Field trip to White Pass and Naches—long version

updated May 31, 2013

Make comments and descriptions and ask questions as you go. Draw sketches at every field location of any geologic features or landscapes. Use a scale. Ask your instructor if you don’t understand what to do. Also, elaborate on at least two of the field sites in a one page word-processed summary on a separate piece of paper. Consult web references and any of the references listed at the end of this guide. They can be found on hold in the library. THIS FIELD TRIP WRITE UP IS DUE ON MONDAY JUNE 10.

Leave Centralia College—go south on I-5 to Exit 68

Head east on US-12 to Packwood and then up over White Pass to US 410 near Naches.

Assignment: Write a field trip report as requested. At the end of your report don’t forget to include a summary section in which you share any feelings, observations, and new perceptions about what you saw and learned about the geology. DOES LEARNING ABOUT THE GEOLOGIC HISTORY CHANGE THE WAY YOU LOOK AT THINGS? Explain. Check the online rubric for help.

PART I: Interstate 5 to State Route 123 via U.S. Highway 12 (Pringle, 2008)

This approach, the western segment of the White Pass Scenic Byway generally follows the Cowlitz River valley to the east (Fig. [leg_c_i]), where it meets State Route (SR) 123 (see Leg G). Leg H is a continuation of this route that extends across the Cascades to SR 410, Naches, and Yakima.

This 71 mi (115 km) route along U.S. Highway (US) 12 begins on the Jackson Prairie (underlain by the Pleistocene Logan Hill Formation) and descends a series of progressively younger glacial terraces in stair-step fashion: Jackson, Lacamas, Cowlitz, and Layton Prairies or their physiographic equivalents. The older surfaces are at higher elevations and the younger ones are lower because erosion, possibly combined with a minor component of regional tectonic uplift, has deepened the Cowlitz River valley before each succeeding episode of deposition. Dethier (1988) evaluated the soil formation on the terraces as related to their age; the deposits will be described en route. Dethier observed that during at least two glacial events the Cowlitz glacier terminated more than 60 mi (100 km) from Mount Rainier, thus making it one of the largest valley glaciers in the conterminous United States.

About 4 mi (6.4 km) west of Morton, the road crosses a divide into the Tilton River drainage and remains in that watershed for about 9 mi (14.4 km) before descending back into the Cowlitz River valley near Fern Gap. East of Mayfield Lake, dark-colored rocks of the Hatchet Mountain Formation of Tertiary age crop out along the highway, indicating that we have entered the eroded and folded Cascade Range. Lava flows and fragmental volcanic rocks of this age crop out all along the route.

At Randle, this route accesses SR 131 and Forest Roads (FRs) 25 and 23, which lead to the east side of the Mount St. Helens area, as well as to the Cispus River drainage and the north side of Mount Adams (see Pringle, 2002). Hummocky glacial deposits (end moraine and ice-marginal deposits) of Evans Creek age (~22,000–15,000 yr B.P.) near here suggest this is the terminal position of the large valley glacier that occupied the Cowlitz River valley.

Distances along the route are given in miles, followed by kilometers in italics. If we take any of the optional trips or sidetrips along the way, we'll have to keep track of, and add those miles to all remaining miles listed in each log. Having a pencil and paper handy, and even a calculator will be helpful.


0.0 0.0

Mileage starts on US 12 at its junction with Interstate 5 (I-5). The road passes east over Jackson Prairie, a gently rolling terrace of the lower(?) Pleistocene Logan Hill Formation. This surface, which may be about a million years old, displays as much as 40 ft (12 m) of relief. Here, the Logan Hill Formation consists mainly of a compacted mixture of cobbles and pebbles in a sandy clay matrix. The sediment is outwash from an ancient glacier whose source was in the southern Washington Cascade mountains near Mount Rainier.


2.4 3.8

Milepost (MP) 67.


2.5 4.0

Marys Corner.


3.2 5.1

Roadcuts expose the clayey, reddish, deeply weathered top of the Logan Hill Formation. Weathering reaches depths greater than 50 ft (15 m).


3.3 5.3

MP 70


4.1 6.6

Good exposures of red soils of the Logan Hill Formation are in a roadcut here.


4.5 7.2

We descend from Jackson Prairie to Lacamas Prairie, the surface of an outwash plain of the middle(?) Pleistocene Wingate Hill Drift. Notice that Lacamas Prairie is flatter than the older Jackson Prairie—it displays relief of only a few feet. Colman and Pierce (1981) estimated the age of the Wingate Hill Drift to be about 600 to 300 ka.


7.3 11.7

On a fine day, there is a good view of Mount St. Helens ahead and slightly to the right. Mount Adams volcano is also visible (east of Mount St. Helens), as is Goat Rocks, a mountainous remnant of an extinct Pleistocene volcano. (For more information on Goat Rocks, see Leg H, p. ???.)


