of the late Pleistocene Suttle Lake (LGM) glacial episode, each possibly related to a major ice advance.

16.1 (3.0)     Refer to Map 4B.2.  Junction of FS Rd 1514 and FS Rd 16.  Turn right (southwest) onto FS Rd 16.  At this point, the road is underlain by outwash, but you soon begin to climb onto the outer (northeastern) slope of Suttle Lake morainal deposits encircling Whychus Creek’s valley.

17.1 (1.0)     The road cuts beginning here to either side of the road and extending upslope to the end of the pavement on FS Rd 16 are in till of the late Pleistocene Suttle Lake glaciation.

17.6 (0.5)     Road cuts to the right reveal volcanic breccias associated with a small Pleistocene-age cinder cone.  Much of the cinder cone is covered in glacial till indicating that it erupted prior to the Suttle Lake late Pleistocene glacial maximum.

20.3 (2.7)     Refer to Map 4B.3.  The road cut here exposes Pleistocene basaltic andesite lavas associated with construction of Broken Top Volcano.

22.9 (2.6)     Refer to Map 4B.4.  End of the pavement on FS Rd 16.  The road becomes progressively more rutted and wash-boarded after this, although it remains suitable for passenger cars if you drive slowly and carefully.

23.2 (0.3)     Parking area for the Park Meadow Trailhead to the right.  The trailhead provides access to trails taking you to Park Meadow (a long out and back; difficult to complete in one day) or Little Three Creek Lake (a shorter, more scenic hike), a tarn tucked up against the base of Tam McArthur Rim’s multifaceted cirque head-wall.

The Park Meadow Trail can be used as an alternate route for reaching exceptional camping at Golden Lake (a relatively short hike upslope from Park Meadow); and this location provides fairly easy cross-country access to the Bend Glacier on a day-hike (described on the Broken Top Loop Trail in Field Trip 2A).  If you choose to reach this goal, explore the wonderful late Holocene neoglacial moraine record left by the Bend Glacier.

23.5 (0.3)     Refer to Map 4B.5.  If you are planning to hike the Tam McArthur Rim Trail, an overnight camp before or after is an added pleasure.  The entrance road to Three Creek Meadow Campground is on the right here; however, better camping awaits.

24.6 (1.1)     The entrance road to Driftwood Campground is to the right.  Wonderful views of Tam McArthur Rim can be had from the northern shoreline of Three Creek Lake adjacent to this campground, especially at sunrise (Figure 4B.5).  The trails for Little Three Creek Lake and Park Meadow can also be accessed from this campground.

Figure 4B.5 - Tam McArthur Rim from TCL copyrightedFigure 4B.5.  Three Creek Lake and Tam McArthur Rim, an ideal lake-filled cirque basin, as viewed from Driftwood Campground.

Just ahead on the left is the trailhead for the Tam McArthur Rim Trail (see Tam McArthur Rim Trail under Optional Hiking Trails at the end of this road log for a complete description of this hike).  Park along the inner shoulder of the road here.  Hiking this trail is an exceptional experience and after the short up-hill section at the beginning, it is quite family-friendly.  If you just make it to the first several overlooks from Tam MacArthur Rim, the views of the eastern flanks of the Three Sisters Volcanoes and the glaciated Whychus Creek basin are amazing.  If you push onward along less-well-traveled routes, the unfolding scenery of glacially sculpted Broken Top Volcano and its record of late Holocene neoglacial moraines is fabulous!

Tam McArthur Rim and Broken Top comprise the glacially sculpted remains of a middle to late Pleistocene stratovolcano composed of intermediate to mafic lava flows and pyroclastic deposits that bury an early Pleistocene silicic volcanic complex.  The plateau of Tam McArthur Rim is primarily composed of younger basaltic andesite lava flows that overlie older dacitic and rhyolitic lavas and pyroclastic material.  The younger, more mafic volcanic rocks are mainly exposed on the western portion of Tam McArthur Rim and were erupted from vents on Broken Top Volcano.   The older, more silicic volcanic rocks are mainly exposed along the eastern portion of the rim around Three Creek Lake, and where they are not obscured by glacial sediments, further down the eastern flanks of Tam McArthur Rim.  These rocks are associated with the Tumalo silicic volcanic complex, the source of several widespread ash flow tuffs near Bend, Oregon (Taylor, 1978; Hill and Taylor, 1989; and Hill and Scott, 1990).  Chiefly mafic volcanic rocks are exposed on the eroded flanks of Broken Top Volcano and are composed of interlayered basaltic andesite lava flows, basaltic to andesitic tuffs, near-vent basaltic breccias, and basaltic dikes and sills.  The more mafic volcanic eruptions of Broken Top Volcano occurred later in the volcano’s construction, while the older silicic volcanism of the Tumalo Volcanic Center on Tam McArthur Rim’s eastern flanks occurred between 650,000 and 300,000 years ago (Sarna-Wojcicki et al., 1989), preceding the construction of the Three Sisters and Broken Top stratovolcanoes.

After your long hike, you may wish to find a campsite down the road at Three Creek Lake Campground (about one mile ahead).  Both Driftwood Campground and Three Creek Lake Campground are very popular in the summer, so beware; this author suggests that you get your campsite before you hike the Tam McArthur Rim Trail.  Otherwise, save yourself the remaining bumpy ride and after your hike, turn around here and return to Sisters, OR.

33.1 (8.5)     Refer to Map 4B.2.  Back to the junction of FS Rd 16 and FS Rd 1514.  Remain on FS Rd 16 all the way into Sisters, OR.

35.3 (2.2)     Here, the road begins descending a bluff formed where Whychus Creek has dissected substantially into older basaltic andesites of Broken Top’s volcanic platform.  There is a great view to the left of the Cascade Crest from Three Sisters to Mount Jefferson with Black Butte to the right in the middleground.  In the foreground, lying below the bluff is the outwash fan associated with the Suttle Lake-age end moraine complex on Whychus Creek.

40.2 (4.9)     Refer to Map 4B.1.  Intersection of FS Rd 16 (Elm Street) and US Highway 20/OR Highway 126 in Sisters, Oregon.  This is the end of Field Trip 4B.

Road Route Maps

Map 4B.1 - Sisters web version

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

Map 4B.2 - Three Creek Butte Map 4B.2.  Color shaded-relief map of the Three Creek Butte 7.5” Quadrangle containing several segments of Field Trip 4B.

Map 4B.3 - Trout Creek Butte

Map 4B.3.  Color shaded-relief map of the Trout Creek Butte 7.5” Quadrangle containing segments of Field Trip 4B.

Map 4B.4 - Broken Top Map 4B.4.  Color shaded-relief map of the Broken Top 7.5” Quadrangle containing segments of Field Trip 4B.

Map 4B.5 - Tumalo Falls

Map 4B.5.  Color shaded-relief map of the Tumalo Falls 7.5” Quadrangle containing a segment of Field Trip 4B.