9.3 14.9

Wingate Hill outwash is exposed in a roadcut about 300 ft (90 m) west of MP 76. The reddish-brown weathering of this deposit extends to depths that range from 16 to 32 ft (5–10 m) and does not have the deep red hue of the older Logan Hill deposits.


11.3 18.1

MP 78


11.4 18.2

Near Salkum, the road descends to a terrace underlain by outwash deposits of Hayden Creek age (~170 to 130 ka ) and then crosses Mill Creek. This terrace is an equivalent of Cowlitz Prairie to the southwest. Till of Wingate Hill age is visible to the right just after we pass Mill Creek. The Wingate Hill terminal moraine is about a mile (1.6 km) west of here, whereas the maximum extent of the Hayden Creek glacier was about a mile (1.6 km) to the east. The hills and mountains of the Cascades come into view as we continue farther east.


15.2 24.3

Cross Mayfield Lake (reservoir). MP 82 is on the bridge. The concrete arch dam was completed in 1962. Glacial outwash deposits are visible north of the highway on both sides of the reservoir.


16.6  26.6 font?

The first outcrops of dark Cascade Range volcanic rocks (Oligocene-Eocene basaltic andesite) begin to appear  in roadcuts near here.


17.3 27.7

There is an outcrop of amygdaloidal basaltic andesite on the right (south) side of the highway, but no place to safely pull over and view it.

19.8  31.7

On our left is another outcrop of basaltic andesite.


21.2 33.9

Cross the Cowlitz River.


22.2 35.5

The cliffs on the left (north of the highway) are more Oligocene-Eocene  basaltic andesite.


22.9 36.6

Landslide with riprap to prevent its moving onto the roadway.


23.3 37.3

Riffe Lake Viewpoint. Riffe Lake (a reservoir) is situated in the glacially carved valley of the Cowlitz River. In this area, the Goble Volcanics (Tertiary) are visible in roadcuts on the north side of the highway; the layers dip to the east at the viewpoint. The outcrop north of the highway is mapped as andesite; however, the rocks contain common ovoid-shaped oxidized remnants of olivine crystals that are more typical of basalt. Beds of fragmental volcanic rocks crop out about 0.5 mi (0.8 km) farther to the east. Note the reddish soils in some places that were baked by lava or pyroclastic flows that buried them and also the steeply dipping hyaloclastite beds where lava flowed into water to trigger phreatomagmatic explosions. Vugs in the lava contain opal. Opal is composed of amorphous (noncrystalline) silica and commonly contains a small percentage of water. The opal was deposited by ground water that percolated into cavities in the rock. Vertical dikes of basaltic andesite are also visible locally.


South-southeast of Riffe Lake, Mount Margaret and other peaks in the Mount Margaret Wilderness north of Mount St. Helens are composed of granodiorite and granite of the Spirit Lake pluton, whose age has been estimated at 22 to 20 Ma by Evarts and others (1987)(Fig. [riffe_lake_view_new]).



23.7 37.9

There is a good exposure of hyaloclastite beds on the left (north), but no safe place to get out and examine them.


24.3 38.9

MP 91.


25.5 40.8

Viewpoint for Riffe Lake. The outcrop on the left (north) is basaltic andesite.


26.2 41.9

The road climbs past glacial drift, out of the Cowlitz drainage, and then crosses a drainage divide into the Tilton River watershed.


27.2 43.5

OPTIONAL SIDE TRIP: Short Road Crater View (1.3 mi [2 km] round trip). If the weather is good, this viewpoint near MP 94 affords a distant view of the Mount St. Helens crater, neighboring mountains, and the valley of the Cowlitz River. As we climb Short Road, we’ll see Bellicum Peak, a mountain to the northwest that was not covered by glaciers during the Pleistocene Epoch. Imagine this lonely peak sticking up in the middle of a broad sheet of glacial ice during the extensive Hayden Creek glaciation.


Remember to adjust the odometer to compensate for mileage along the side trip.


28.2 45.1

As the road descends into Morton, a zone of reddish altered rocks is exposed on the left. Slickensides are common on these rocks. Although no trace of a fault is visible, this may be a shear zone.


The smooth southwest-facing hillslope visible northwest of Morton is the dip slope of a limb of a northwest-trending anticline. Folding near Morton is more complex and tightly spaced than the gentle folding typically found in the region. Whereas the crest spacing (wavelength) of the folds in the younger rocks south of Rainey Creek ranges from about 6 to 18 mi (18–30 km), the spacing in the older rocks north of Rainey Creek ranges from about 2 to 6 mi (3–10 km). Because of this more intense folding, the rocks are generally more shattered and altered, and erosion has cut a window through the volcanic rocks into the older sedimentary rocks of the Eocene Puget Group. Landslides abound in this area, which lies between two active, north-northwest-trending seismic zones, the St. Helens zone and the West Rainier zone. The West Rainier zone is about 12 mi (20 km) north-northeast of the St. Helens zone. (See p. ?.)