Optional Hiking Trails

Camp Lake Trail (Tr 4B.1)

      The alpine marvel that is Camp Lake (within the greater Chambers Lakes area) is not a destination easily surpassed.  This small lake clings to the sparsely vegetated, hummocky morainal topography wedged between South and Middle Sister Volcanoes, only adding to the stark beauty of this often eerie, cool landscape.  Aside from unprecedented scenery and opportunities to observe a spectacular record of Holocene neoglaciation, Camp Lake provides access to nontechnical climber’s routes that creep up the steep, scree-covered slopes to the summits of both peaks, and opportunities galore to simply strike out over one of the largest areas of alpine terrain in the Oregon Cascades.  This hike is intended as a multiday backpacking trip with the ultimate objective of ascending to the top of both volcanoes (there’s nowhere else to go but up).

Day One

Begin at the large message board at the left edge of the parking area (Map 4B.6).  The hike begins with a long, hot and dusty, uphill slog through viewless lodgepole pine forest.  It’s about one and a half miles to a junction with the eastern branch of the Pacific Crest Trail (another branch traverses west of the Cascade Crest); head left on the PCT.  After another seven-tenths of a mile, you reach Soap Creek; cross on a log footbridge and immediately arrive at another trail junction.  The PCT veers left; take the Chambers Lake Trail to the right and climb steadily upward another two and a half miles to the North Fork of Whychus Creek.  As you ascend, the trail gradually enters the cooler, moister climes of western hemlock- and subalpine fir-dominated forest.  Basaltic andesite, lavas from North Sister Volcano smoothed and striated by glacial erosion, crops out along the trail as you approach Whychus Creek.  On the banks of the stream you are offered the first good view of Middle Sister’s massive volcanic form.  Note the bouldery, grayish debris along the stream banks here, this material was deposited by a debris flow that ripped down the stream channel recently (Figure 4B.6).

Figure 4B.6 - Debris Flow copyrighted

Figure 4B.6.  Coarse, grayish debris from a recent debris flow coats the banks of the North Fork of Whychus Creek near the Chambers Lakes Trail crossing.

There is no bridge, so take care fording Whychus Creek’s rushing water on slippery rocks.  Once across, a trail junction on the far bank leaves you with two options.  The left trail offers a shorter one and a half mile round-trip excursion to Demaris Lake, less scenic, but also considerably more sheltered if nasty weather threatens.  The right trail heads upslope the last two and half miles to the shore of Camp Lake (Map 4B.6); this is your real goal!

The trail to Camp Lake having been selected, it rapidly ascends through a series of switchbacks negotiating bedrock outcrops of andesite and basaltic andesite of Middle Sister Volcano; watch for evidence of glacial sculpting.  Midway through this climb, a rocky promontory offers your first superb view of Middle and North Sister, their eastern flanks much affected by glacial activity (Figure 4B.7).  In a little more than a quarter-mile, you reach a gradual ascent along a more open ridge and fabulous views of Broken Top Volcano and all three of the Sisters stratovolcanoes begin to unfold.  Keep climbing another mile, you soon pass a small tarn on the left below the trail and then climb to the crest of a distinctive, arcuate moraine (Map 4B.6).  Given its position, preservation and lightly weathered surface; this moraine is almost assuredly correlative with the Canyon Creek advance of the Cabot Creek glaciation of Scott (1977).

Figure 4B.7 - First view of Middle&North Sister copyrighted Figure 4B.7.  The first awe-inspiring view of Middle and North Sister from the Camp Lake Trail; note the large U-shaped valley in the foreground on Middle Sister’s eastern slope with the Diller and Hayden Glaciers encircled by youthful-looking moraines at its head.

Continue higher into this alpine fairyland studded with glacially carved rocky knobs, small tarns, and copses of stumpy Douglas fir and subalpine fir.  In slightly more than one mile, you are eventually confronted with a large basin containing Camp Lake, the easternmost and closest of the Chambers Lakes (Map 4B.7).  The lake is dammed on the north and east sides by a low morainal ridge, probably a recessional moraine of Canyon Creek age.  Several tenths of a mile further brings you near to the lake; begin looking for a campsite with as little exposure to the commonly chilly winds as possible.  Remember that this lake is a very popular destination on weekends in August and September, and that campfires are not permitted in the area.  During your coming stay, be sure to take in those sunrises and sunsets from the lake shore, there are few better to be had in the central Oregon Cascades (Figure 4B.8).  Explore the area around the lake, you are in a premiere wilderness with abundant evidence of neoglacial activity and there are plenty of features to casually observe.

Figure 4B.8 - South Sister sunrise at Camp Lake copyrighted

Figure 4B.8.  A sunrise on South Sister from Camp Lake; one of life’s simple pleasures not to be missed.

Day Two

Now for the reason you are here; climbing volcanoes!  Climber’s routes lead out of the west end of Camp Lake’s basin up both peaks, so take your choice.  Both peaks can be tackled from multiple paths and this author choose slightly different out and back routes to maximize geological observation.  Let’s begin with a description of the Middle Sister ascent; Figure 4B.9, aside from being a great photograph of Middle Sister in the morning sunlight, shows nearly the entire length of your route.  First, cross the basin northwest away from the lake to the foot of the slope making up the volcano’s southern flank (Map 4B.7).  Begin climbing along the outer perimeter of the bowl that slants upward to the west.  About midway up, curve to the right across the snow-filled depression and ascend the steep slope before you toward the ridge at the east end of the prominent bench above.  As you climb, don’t hesitate to look around; a volcanic landscape, modified by the affects of recent glaciation is astonishingly arrayed before you.  Figure 4B.10 offers a spectacular view of South Sister Volcano’s glaciated northeastern flank, the freshly gray, sharp-crested neoglacial moraines are not difficult to spot.  The multiple nested moraine crests superimposed on the hummocky morainal topography draped over South Sister’s northeast slope indicate several periods of Holocene neoglaciation.   Broken Top Volcano peers over the ridge above Camp Lake; the dark gray mass at the end of this ridge is an older Pleistocene lava dome and associated lava flows comprised of rhyodacite and dacite.

Figure 4B.9 - Middle Sister at sunrise copyrighted

Figure 4B.9.  This photograph, taken from near a favorite campsite on Camp Lake (its out of the wind!), shows much of the Middle Sister climbing route used by this author.

Figure 4B.10 - Neoglaciation on South Sister copyrighted

Figure 4B.10.  Camp Lake and South Sister’s spectacular record of neoglacial activity, readily observed from a ridge on Middle Sister’s southern flank.

When you reach the ridgeline, head upslope toward the rocky promontory ahead.  The ridge offers superb views of Diller Glacier, lying to your immediate right (Figure 4B.11).  Look up and down the length of the glacier.  Here, you are above the glacier’s ELA and within its accumulation zone (wind-drift of snow from the bench above probably aids this) where erosion is occurring, but downglacier to the northeast, moraines can be readily observed. These arcuate piles of bouldery debris encircle the ablation zone and mark a transition into that portion of the glacier where deposition is occurring; a pattern consistent with all glaciers, see if you can pick it out it the course of your travels for the next few days.

Figure 4B.11 - Diller Glacier and Middle Sister copyrighted

Figure 4B.11.  Diller Glacier clings to the east face of Middle Sister Volcano.

As you approach the promontory at the east end of the rocky bench, you’ll need to angle to the right; stick to the steep slope between the glacier and its headwall until you see a broken down section of the low cliff that can be safely climbed.  It looks rather imposing at first, but is really quite easily negotiated.  Now at just over one mile, you have reached the top of the bench (Map 4B.7).  The rock capping this bench is a basaltic andesite lava flow.  In places, it retains a pahoehoe-textured surface, and it does not exhibit signs of glacial sculpting, suggesting that it remained above the level of ice during late Pleistocene glaciation as a nunatak.  Fine views of North Sister and Middle Sister (including the now obvious path to the volcano’s summit) lie to the north and northeast (Figure 4B.12).  To the south and southeast, the entire northern flank of South Sister, Broken Top, and Tam McArthur Rim is marvelously displayed.

Figure 4B.12 - North Sister and pahoehoe lavas copyrighted

Figure 4B.12.  North Sister volcano as first seen from a rocky bench on Middle Sister’s southern slope high above Camp Lake.  Note the lovely pahoehoe-textured surface of the basaltic andesite lavas comprising the bench displayed in the foreground of the photograph.

Hike northwestward, upslope along the bench, and then angle gently more to the west below the snowfield ahead.   A steep climb between this snowfield and another to your west ensues; above which you eventually reach a northwest-trending ridgeline sweeping down from the volcano’s summit in about three-quarters of a mile (Map 4B.7).   Follow this rocky ridge to the peak’s summit along an anastomosing system of climbing trails, the goal is obvious.  Climbing this ridge brings unparalleled views of Diller and Hayden Glaciers encasing the eastern face of Middle Sister.  Note the fresh-looking terminal moraines below both glaciers; these were formed during the most recent Holocene neoglacial event of the Little Ice Age.

The summit of Middle Sister Volcano is reached in about two miles and 3000 feet of elevation gain from your campsite; not bad, but you still have to climb back down.  The summit area is much smaller than South Sister, much of the eastern side of the mountain having been carved away by glacial activity (Figure 4A.38); the 360° panoramic views from here are almost overwhelming.  After the initial shock of taking it all in, find a seat, enjoy lunch, and absorb.  Examine Middle Sister’s flanks; as indicated, her eastern face is heavily dissected by Hayden and Diller Glaciers.  Her northwest slope is notched, albeit less severely, and covered by the remnants of Renfrew Glacier.  The healthier Collier Glacier, Oregon’s largest, occupies the valley heading between Middle Sister’s northern slope and North Sister’s western side, having eroded most significantly into this latter slope.  Middle Sister’s southern and western flanks are the least modified, glaciers finding a difficult hold there as these face most directly into the sun.

Middle Sister’s siblings, as their names imply, lie immediately to the north and south of your perch, respectively.  Compare the three volcanoes, based on their state of symmetry and degree of erosion, it is readily apparent that North Sister is the eldest and South Sister is the youngest of the fiery siblings (compare Figure 4B.13 and Figure 4A.39, respectively).  Although younger, South Sister displays an amazing record of late Pleistocene glaciation (Figure 3.4A.39).  These peaks formed during the Quaternary, overlying Pliocene and early Pleistocene volcanics filling the central Oregon Cascades graben.  North Sister is a basaltic andesite and andesite composite volcano constructed on a broad shield volcano base of basaltic andesite flows (Taylor, 1968 and 1981; Taylor et al., 1987).  Its flanks have been considerably eroded by glaciers and expose the central volcanic plug, dike systems, and near-vent volcanic breccias (observed here on its craggy southwestern flank).  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.  Massive lava domes and flows of somewhat older rhyodacite and dacite crop out around their lower flanks (Taylor, 1968 and 1981; Taylor et al., 1987).  Middle Sister is comprised chiefly of basalt porphyry lava flows and lesser amounts of basaltic andesite, andesite, dacite, and rhyodacite lavas and pyroclastic deposits.  South Sister’s composition is more intermediate, mainly comprised of andesitic lava flows and a lesser amount of andesitic pyroclastics; although its southern flanks are blanketed in Pleistocene and Holocene rhyodacite domes, lava flows, and tephra.

Figure 4B.13 - North Sister from Middle Sister copyrighted

Figure 4B.13.  North Sister Volcano, oldest of the Three Sisters, has been significantly impacted by glaciation since its initial formation as much as 300,000 years ago.

Broken Top Volcano lies east and a bit south of South Sister (Figure 3.4A.39), a stratovolcano with an age and compositional affinity to North Sister, chiefly comprised of basaltic andesite, although it is built on a base of older rhyodacite and dacite similar to Middle and South Sister (Taylor, 1968 and 1981; Taylor et al., 1987).  Broken Top’s flanks are considerably eroded by glaciers, exposing its dike systems, and near-vent volcaniclastics.  Mount Bachelor, a late Pleistocene to Holocene composite volcano and one of the youngest in the Oregon Cascades, peeks out between South Sister and Broken Top (Figure 3.4A.39).  Two large Holocene cinder cones are superimposed on the ridgeline stretching north of North Sister; Collier Cone somewhat nearer the base of North Sister and Yapoah Crater slightly east and nearer McKenzie Pass.  Vents at each location produced voluminous outpourings of basaltic andesite lavas that moved down the northwest flank of the ridge in several lobes some coalescing near the pass, while others poured off the ridge to the west and into the glaciated trough of the White Branch of the McKenzie River.  Four-in-One Cone, a unique ridge cone formed of coalescent basaltic cinder cones, lies below and to the west of the ridge, sandwiched between the lava flows from Yapoah Crater and Collier Cone.  This cone complex erupted andesitic lavas that generated two flow lobes that generally flowed down slope to the northwest, wrapping around the northeast and southwest sides of a ridge punctuated by Condon Butte.

To the west lies the older, more highly glaciated terrain of the volcanic platform comprised of basalt and basaltic andesite lavas and pyroclastic material originally built on the central High Cascades graben during the late Pliocene and early Pleistocene.  This broad plateau dips gently westward, dissected by headwater tributaries of the McKenzie and Willamette Rivers, and punctuated by occasional resistant mafic knobs such as The Husband and The Wife, the remnant summit cones of much-sculpted shield volcanoes and the source of the volcanism that formed this landscape.

All good things need come to an end, and so you must descend from your throne.  Return to approximately the same position on the northwest trending ridge you used to reach the summit, about midway between the two large snowfields (Map 4B.7).  At this point, climb down the steep slope along an almost due south trajectory, first between the two snowfields immediately below, then keeping to the left of the next larger snowfield.  As you descend pass the upper snowfields, note the relatively fresh pahoehoe lavas cascading in narrow, wax-like ribbons downslope nearby; close examination reveals a vesicular basalt porphyry composition.  Continue downslope, the bench capped by lava flows you climbed early should remain well to your right as you drop into the upper end of a bowl-shaped, glacially-carved depression about a third of a mile below the ridge (Map 4B.7).  Pass the second snowfield; this is one of several remnants of the former Irving Glacier, now succumbed to an early death by recent climate change (Figure 4B.14).  Larger snowfields lie below and to your left, preserved by wind-drifted snow from the west.