31.0 49.6

Junction of US 12 with SR 7 at Morton. We will continue east on US 12.


31.3 50.1

MP 98


33.3 53.3

Slightly east of MP 100 sedimentary rocks of the Puget Group crop out on the south side of the highway. These rocks for the most part predate the Cascades, but they do interfinger with the earliest Cascade volcanic rocks. Clevinger (1968, 1969) provided details about the fossils and minerals associated with rocks in this area in two useful guidebooks (Fig. [clev-morton-xsec]).


34.6 55.4

There is an outcrop of volcaniclastic rocks of Tertiary age on the right (southwest), but no pulloff.


34.8 55.7

Junction of Davis Lake Road. Davis Lake. This road extends to the north and then loops back to the northwest for a few miles at the base of the foothills before its junction with SR 7 in Morton. Davis Lake and Lake Creek sit in an extension of the same northwest-trending valley occupied by the Tilton River northwest of Morton. (See Leg I, p. ?). The orientation of the valley seems influenced by the orientation of nearby folds and faults, many of which trend northwest.


Cathy Whitlock (Barnosky, 1981) developed an extensive record of paleoclimate and environmental change using pollen and plant fossils she obtained by coring at Davis Lake and at other sites in this region, including Mineral Lake, 12.7 mi (20 km) to the north-northeast. Her radiocarbon dates for the Davis Lake core spanned that last 26,000 years and cut through eight recognizable tephra layers. She concluded from the pollen evidence that tundra-parkland vegetation likely existed in this area from 26,000 to 16,000 yr B.P. These cold conditions were followed by warming from 16,000 to 15,000 yr B.P. and then a return to a colder climate until 12,000 yr B.P., when the pollen of several subalpine lowland tree species appeared, representing a shift to warmer, interglacial environment.


35.2 56.3

About 4.2 mi (6.7 km) past the SR 7 junction, the road crosses a northeast-trending normal fault, and we are back in the Goble Volcanics. The fault is not visible in outcrop. Although the fault motion has been interpreted as down-to-the-southeast, the topography appears inverted because the down-dropped rocks now stand higher than those rocks across the fault. If the interpretation of the fault is correct, this situation resulted perhaps because the Goble Volcanics are more resistant to erosion than the sedimentary rocks of the Puget Group.


The highway starts to climb once it reaches the volcanic rocks, passes again into the Cowlitz River drainage and the valley of Rainey Creek at Fern Gap, and then descends to Glenoma.


36.4 58.2

Light tan to brown volcaniclastic rocks of Tertiary age crop out in the cliff on the left (northeast) side of the highway about 260 yards (240 m) east of the Kosmos Road. 


36.6 58.6

A lava  flow of Tertiary age crops out on the inside of a curve on the left (north) side of the road.


36.9 59.0

Basalt flows of Tertiary age are exposed here.


37.3 59.7

MP 104, Udan Road. Glacial drift exposed in this area is of Hayden Creek age (~170 to 130 ka).


37.9 60.6

Glenoma. The south valley wall along Rainey Creek (the elongated ridge south of Glenoma) is composed of upper Oligocene basaltic rocks, mostly lava flows. These rocks were derived from a volcanic center south of Riffe Lake.


41.0 65.6

Rainy Creek Road.


41.4 66.2

Although not readily visible from the highway, deposits of yellowish tephra are common in roadcuts and stream banks near where the road crosses Rainey Creek. This pebble-size pumice is the Yn tephra layer from Mount St. Helens, erupted about 3,600 yr B.P. The deposit of pumice and ash from this eruption extends to the northeast to Alberta, Canada. It was probably the largest volcanic eruption in this region in the past 4000 years. Scientists studying the settlement sites and patterns near this area found that the native peoples moved far away from Mount St. Helens area at about this time and probably stayed away for nearly 2000 years (McClure, 1992).  WHY DO YOU THINK THE TEPHRA FALLOUT OF Yn WAS SO HEAVY HERE? 


43.0 68.8

Cliffs on the north side of road are Oligocene andesite flows.


43.8 70.1

Kiona Creek. 


44.3 70.9

MP 111. Junction of US 12 with Savio Road. We continue ahead on US 12. Lake Scanewa (610 acres; 2469 m2), a reservoir impounded in 1994 by Cowlitz Falls Dam, lies several miles to the south. It is closed to fishing from March 1 to May 31 to allow out-migration of juvenile steelhead.


44.6 71.4

Here we cross the drainage divide between Rainey and Kiona Creeks, located on coalescing alluvial fans from those two creeks. LOOK IN THE STREAM CHANNELS--DO YOU SEE ANY EVIDENCE FOR RECENT DEPOSITION?