Figure 4B.14 - Irving Glacier copyrighted

Figure 4B.14.  The remnants of Irving Glacier, now little more than perennial snowbanks preserved by wind drifted snow from the west; South Sister Volcano lies in the background, across the valley to the south.

After entering the former Irving Glacier’s cirque, descend southeasterly to a broad ridge on the cirque’s eastern perimeter, about six-tenths of a mile distant (Map 4B.7).  Outcroppings of rock on this ridge exhibit beautiful glacial striations, indicating that this whole area below the lava flow bench was once ice covered.  From the ridge, traverse the upper end of another snowfield; in about four-tenths of a mile you’ll connect with a distinctive trail that heads east-west through the valley containing the Chambers Lakes (Map 4B.7).  Hike downslope on this trail another four-tenths of a mile back to Camp Lake and your campsite for a much needed rest and preparation for South Sister’s climb tomorrow, an even bigger fish to fry.

Day Three

Wake early.  As with your trek yesterday, this day hike to South Sister’s summit is described as an out and back route, sections of which can be taken in either direction; this author recommends that you take all sections described in order to maximize geological observations, although the return route as described does perhaps offer less “tricky” spots to negotiate.  To begin your ascent of South Sister, head around the north side of Camp Lake and to the west back up the trail you descended yesterday.  When you reach the position where you joined the trail, continue on it upslope to the west.  In just a bit over a quarter-mile, you reach the crest of a ridge separating the Camp Lake basin from the next Chambers Lakes basin to the west (Map 4B.7).  Make a right turn and hike southward along the ridge, avoiding the scrubby trees to its west side.  The ridge is capped with a veneer of reddish, oxidized scoria and small lava blocks and bombs, the remnants of an eroded cinder cone.  In about three-tenths of a mile, you arrive at a highpoint on the ridgeline with a USGS benchmark indicating an elevation of 7525 feet (you have a long way to go).  Traverse a short swale and another highpoint, and then drop off the ridge to your right and cross the lower end an often snow-filled basin.  This basin is encircled by fresh-looking, sharp-crested moraines and recently contained the Skinner Glacier (Figure 4B.15).  The glacier is now just a vestige of its former self, another casualty of global warming in the 20th and 21st centuries.   Stick to the outer margin of the cirque, north of a small tarn and near to the steep drop-off to one of the Chambers Lakes below.  The view northward from this vantage point is awesome (Figure 4B.16).  North Sister can be seen poking her head above Middle Sister’s northeastern shoulder in the distance; Middle Sister lies to the fore at center; the basins containing the two largest of the Chambers Lakes rest at Middle Sister’s feet.  The knife-edged ridge of rock above the nearer lake is rhyodacite and dacite, while the cliffs to the west are comprised of andesite; these are much glaciated older Pleistocene lava domes and flows forming Middle and South Sister’s foundation (Taylor, et al., 1987).  The upper flanks of Middle Sister Volcano consist of younger basaltic andesites.

Figure 4B.15 - Middle and North Sister copyrighted

Figure 4B.15.  The remnants of Skinner Glacier on the northern flank of the South Sister volcano; note the youthful, late Holocene neoglacial moraines encircling the glacier well below its terminus.

Figure 4B.16 - Skinner Glacier and South Sister copyrighted

Figure 4B.16.  The outer rim of the former Skinner Glacier’s cirque offers an expansive view across the Chambers Lakes to Middle and North Sister stratovolcanoes.

Make for the morainal ridge sweeping down from the basin’s western side; it’s about four-tenths of a mile to a low hillock at the base of this left-lateral Little Ice Age moraine (Map 4B.8).  Climb the moraine crest past a sharp right-hand jog and to the base of a dramatically steepened length of slope before you.

From here, head west and gradually upslope across the upper end of a snowfield to a meltwater stream-cut breach in the neoglacial moraine below the Eugene Glacier (Figure 4B.17); you’ve come another half-mile.  Now head into the breach and trace a circuitous route around the inner slope of the neoglacial moraine to its left-lateral margin; climb to the moraine crest and continue upslope (Map 4B.8).  Note the stratigraphy exposed in the cirque headwall, a thin drape of gray andesite talus overlies thick, reddish, oxidized volcaniclastic deposits erupted from South Sister.  As you approach the headwall above the glacier, the slope steepens considerably.  Where the moraine butts up against the headwall, cross onto the perennial snow at the top of the glacier; watch your step.   Continue upward until you locate a convenient place to climb the scoriaceous scree above the headwall and onto the ridge above.  Look back the way you have come, that last half-mile was a bit dicey (Map 4B.8).  As you climbed, it probably wasn’t difficult to notice the sad state of the Eugene Glacier, clearly this glacier is sharing a similar fate to its neighbors.

Figure 4B.17 - Eugene Glacier copyrighted Figure 4B.17.  The remnants of the Eugene Glacier; the climbing route used by this author passes around the neoglacial moraines encircling the outer margin of the Eugene Glacier and up onto the headwall in the background.

When you first reach the ridge, there is a lot to take in.  The ridge is a narrow spine of steeply dipping, interlayered andesitic lava flows and pyroclastic material with a thin veneer of bouldery talus literally clinging to the side of the mountain (Figure 4B.18).  The Eugene and Skinner Glaciers to the north and the Lost Creek Glacier to the south have steadily eaten into the volcano’s flank, leaving only this fin of none-too-resistant rock preserved between them; it is only a matter of time.  The view north to Middle and North Sister Volcanoes, the Chambers Lakes, and the Eugene Glacier’s cirque is unforgettable.  You’ve been climbing over and around its products all morning, but from here, you can see the big picture of glacial influence on the north flank South Sister.  To the west, The Husband, the central core of a much eroded shield volcano, rises above the gently westward-dipping volcanic platform of the early High Cascades (Figure 4B.18).

Figure 4B.18 - Final ascent of South Sister copyrighted

Figure 4B.18.  This is your route to South Sister’s summit; a ridge of steeply dipping lava flows and pyroclastic material with a thin veneer of bouldery talus seemingly “poised” on the side of the mountain and ready to leap into space, located between the Eugene and Skinner Glaciers to the north and the Lost Creek Glacier to the south.

Continue ascending the ridge; its steep, boulder-strewn surface doesn’t let up, but the goal soon becomes obvious, a notch in the volcano’s summit rim about seven-tenths of a mile ahead (Map 4B.8).  Before reaching the notch, be sure to climb over to a position near the Lost Glacier’s headwall, about midway through your ascent of the ridge; the close-up view of South Sister’s volcanic stratigraphy exposed here is impressive.  A little further on affords a nice view directly down the axis of the glacier.  The glacier’s surface is crevassed, revealing that it is still undergoing flow (and hence, still a glacier).  Notice the debris cover on the upper part of the glacier; a typical source of the material transported by alpine glaciers is via frost action and massive wasting onto its surface from surrounding ice-free slopes.