45.4 72.6

Glacial drift of Hayden Creek age is exposed on the left (north) side of the highway.


46.6 74.6

Slightly east of here the highway passes through a terminal moraine constructed by the large valley glacier that occupied the Cowlitz River during the Evans Creek time of alpine glaciation (~22–15 ka). South-dipping dip slopes in andesite flows are visible to the north.


47.2 75.5

MP 114. A significant landslide more than 197 ft (60 m) wide blocked the highway here at about 6:00 p.m. on Nov. 22, 1995, narrowly missing a vehicle (Fig. [sw_12_ls-at_randle]). During heavy rains, a large mass of rock toppled onto the clay-rich slope to the north, overloading it and causing rock debris and soil to move rapidly some 985 ft (300 m) downslope. Inspection of the rock units above revealed a north-trending fault and an associated zone of gouge in the altered rock material along the fault zone—a zone of slippery, clay-rich, and mechanically weak rock debris. The rocks are sandstones and conglomerates of Oligocene age. The landslide deposit is 3.3 to 30 ft (7.5–9 m) thick, and its volume is 150,000 to 200,000 yd3 (115,000–150,000 m3). The Washington State Department of Transportation removed about 70,000 yd3 (52,000 m3) of


49.4 79.0

Cowlitz Valley Ranger Station


50.0 80.5

The broad flood plain of the Cowlitz River is contains numerous layers of flood silts and sands. USGS geologist Norm Banks located a stack of flood deposits near here that overlie the white pumice layer Wn, erupted from Mount St. Helens in late A.D. 1479 or early 1480 (Fig. [randle_floodseds97]). At least 13 flood layers lie on top of the Wn pumice, and the 1996 flood, which was a rain- landslide debris. Several days after the initial slide, the landslide began to flow 20 to 30 ft/day (6–9 m/day ). The area had received 16.82 in. (42.7 cm) of rainfall in the previous 42 days, 8.68 in. (22.1 cm) in the previous 22 days, and 1.96 in. (5 cm) in an intense storm two days before the event. The Department of Transportation has stabilized this slide and moved the road away from the base of the slope.  On a clear day, Mount Adams volcano is visible 33 mi (53 km) to the  southeast.


48.3 77.3

Randle and intersection of US 12 with SR 131. Note: If heading south from here to visit the Mount St. Helens area, there are no service stations between here and the town of Cougar (~100 mi; 160 km), so make sure to have plenty of fuel.


.SR 131 accesses the east side of the Mount St. Helens National Volcanic Monument. The road crosses the Cowlitz River, the main fork of which originates on Mount Rainier. The greenish gray color of the water is caused by “rock flour”, the silt and clay carried in suspension by the river. Rock flour is created by the grinding action of rocks at the bed of a glacier.

on-snow event and inundated the valley floor from wall to wall, left the uppermost layer.


51.2 81.9

MP 118. Cliffs to the left (north) are Miocene diorite and granodiorite intrusions.


54.0 86.4

Pullout to the right (south). The outcrop on the left is granodiorite with inclusions of andesite.


55.1 88.7

MP 121. The road curves to the left. Note the big scarp visible in the distance to the southeast on the face of Castle Butte. Strata of upper Oligocene volcaniclastic rocks are visible in the scarp.


58.8 94.1

Near MP 125 we notice some large boulders near the mouth of the canyon to the south. They might have been carried down to the flood plain by debris flows.


59.1 94.6

MP 126. Rest area. The hummocky terrain and large, angular to subangular boulders are typical of a landslide deposit, although Swanson and his USGS colleagues Norm Banks and Richard Moore (1997) have mapped this area as “alluvial fan”. The source area of the debris is Goat Dike, a rock promontory to the south composed of volcani­clastic rocks. Light-gray granules of Mount St. Helens Wn tephra (A.D. 1479) lie atop the debris; however, there is no trace of the 3,500 yr B.P. Yn tephra from Mount St. Helens whose fallout was heavy in this area. Therefore, the age of this hummocky deposit is bracketed by the ages of these two layers.


Cross sections of the geology in this area as interpreted by Swanson and his colleagues are shown in Figure [packwoodxsec]. Their studies revealed that the area is underlain by bedded volcaniclastic rocks of late Eocene and Oligocene age, mostly laharic units, that interbed with andesite and basaltic andesite lava flows on the south side of the Cowlitz River valley between Smith and Johnson Creeks. Those flows were part of a large shield volcano that was located near Angry Mountain, about 8.7 mi (14 km) to the east-southeast.


60.7 97.7

FR 20. More large rocks south of the highway may also be part of the same landslide noted at the rest area.


61.1 97.8

MP 128.


61.2 97.9

More large rocks, these at the mouth of Dry Creek, were probably deposited by debris flows.


61.5 98.5

Chambers Lake turnoff. FR 21 .


61.6 98.6

Mount Rainier is visible to the left on a clear day.