When you reach South Sister’s summit rim, your first view is of its snow- and ice-filled crater.  You may wish to walk the perimeter of the crater basin (you’ve earned that privilege); it’s a mere eight-tenths of a mile to circumnavigate the entire crater (Map 4B.8).  Take note of the notch’s location and begin hiking clockwise around the rim to the highest point on the mountain; fine views unfold along the north side of the summit area.  First, looking westward, one can observe the older, highly glaciated terrain of the volcanic platform comprised of basalt and basaltic andesite lavas and pyroclastic material originally built on the central High Cascades graben during the late Pliocene and early Pleistocene (Figure 4.2; Taylor, 1968 and 1981; Taylor et al., 1987).  This broad plateau is dissected by the headwater tributaries of the McKenzie and Willamette Rivers, and its gently westward-dipping slope is dimpled with resistant knobs such as The Husband and The Wife, the remnant summit cones of much-sculpted shield volcanoes and the source vents for the volcanism that formed this landscape.

South Sister’s two younger siblings make up much of the northern skyline.  All three of these immense stratovolcanoes were built upon the older shield volcano material that initially filled the developing High Cascades graben beginning in the early Pleistocene (Figure 4.2; Taylor, 1968 and 1981; Taylor et al., 1987).  Comparing their state of symmetry, degree of erosion, and compositional differences, it is apparent that the three volcanoes decrease in age and become more intermediate southward.  North Sister is a basaltic andesite and andesite composite volcano constructed on a broad shield volcano base of basaltic andesite flows.  Its flanks have been considerably eroded by glaciers and expose the central volcanic plug, dike systems, and near-vent volcanic breccias.  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.  Middle Sister is comprised chiefly of basalt porphyry lava flows and lesser amounts of basaltic andesite, andesite, dacite, and rhyodacite lavas and pyroclastic deposits.  South Sister’s composition is more intermediate, mainly comprised of andesitic lava flows and a lesser amount of andesitic pyroclastics; although its southern flanks are blanketed in rhyodacite domes, lava flows, and tephra.  Massive lava domes and flows of rhyodacite and dacite crop out around the lower flanks of each volcano, although South Sister more so than Middle Sister.  Middle Sister’s silicic volcanism ended prior to late Pleistocene glaciation as all of its exposed silicic rocks show signs of the passage of ice; on the other hand, South Sister exhibits an abundance of Holocene silicic eruptive centers.

To the northeast lies the Whychus Creek drainage basin (Figure 2A.51), from Tam McArthur Rim and Broken Top to the east, to the glacially scoured east face of South Sister before you, to the Chambers Lakes area and on to Middle Sister, North Sister, and finally Black Crater in the north.  At its maximum 24,000 to 18,000 years ago, the Suttle Lake ice mass covered this entire watershed (Figure 4B.2).  Climb to the volcano’s summit for a good view down onto Prouty Glacier to the east.  Looking down the axis of this glacier, notice Carver Lake, encircled by narrow, arcuate piles of loose debris, the terminal moraine of the Prouty Glacier at its Little Ice Age maximum.

Now walk to the southeast along the summit rim to more amazing views of this world of fire and ice.  As you walk, Teardrop Pool, an annual summer-season meltwater pond formed within South Sister’s crater, shimmers to you right (Figure 2A.50).  Heavily glaciated Broken Top Volcano lies to your left, an older stratovolcano with an age and composition similar to that of North Sister.  This stratovolcano is chiefly comprised of basaltic andesite, although it is built on a base of older rhyodacite and dacite similar to Middle and South Sister (Taylor, 1968 and 1981; Taylor et al., 1987).  Broken Top’s flanks are considerably eroded by glacial activity which has exposed its dike systems and near-vent volcanic breccias.  The Green Lakes, some of Oregon’s finest, lie in the glaciated valley between Broken Top and South Sister; note the rough, gray rhyodacite lava dome and flows on the valley’s nearer right-hand side, one of several major Holocene silicic eruptive centers on South Sister’s southern flank.  South of Broken Top lies Mount Bachelor, a basaltic andesite stratovolcano erupted near the close of the Pleistocene.  Note this volcano’s more nearly perfect symmetry, with only a relatively minor cirque developed on its northern flank beneath the summit cone.

In a little over half a mile from the notch, you arrive at a trail intersection; continue your circumnavigation, the trail to the left here is used by climber’s coming up from the Green Lakes, Moraine Lake, or Devils Lake.  Shortly after this trail intersection, Rock Mesa, a complex obsidian dome and associated flow another product of Holocene silicic volcanism, enters your field of view to the left.  From there, a short distance to the notch completes your hike around South Sister’s summit crater.

When you are ready to come down from this high, hike down the ridgeline you ascended earlier in the day; roughly seven-tenths of a mile of rapid descent will bring you to the point above Eugene Glacier were you first climbed onto the ridge.  From this location, keep climbing down the ridge (Map 4B.8); angle to the left until you reach the fresh-looking Little Ice Age moraine at the terminus of the still-active portion of Lost Creek Glacier.  Figure 4B.19 presents an excellent view of the volcanic stratigraphy beautifully displayed in the cirque headwall above.  Continue down slope from these moraines, traversing a large snowfield, the now detached remnants of Lost Creek Glacier (Map 4B.8).  Midway through your snowfield descent, multiple lava flows are exposed in the wall of the older part of Lost Glacier’s cirque (Figure 4B.20).  Look closely at the cliff face, a typical aa lava flow cross-section is displayed including the rubbly flow top and base, as well as the denser middle section of the flow.

Figure 4B.19 - Lost Creek Glacier copyrighted

Figure 4B.19.  Interlayered andesitic lavas and volcaniclastic deposits intruded by mafic dikes nicely displayed in the cirque headwall above the active portion of the Lost Creek Glacier on South Sister’s western flank.

Figure 4B.20 - Interlayered volcanics copyrighted

Figure 4B.20.  Lava flows displaying a typically dense interior and rubbly base and top are well-exposed in the lower cirque of the former Lost Creek Glacier.

Continue beyond the snowfield about three-quarters of a mile, keeping to the inner slope of Lost Creek Glacier’s neoglacial right-lateral moraine until you can conveniently cross to the northeast where its height has decreased considerably (Map 4B.8).  Once past the meltwater stream on the far side of the moraine, trend slightly downslope in a northeastward direction; in about two-tenths of a mile you’ll reach a trail heading east-west through the valley containing the Chambers Lakes.  Take this trail to the northeast, it reconnects with the point on the ridge where you left it earlier in the day in just over a mile (Map 4B.8).  Climb over the ridgeline ahead and into the basin beyond containing the westernmost of the Chambers Lakes.  The tall cliff northwest of this basin is comprised of older Pleistocene silicic rocks and exposes the foundational bedrock on which Middle Sister Volcano was constructed; slopes to the southeast side of the lake basin are covered in neoglacial debris from the former Skinner Glacier.  At the eastern end of this basin, cross a second ridgeline into a basin containing the largest of the Chambers Lakes (Map 4B.7); the dark gray cliffs northwest of the ridge and basin expose a cross-sectional view of the core and flanks of a rhyodacitic to dacitic lava dome on Middle Sister’s southern flank (Figure 4B.21).  Parallel the southeastern lake shore and ascend the trail to the top of the next ridge, the point where you left this trail that morning.  From this position, simply backtrack to your campsite at Camp Lake.

Figure 4B.21 - Chamber Lakes copyrighted Figure 4B.21.  The cliffs flanking the north side of the Chambers Lakes expose more silicic dacites and rhyodacites, such as this volcanic plug, the older Pleistocene foundation on which Middle Sister Volcano was formed.

Day Four

One more good night’s rest behind you, and from Camp Lake, it is roughly seven miles back to the trailhead and your vehicle.  But before you depart this wonderland of volcanic rock, snow, ice, and water, consider taking a morning hike to Carver Lake, perched on South Sister’s heavily glaciated northeast flank below the Prouty Glacier.  From your campsite, head south across the outlet stream from Camp Lake and over the broad ridge beyond; in about three-tenths of a mile you enter a pretty meadow (Map 4B.7).  Cross a small stream and initially climb the steep slope to the southeast until beyond the scrubby trees where you can begin angling south and upslope parallel to the ridgeline.  This ridge is comprised of older Pleistocene rhyodacite and dacite, part of a large lava dome and associated lava flow.  Continue to climb, about a half-mile above the meadow, numerous rock outcrops begin to present themselves, exposing exquisite layers of platy rhyolite and snowflake obsidian within the silicic mass of the ridge (Figure 4B.22).  Eventually, you’ll traverse a small swale in the ridge, beyond which marvelous views of Middle and North Sister open up to your right.

Figure 4B.22 - Silicic volcanic rocks

Figure 4B.22.  Layers of platy rhyolite (A) and snowflake obsidian (B) contained within the larger silicic volcanic mass that comprises the ridge southeast of Camp Lake.

Continue your gradual ascent, at first sticking to the inner slope of the ridge, then climbing a dry stream valley to the foot of the moraines encircling Carver Lake (Map 4B.8).  This wash was once an outlet channel draining the lake which had cut its way through the moraine above, a position you’ll reach in about three-quarters of a mile.  From the moraine breach, you can finally see Carver Lake tucked down behind tall, sharp-crested, fresh-looking moraines (Figure 4B.23).  These moraines likely formed during a late Holocene neoglacial advance of the Prouty Glacier, the rapidly melting remnants of which you can see clinging to South Sister’s northeast face.  Notice that another set of arcuate moraines lies upslope from Carver Lake, probably the product of Prouty Glacier’s more recent Little Ice Age advance. The current outlet stream for Carver Lake has dissected deeply through the neoglacial terminal moraine, draining to the east into Park Creek.  Recall the sign warning of a potential glacial-outburst flood from Carver Lake at mile 10.3 of Field Trip 4B; morainal dams such as these are very unstable, a substantial rise in lake level due to rapid melting of snow and ice could trigger such a flood event.

Figure 4B.23 - Carver Lake copyrighted

Figure 4B.23.  Carver Lake, encircled by neoglacial moraines, lies at the foot of Prouty Glacier on South Sister’s northeast slope.

Now climb the perennial snowbank and moraine crest to the right to a highpoint a few hundred yards ahead (Map 4B.8).  This location affords a dizzying array of opportunities for observation.  An excellent aerial survey of Carver Lake and its moraines, as well as the record of neoglacial activity on South Sister’s northeast face can be viewed to the south.  To the southwest, classically interlayered lava flows and pyroclastic deposits, typical of composite volcanoes such as the South Sister, are revealed in the cirque headwall above the Prouty Glacier.  Immediately to the north, a superb display of neoglacial deposits of the former Carver Glacier lie in the valley below, encircling the uppermost of the Chambers Lakes (Figure 4B.24); further afield, neoglacial moraines can be seen below Diller and Hayden Glaciers on the eastern flank of Middle and North Sister Volcanoes.  Imagine, all of these moraines were deposited during the Little Ice Age, in most cases indicating the ice position in the early 1900s, and thus suggestive of substantial warming in the 20th and 21st centuries.

Figure 4B.24 - Middle and North Sister from Carver Lake hike copyrighted Figure 4B.24.  A fine view to the north from the moraine crest encircling Carver Lake; the highest and smallest of the Chambers Lakes lie in the foreground, nestled behind the compound neoglacial moraines of the former Carver Glacier; while in the distance, Middle and North Sister preserve a similar neoglacial record below the Diller and Hayden Glaciers.

From your current perch, descend a narrow valley just to the east of the former Carver Glacier’s cirque to the easternmost of the Chambers Lakes; it’s a bit over eight-tenths of a mile to that lake’s outlet and another half-mile to that pretty little meadow above the ridge you first climbed as you left your campsite (Map 4B.7).  As you pass the outlet, another outcrop of platy dacite is exposed in its right wall.  Notice the degree of stream channel downcutting here, indicating a significant volume of meltwater must have once poured through this notch given how recently the area was glaciated.  Once back in the meadow below, stay west of the stream, round the broad ridge to the left, and descend to your campsite.

      It’s time to pack up and return to the Pole Creek Trailhead and your vehicle and its downhill all the way.

Tam McArthur Rim Trail (Tr 4B.2)

      This day-hike, tailored long or short by you, offers some of the most exceptional views of the Three Sisters and Broken Top volcanoes in all of the Three Sisters Wilderness.  The trail first climbs steeply onto Tam McArthur Rim, but then traverses the rim along much more gentle grades.  Several overlooks peer into the cirques on the glacially-scoured north face of Tam McArthur Rim.  Extending the hike offers opportunities for more hardy souls to traverse less trammeled ground to observe the volcanic stratigraphy exposed in Broken Top Volcano’s dissected eastern flank and its exceptional record of Holocene neoglaciation, or to reach the furthest limit of Tam McArthur Rim and its unparalleled views.

Tam McArthur Rim and Broken Top comprise the glacially sculpted remains of a middle to late Pleistocene stratovolcano comprised of intermediate to mafic lava flows and pyroclastic deposits superimposed on an early Pleistocene silicic volcanic complex (Figure 4.1).  The plateau of Tam McArthur Rim itself is primarily composed of younger basaltic andesite lava flows that overlie older dacitic and rhyolitic lavas and pyroclastic material.  The younger, more mafic volcanic rocks are mainly exposed on the western portion of the rim and were erupted from vents on Broken Top Volcano; whereas, the older, more silicic volcanic rocks are mainly exposed along the eastern portion of the rim around Three Creek Lake and further down the eastern flanks of Tam McArthur Rim.  The older silicic rocks are associated with the Tumalo volcanic complex of Taylor (1978).