62.0 99.2

The terrace or bench on the south side of the highway near the curve, slightly after Hall and Johnson Creeks, is supported in part by southwest-trending andesitic dikes. Hager Creek is a “disappearing stream” that drains into the coarse rocky debris of a large alluvial fan. What could be the source of all this rocky debris? Hager Lake, which lies some 2.4 mi (3.8 km) southeast of the highway up Hager Creek, is dammed by a large landslide from Hall Ridge. USGS Geologist Bob Schuster found standing snags in the lake and suggested it formed within the last several centuries (Swanson and others, 1997). Perhaps partial breaching of the newly dammed lake deposited the bouldery debris of the alluvial fan at the base of the valley wall.


62.1 99.4

MP 129.


62.5 100.0

Chambers Lake turnoff and FR 21 to Glacier Lake. For a good view of Mount Rainier on a fine day, we could drive about 1.5-2 mi (2.4-3.2 km) up this forest road (Fig. [mora_wa_packwood_FR21_820 levels]). Glacier Lake, accessible about 6 mi (10 km) up this valley and via a vigorous hike, is dammed by a large rock slide–debris avalanche deposit. Bob Schuster dated a subfossil tree exposed in the lakebed upstream of the blockage at about 600 yr B.P., and a provisional tree-ring analysis of one subfossil snag drowned behind the landslide dam indicates that its outermost ring is about A.D. 1453. However, the bark was missing from the tree, so the precise year of its demise is uncertain—it is unclear if there are missing outer rings. The Glacier Lake rock slide may record a significant earthquake, owing to its proximity to two other landslide-dammed lakes and because it is not uncommon for landslides of this type and size to be triggered by seismic shaking. Packwood Lake, only about 2.5 mi (4 km) to the northwest of Glacier Lake, is also impounded by a large landslide (Pringle and others, 1998), and Hager Lake, mentioned above, is only about 3 mi (4.8 km) from the others.


Swanson (1995) found no significant evidence of active faulting in this area, although he did identify many small, mostly north-northwest-trending shear zones that cut Tertiary rocks. He constrained the age of most folding in this area to “between 18 Ma (possibly 15.6 Ma) and 12 Ma” based on paleomagnetic data, several K-Ar ages, and field observations.


64.1 102.6

MP 131, downtown Packwood. This small town may be located closer to more volcanoes than any other town in Washington. This location provides easy access to Mounts Adams, St. Helens, and Rainier, as well as the Indian Heaven volcanic field to the south and the White Pass area. Buried trees and gravel and sand layers rich in Mount Rainier andesite fragments (Figs. [arkle_packwood_tree010118] and [packwd-snag-Wnash-cowltz]) indicate that lahar runouts and volcanic floods from Mount Rainier eruptions have flowed along the valley bottom. The configuration of the Cowlitz River’s upper tributaries at Mount Rainier, however, suggests that other river systems face a higher probability of future lahars than the Cowlitz valley.


Glacial grooves and striations on rocks locally offer further evidence of the huge glaciers that have carved this broad valley during past ice ages.


64.3 103.5

We depart Packwood heading east. Note landslides visible ahead and slightly to the left, on the south face of Tatoosh peak (the local name for the exposed rocky face to the north that is the southernmost peak of the Tatoosh Range). The Tatoosh pluton was named for the Tatoosh Range by Fiske and others (1963).


64.6 103.4

The outcrop on left is a Miocene intrusive andesite sill.


64.9 103.8

The black sedimentary rocks that crop out on north side of road are lake deposits. On the south side of road is an andesite sill.


66.6 106.6

Cross Lake Creek, which drains landslide-dammed Packwood Lake.


66.9 107.6

MP 134. Columnar-jointed rock exposed south of the road is an andesitic sill of the Packwood complex of Oligocene or Miocene age (Swanson and others, 1997).


67.2 107.5

Thin shale laminae under the sill are lake deposits that contain some leaf fossils (Figs. [pack_laptuff_shale839] and [leaf_fossil_cu_mosaic]).


67.6 108.2

Grizzly Road. The highway is cut through an andesite sill.


67.5 108.6

Gray, intrusive andesite of Miocene or Oligocene age.


68.3 109.3

Coal Creek and Coal Creek Road


68.4 109.4

Enter Gifford Pinchot National Forest.


68.7 109.9

The small outcrop of tuffaceous sedimentary rock on the right contains fossil wood.


69.0 110.4

Near MP 136, FR 46 is on the southeast side of the road, and slightly past here, FR 1270 goes to Backbone Lake Trail #164.


69.4 111.7

Gray tuff on the right (southeast) that overlies a resistant andesitic sill is probably the Purcell Creek tuff of Miocene age, identified by Swanson (1996) in this area.


69.6 111.4

Green to light-gray tuffs crop out to the right (southeast) of the road.