Begin at the Tam McArthur Rim Trailhead on the left side (south) side of the road (Map 4B.9) across from the entrance to Driftwood Campground.  The trail ascends fairly steeply from the parking area for a little under a mile, generally near the cliffs that eventually become Tam McArthur Rim.  These cliffs form the headwall of several amphitheater-like cirques carved into the north side of the volcanic plateau that now exists as Tam McArthur Rim.  The trail switchbacks in the first quarter mile near the headwall cliffs provide good views of Three Creek Lake and its basin, the first major cirque of Tam McArthur Rim that you’ll see on this hike.  Rock outcrops along this section of trail are comprised of platy andesite.   A switchback near three-quarters of a mile affords a gorgeous view of Three Creek Lake.  This lake is known as a tarn, a water-filled basin scoured from bedrock by glacial erosion.   Near the one mile mark, the trail levels out for a short stretch as it passes into the Three Sisters Wilderness, then climbs another steep section.  The trail begins to traverse thin patches of glacial till and scattered outcrops of dacite and rhyodacite.

In a little over one and a half miles, the Tam McArthur Rim Trail intersects with a horse trail coming in from the south; shortly thereafter, the trail passes very near the headwall of the Three Creek Lake cirque (Map 4B.10).  Take a short side trip upslope to the right for a superb view of the glaciated amphitheater in which Three Creek Lake rests.  Briefly consider the formation of a cirque.  Initially, snowfall and wind-drifted snow accumulates in a natural depression in the landscape.  Ideally, this location is in the head of a stream valley on the northeast-facing side of a mountainous area where the average air temperature is in general cool; a location which is relatively protected from the sun, and that is on the lee side of the prevailing wind (and moisture source) direction.  Eventually, enough snow accumulates and is converted to ice with depth and passage of time.  The perennial snow and ice increase in mass and aerial extent to a certain threshold, and begin to flow downslope under the force of gravity.  A glacier is born!  The glacier picks up loose debris along its base and uses this material to scratch, gouge, pluck, and polish the surface over which it glides.  As that first depression is accentuated, debris can be added to the glacier from frost-action and mass wasting along its ice free margins and by freeze-thaw processes on the glacier’s bed, helping to maintain the supply of abrasive material.  Over long periods of time, multiple glacial episodes work to carve a cirque from that initial humble depression in the landscape.

Return to the main trail and hike west near the rim (Map 4B.10).  This part of the rim is formed of basaltic andesite.  One distinct trail becomes hard to follow in some places because the passage of so many people wandering the rim has created several anastomosing options.  Almost immediately, a well used alternate trail branches from the main one, paralleling it just a few hundred feet to the left (further from the rim).  Continue on the main trail, as it heads in the direction you wish to go.  The trail soon crosses a less-vegetated swale filled with a thin veneer of glacial till; Mount Bachelor can be seen to the southwest.  The trail eventually ascends from the swale and returns toward the rim and more rocky outcrops of rhyodacite.  The alternate, parallel trail rejoins the main trail just past two and a half miles; continue west, and almost immediately, you encounter a “Y” fork in the trail.  Take the trail that heads off to the right (north) which remains close to the rim (you will return via the main trail on the left later).  In about two-tenths of a mile, the trail climbs to a promontory on Tam McArthur Rim located on a protrusion of the headwall separating two cirques (Map 4B.10).  As you ascend to the rocky spur, a breath-taking view into the glacially sculpted basin containing Three Creek Lake is offered to the right (Figure 4B.25).  Prominent lobes of rocky debris occur at the base of the nearer cliffs.  These low, arcuate, bouldery ridges are protalus ramparts generated by the most recent neoglacial activity in these cirque basins.  They form as frost-action-induced rock falls cascade from the cliffs encircling the cirques and down across perennial snow fields perched against the base of the headwalls.  On the far side of the promontory, at roughly two and eight-tenths of a mile, views of Tam McArthur Rim expand to the west and giving you a first glimpse of Broken Top and the Three Sisters volcanoes.  Bedrock exposed here is the Rhyodacite of Tam McArthur Rim (Sherrod et al., 2004); beneath this rock unit, a layer-cake of basaltic andesite lava flows are revealed in the cirque headwalls (Figure 4B.26).

Figure 4B.25 - Protalus Ramparts along Tam McArthur Rim copyrighted

Figure 4B.25.  These low, arcuate, bouldery protalus ramparts found at the foot of the cirque headwall above Three Creek Lake probably formed during Holocene neoglaciation as frost action produced rock falls that cascaded from the cliffs and down across perennial snow fields perched against the base of the headwalls.

Figure 4B.26 - Tam McArthur Rim copyrighted

Figure 4B.26.  The western portion of Tam McArthur Rim; the Three Sisters appear on the horizon.  Here the rim rock is the Rhyodacite of Tam McArthur Rim overlying basaltic andesite lavas.

Your promontory trail rejoins the main trail coming in from the left in another two-tenths of a mile (Map 4B.10).  Your visit to the promontory can be bypassed on your return journey by using the main trail here that cuts off to the left.  In just shy of three and a quarter miles total, the Tam McArthur Rim trail descends into another larger, till-floored swale; spectacular views of Mt. Bachelor abound to your left (Figure 4B.27).  Mt. Bachelor is a late Pleistocene to early Holocene stratovolcano comprised of basalt and basaltic andesite lava flows and pyroclastic deposits erupted onto a silicic platform of earlier Pleistocene rhyodacite and dacite, similar to the silicic volcanic rocks found on Tam McArthur Rim and Broken Top.  The trail lies near the rim here, and an option to extend your hike out to the tip of the final promontory of Tam McArthur Rim returns to the main trail near this location (although no official trail exists).

Figure 4B.27 - Mt Bachelor copyrighted

Figure 4B.27.  A swale on the western portion of Tam McArthur Rim affords a great view of Mount Bachelor, a youthful stratovolcano barely touched by glaciation (unlike Tam McArthur Rim).

Continue west on the trail, crossing the swale; you have reached an elevation on Tam McArthur Rim that is both high enough and cool enough to preserve perennial snow banks, such as the one to the right of the trail at the far end of the swale (Map 4B.10).  Notice the exposed hollow where the snow bank lies, bare gray rock surfaces and a lack of vegetation suggest recent melting back of the snow field, quite possibly related to atmospheric warming due to recent climate change.  This depression is called a nivation hollow, an incipient cirque if you will, indicating that until recently, climatic conditions were right for the permanent accumulation of snow.