69.8 112.3

At a curve to the right slightly before MP 137, note a large cliff of greenish-brown volcaniclastic rocks, possibly Purcell Creek tuff.


70.4 112.6

Fine-grained tuffs on the right here contain lapilli and larger clasts.


70.6 113.6

Chain-up area.


71.1 113.8

La Wis Wis Campground.



Intersection of US 12 with SR 123. We can continue east on US 12 via Leg H or turn left here and follow Leg G (in reverse order) toward Chinook Pass (16.3 mi or 26.2 km). Alternatively, we can intersect the end of Leg B and the Ohanapecosh Entrance to Mount Rainier National Park by going north on SR 123 5.4 mi (8.6 km).




PART II: Ohanapecosh (State Route 123) to Naches (State Route 410) via U.S. Highway 12  (from a draft by P. T. Pringle, P. E. Hammond, N. P. Campbell, and W. J. Gerstel)

This route is the eastern segment of the White Pass Scenic Byway and was featured in Newell Campbell’s geologic road guide (1975). The byway passes through one of the most scenic and geologically interesting areas in Washington (Fig. [loc_map_east]?). There are tens of volcanic centers that range in age from earliest Miocene to Pleistocene. Late Paleozoic/Mesozoic sedimentary rocks and metamorphic rocks, glacial deposits, and large active landslides are also exposed along the way. We begin the 46-mi (75 km) journey by ascending from the valley of the Cowlitz River (elev. ~1580 ft or 480 m) through the valley of its tributary, the Clear Fork Cowlitz River, toward White Pass (elev. 4470 ft or 1363 m). En route we pass the west-dipping Ohanapecosh Formation beds (about 36 to 28 Ma, or late Eocene to mid-Oligocene age [Vance and others, 1987]) and cross a major fault boundary along the western margin of the Rimrock Lake inlier. The Rimrock Lake inlier is a body of older rocks that pokes up through the younger rocks near White Pass. East of the pass, we thread our way through andesitic lavas from Ice Age volcanoes that erupted both north and south of the highway and drive past more outcrops of the pre-Tertiary Rimrock Lake inlier rocks before passing into a section of younger rocks near Tieton Dam. Before arriving at the junction of U.S. Highway (US) 12 with State Route (SR) 410 (elev. 1608 ft or 490 m), we pass some textbook-quality displays of columnar jointing in lava flows and  examine a complicated interplay of volcanic deposits and flows buried by younger lavas of one of the world’s largest lava flow complexes, the Grande Ronde Basalt. At other locations, invasive flows that entered stream sediments are exposed, and in places river valleys have been displaced or buried by lavas. The distinctive vegetation zones traversed en route have evolved and adapted to both climate and geology.


The highway is sometimes closed in winter, so it is wise to check on road conditions before traveling. Road status can be checked at Washington State Dept. of Transportation via their web site or by phone. (See Web Links and Phone Numbers, p. ?).


Distances along the route are given in miles, followed by kilometers in italics. If we take any of the optional trips or side trips along the way, we'll have to keep track of, and add those miles to all remaining miles listed in each log. Having a pencil and paper handy, and even a calculator, will be helpful.




0.0 0.0

Junction of SR 123 and US 12. Sills and pyroclastic flows are visible here in Ohanapecosh rocks. Fiske and others (1963) measured a stratigraphic section of the Ohanapecosh Formation volcaniclastic rocks from Stevens Canyon to the Ohanapecosh River and then to the east along US 12 at 6000 ft (1829 m) thick.


0.3 0.5

Milepost (MP) 139.


0.6 1.0

A poorly sorted deposit or diamicton here is till of Evans Creek age, 22 to 15 cal. yr B.P. Evans Creek is the youngest major episode of alpine glaciation in this area.


1.2 1.9

Near MP 140, Forest Roads (FRs) 45 and 4510 lead to Soda Springs Campground. The historic Cowlitz Trail heads at the campground. This route linked local tribes east and west of the Cascade crest, a distance of more than 40 mi (64 km) between settlements. Outcrops of Ohanopecosh Formation volcaniclastics are visible before and after FR 45.


1.9 3.1

Ohanapecosh sedimentary rocks dip about 30 degrees to the southwest several hundred yards (meters) west of MP 141 (Fig. [us12ohan_dipswavemp140p6-38]). The sill is a plagioclase-phyric olivine basalt. Note the gentle 10- to 13-ft- (3–4 m) wavelength folds in the rocks above the sill. The deformation likely occurred during emplacement of the sill.


2.3 3.7

Palisades rest area. Here are some spectacular columns in the Clear Fork andesite, an intracanyon lava flow of Pleistocene age from a vent near Goat Rocks (Fig. [clear_fk_andesite_col]). Clayton (1983) dated the Clear Fork flow at 0.65 Ma.


Why are the columns here wider at the top? The columns develop at different rates as cooling progresses both upward from the base and from the top down. (See Fig. [flow anatomy/structure] in Leg F). The flow-top rubble at this site has been removed by subsequent glaciation. Hammond has suggested that this same flow may have been ponded by ice in the Cowlitz River valley to the west. He originally interpreted this unit as a dacite (Hammond, 1980).


2.5 4.0

FR 1276.


3.0 4.8

West dipping water-laid beds of the Ohanopecosh Formation crop out here.


4.1 6.6

.Note that the beds of the sedimentary rocks here dip at least 10 degrees more steeply to the southwest than the rocks 2 mi (3 km) to the west; beds become steeper over the next several miles (kilometers) and are nearly vertical as we approach the rocks of the pre-Tertiary Rimrock Lake inlier.


 4.2 6.7

MP 143.  Here we see an outcrop of hydrothermally altered volcanoclastic rocks of the Ohanopecosh Formation. At the east end of the outcrop a small amount of coal is exposed. Also visible in the outcrop are the vesicular olivine basalt flow of Hogback Mountain and a flow breccia. This outcrop is continuous to mile 4.5.


4.5 7.2

Columnar Hogback Mountain olivine basalt is exposed on the north side of the highway. This lava flow originated at Hogback Mountain to the southeast. Clayton (1983) estimated (by whole-rock K-Ar dating  techniques) the age of a lava flow near the top of the Hogback Mountain volcano to be about 1.53 Ma; using magnetic polarity reversals, he dated the lower part of the volcano at between 2.47 and 3.40 Ma.


4.6 7.4

The pullout to the right offers an excellent view of Lava Creek Falls as the stream goes over the Palisades columns and plunges into the Clear Fork River. The river incised its course along the contact between the valley dacite flow and the older Ohanopecosh Formation. Lava Creek was not able to erode the valley dacite flow at the same rate as the river, and as a result the waterfall was formed.


4.7 7.5

The contact between the Ohanopecosh Formation and the overlying Hogback Mountain basalt flow.


4.8 7.7

The Ohanopecosh Formation is exposed for about  0.8 mi (1.2 km). Higher up, in cliffs above the roadway  Hogback Mountain basalt can be seen


5.2 8.3

MP 144.


5.5 8.9

Intrusive rocks are exposed on the left near where the road curves to the right (if eastbound). At the turnout slightly past this curve, note the valley slightly east of the high exposure of intrusive rocks, as well as the yellowish alteration in volcanic rocks adjacent to the intrusive body. The intrusion evidently altered and weakened the surrounding rocks.


6.0 9.7

We enter a stretch of the road that passes through colluvium as we approach the margin of the pre-Tertiary rocks.


7.0 11.2

The inferred fault contact  between the  Russell Ranch Formation and Ohanopecosh Formation is near here. Although the fault itself is not visible, the east block is up relative to the west block, which places black carbonaceous shale of the Russell Ranch (east) against light-colored andesites of the Ohanopecosh on the west. This faulting causes chronic landslide problems here.


7.1 11.4

MP 146 area. Hammond (1980) described the steeply west dipping beds of Summit Creek sandstone here that overlie shattered and  sheared argillite rocks of the Russell Ranch Formation. The Summit Creek deposits are arkosic sandstone that underlie the Ohanapecosh Formation in this part of the Cascade Range and thus predate the onset of Cascade volcanism. The Summit Creek rocks are older than the 36 Ma fission-track age on overlying volcanic rocks and younger than the 55- to 44-Ma fission-track ages for the unnamed volcanic rocks below the sandstones (Vance and others, 1987). There are some intrusions exposed here in the Russell Ranch Formation that also cut, and thus are younger than the rocks of Summit Creek. The Russell Ranch rocks mostly consists of sheared, fine-grained oceanic sedimentary rocks that were buried and weakly metamorphosed; the rocks include argillite and arkosic sandstone, along with tuffs and radiolarian cherts. The argillite is highly shattered in this area, and Hammond and others (1994) interpret this shearing as a result of faulting along the west margin of the pre-Tertiary Rimrock Lake inlier.


8.1 13.0

Intrusive andesite with large hornblende and biotite crystals crops out here.


8.2 13.1

Here we see out first good outcroppings of the Russell Ranch Formation. DESCRIBE THESE ROCKS!


8.9 14.2

The pullout to the right has a good exposure of the Russell Ranch Formation.


9.1 14.6

Scenic vista points near here (MP 148) offer views of Goat Rocks to the south-southwest (Fig. [goatrxfr_sliW_mp149wide]), Mount Rainier to the northwest (Fig. [mora_from_m148us12]), and the shattered Russell Ranch Formation across the road to the north (Fig. [RussellRanchargill_US12-MP148]). The Clear Fork andesite flow is visible to the west.


9.6 15.4

The Russell Ranch Formation  is exposed for next 0.4 mi (0.6 km).