As the trail begins to ascend from the swale at this point, it becomes less distinct and anastomosing in places for the next several tenths of a mile.  A good rule of thumb here is to keep to the higher trail and head more or less west and slightly southwest, you’ll hit the western edge of Tam McArthur Rim at roughly the four-mile mark (Map 4B.10).  Some two hundred yards from your destination (a highpoint on the rim to the west), you pass a second perennial snow bank and nivation hollow off to the left.  Note that the ground here is becoming increasingly covered in red scoria and lava bombs, quite unlike the grey rhyodacite of Tam McArthur Rim seen in scattered outcrops.  When you reach the rim’s western edge, be prepared for an incredible view of the Three Sisters and Broken Top stratovolcanoes.  Mt. Washington, Three Fingered Jack, and Mt. Jefferson can be seen further north, as well as the more subdued peaks of Black Crater and Black Butte.  Young, basaltic to andesitic, near-vent breccias, lava flows and pyroclastic material, invaded by dikes, can be seen on Broken Top’s upper eastern flanks.  Below you lies a broad expanse of ice-sculpted bedrock and sparse vegetation forming a compound cirque in the headwaters of Whychus Creek on the north side of Tam McArthur Rim, Broken Hand and Broken Top.  Several perennial snowfields dot this basin.  To your left (southwest), along the rim is another permanent snow field; and beyond that are the glaciated remnants of a red cinder cone, the source of the scoria and volcanic bombs that are scattered about you here.  To your right, the rim you are standing on stretches out about another half mile to the north, ending in a broad platform that forms the terminus of Tam McArthur Rim’s main promontory.  Faint trails lead to and from this pretty location if you desire the extra walking (Map 4B.10); it does make quite an amazing campsite, albeit a potentially windy one.

This is a good turn-around point, making a round-trip excursion of about eight miles (Map 4B.10).  Not for the faint-hearted or those with small children, but this author recommends that if you have the energy and time, continue on this trail in one of two directions.  From here, either hike along the ridge to the southwest, first traversing the cinder cone, then rounding below and to the right of Broken Hand’s jagged spine, and finally negotiating an undulating saddle to the second of two highpoints west of Broken Hand; or bushwack your way northward to the very end of Tam McArthur Rim, the promontory ridge to your right.  You may wish to complete both options, in which case an overnight stay here is in order; the krummholz trees nearby can serve as a nice wind break and the snowbanks provide water for the weary.

Let’s assume that you hike the trail toward Broken Hand (Map 4B.10); initially, it is quite easily followed as it passes along the cinder-strewn ridge.  In about half a mile, you ascend to the highest point on the cinder cone described earlier; the “cone” hardly appears as such due to glacial erosion, but look for the concentration of red cinders, lava blocks, and bombs to mark its position (Figure 4B.28).  Examine the many fine examples of spindle and breadcrust bombs as you walk.  After traversing the second of two highpoints, the trail crosses a saddle to the foot of Broken Hand in about three-tenths of a mile; it divides here, follow the branch to the right (Map 4B.10).  Here you will need to cross a steep scree- and/or snow-covered slope beneath Broken Hand’s northern face, just watch your step and proceed carefully.  Broken Hand is made up of a layer-cake of eastward dipping lavas and pyroclastics erupted from nearby Broken Top Volcano. Shortly, you’ll reach a saddle in the ridge west of Broken Hand, from here the views of Broken Top just get better and better; one feels as if they can literally reach out and touch the volcanic rocks of its glacially carved summit.

Figure 4B.28 - Volcanic bombs  copyrighted Figure 4B.28.  Red cinders, lava blocks, and bombs mark the general position of the glacially eroded cinder cone capping the ridge east of Broken Hand.

When you have reached the second of two krummholz-covered promontories at roughly five and half miles out (Figure 4B.29), your outbound trek will be complete; the amazing plethora of geologic features to observe on this extended hike makes it an effort extremely well-vested.  Eat lunch and enjoy the outrageous views into the cirques on Broken Top’s northern and eastern faces, they are truly fantastic.  A spectacular mafic dike intruding upward through interlayered lava flows and pyroclastic deposits is exposed on the upper northeast side of the peak (Figure 2A.30), an excellent example of the internal volcanic stratigraphy of a stratovolcano.  Below you lie living, breathing glaciers, and abundant evidence of their recently-departed glory in the form of moraines marking their late Holocene neoglacial and Little Ice Age maximums.  The cirque on Broken Top’s northern flank preserves the remnants of a former age, the Bend Glacier, now melting back from distinctive Little Ice Age moraines (Figure 2A.31), and the eroded eastern face of Broken Top forms a fine little cirque basin floored by a proglacial lake dammed by a Little Ice Age moraine (Figure 2A.32).  From this vantage, follow the trail back the way you came; other trails shown on Map 4B.10 leading from this position are described under the Broken Top Loop Trail of Field Trip 2A (Map 2A.17), awesome destinations for extended backpacking trips.

Figure 4B.29 - View of Broken Top from Broken Hand copyrighted

Figure 4B.29.  A view to the west from the “trail” on the north side of Broken Hand.

Now for that little trek to the end of Tam McArthur Rim.  There is no trail to speak of, but the destination is not difficult to find.  Just head north along the ridge, keeping the west-facing cliff near at hand (Map 4B.9).  You’ll reach the promontory’s terminus in about six-tenths of a mile; this author believes that the westward view from this final perch is one of the finest in all of the Oregon Cascades.  There is nowhere else that so graphically displays stratovolcanoes or their linear relationship to the formation of a volcanic arc (Figure 4B.30); you can literally see their snowcapped summits lined up like so many pearls on a string.  If you can tear yourself away, walk northeast about one-tenth of a mile to the east-facing cliff along this promontory for another eye-popping view back along Tam McArthur Rim’s entire length (Figure 4B.31).  When you’re finished, continue back along this margin of the ridge until you link up with the main trail near five and three-tenths miles (Map 4B.10); just another three mile push returns you to the trailhead (Map 4B.9).

Return to the trailhead and your waiting vehicle, it surely has been quite a whirlwind tour.

Figure 4B.30 Three Sisters from Tam McArthur Rim copyrighted Figure 4B.30.  The view westward from Tam McArthur Rim’s furthest promontory offers an unparalleled view of the Three Sisters volcanic platform and composite volcanoes of the High Cascades volcanic arc.

Figure 4B.31 - View east from Tam McArthur Rim copyrightedFigure 4B.31. An eastward view from Tam McArthur Rim’s furthest promontory unfolds the entire line of glacier-carved cliffs.

Optional Hiking Trail Maps

Map 4B.6 - Camp Lake Trail A web version

Map 4B.6.  Color shaded-relief map of the southwest quarter of the Trout Creek Butte 7.5” Quadrangle showing a portion of the Camp Lake Trail (Tr 4B.1).

Map 4B.7 - Obsidian & Scott Loop Trail C, Camp Lake Trail B

Map 4B.7.  Color shaded-relief map of the southeast quarter of the North Sister 7.5” Quadrangle showing a portion of the Camp Lake Trail (Tr 4B.1).

Map 4B.8 Moraine Lake Trail B

Map 4B.8.  Color shaded-relief map of the northeast quarter of the South Sister 7.5” Quadrangle showing a portion of the Camp Lake Trail (Tr 4B.1).

Map 4B.9 - Tam McArthur Rim Trail A web version

Map 4B.9.  Color shaded-relief map of the northwest quarter of the Tumalo Falls 7.5” Quadrangle showing a portion of the Tam McArthur Rim Trail (Tr 4B.2).

Map 4B.10 - Broken Top Loop Trail C

Map 4B.10.  Color shaded-relief map of the northeast quarter of the Broken Top 7.5” Quadrangle showing a portion of the Tam McArthur Rim Trail (Tr 4B.2).