10.6 17.1

Note the hummocky landslide surface in the forest south of the road between here and Knuppenburg Lake.


10.7 17.2

Knuppenburg Lake to the right is dammed by the rock slide–debris avalanche that originated from the north flank of Hogback Ridge, due south of the lake. Radiocarbon ages for the submerged subfossil forest in the lake will reveal the approximate age of the landslide (Fig. [knupenberg-lake-snagscu03]). A few hundred yards (meters) upstream of the lake, debris-flow deposits of the last few decades have partially buried the trees growing adjacent to Millridge Creek.


Hogback Ridge is the source of the Hogback Mountain olivine basalt, which overlies shattered and landslide-prone clastic rocks of the Russell Ranch Formation. According to Swanson and Clayton (1983), the late Pliocene and early Pleistocene Hogback Mountain shield volcano was 3 mi (5 km) wide and 2300 ft (700 m) high. Some of its more than 200 lava flows, which are intercalated with those of the Pliocene to Pleistocene Goat Rocks volcano, poured as far as 18 mi (30 km) down valleys. Clayton (1983) suggested that, during its youth, Hogback Mountain volcano might have looked similar to the Tumac Mountain complex. Tumac Mountain (Fig. [tumac-mtn-clayton]) is a late Pleistocene basaltic volcano located 1.8 mi (3 km) north of Spiral Butte. Clayton estimated its age to be 30 to 20 ka.


11.0 17.7

Leech Lake on the north. Here is another outcrop of shattered Russell Ranch Formation rocks.


11.6 18.6

FR 1284 and entrance to a Department of Transportation maintenance facility.


12.3 19.7

White Pass, elevation 4470 ft (1363 m). Andesite crops out on the north side of road, and Russell Ranch Formation is exposed on the south side. Spiral Butte is ahead to the northeast.



Boundary between Gifford Pinchot (west) and Wenatchee (east) National Forests.


12.9 20.6

Pacific Crest Trail crosses our route here


13.2 21.1

This roadcut is in Spiral Butte dacite.


13.7 22.0

Spiral Butte lavas. Spiral Butte is a dacite dome of Pleistocene age (Fig. [spiral-butte-aerial-clayton]). Clayton (1983) inferred that during the most recent Ice Age advance of alpine glaciers (~22–15 ka), Spiral Butte may have partially dammed the ice that moved south from the Tumac Mountain plateau. This obstacle would have concentrated the flow of ice in the Clear Creek valley. Erosion by the ice would have exposed the fragmental deposits at the summit of Spiral Butte and created the cliffs that have now ravelled to form the talus slopes seen northeast of Dog Lake. Alternatively, an earlier thick ice cap centered on the Tumac Mountain plateau could have diverted the thick lava flow of Spiral Butte to the southeast into the Clear Creek valley. There the lava may have cooled against ice, as Lescinsky and Sisson (1998) have suggested for flows at Mount Rainier (see Fig. [lescinsky-sisson-ice-bound-lava]). Tom Sisson and Marvin Lanphere (USGS, written commun., 2004) report an 40Ar/39Ar age of about 102 ka for Spiral Butte lava.


14.4 23.0

Dog Lake Campground.


14.8 23.8

The turnout on the south side of the road has a scenic view of Clear Creek falls, a view down Clear Creek valley to the southeast, and restrooms (except in winter). Clayton (1983) mapped the silicic andesite lava flow that forms the falls. It was erupted from a vent area about 3 mi (5 km) farther west. The flow overlies a basalt flow that Clayton dated at 0.65 Ma.


15.7 25.1

A broad turnout on the south side of US 12 at a pronounced curve about 0.5 mi (0.8 km) offers a panoramic view of the local terrain (Fig. [morapan154pan]): Round Mountain to the south-southeast and the Clear Creek valley and Rimrock Lake to the east. Note: this turnout may be inaccessible owing to rockfall hazards.


16.0 25.6

We’ll see platy andesite outcrops for the next 1.5 mi (2.4 km).


16.3 26.1

MP 155.


17.6 28.3

At this curve on the left, the contact between Indian Creek Gneiss (to the east), about 154 Ma, and the sheared rocks of the Russell Ranch Formation (>144–146 Ma) is exposed (Clayton, 1983; Miller, 1989). Till deposits of alpine glaciers are extensive in this area.


17.9 28.6

Indian Creek Gneiss and amphibolite crop out in this roadcut. The rocks are highly sheared and deformed by high temperature metamorphism. Pegmatite dikes intrude the outcrop.


18.9 30.4

An outcrop of greenstone of the Russell Ranch Formation is on the south side of road slightly east of MP 158.


19.3 30.9

More greenstone of the Russell Ranch Formation.


 19.9 31.8<<FONT

Tieton Road-Clear Lake area turnoff.


20.1 32.2

Campbell (1975) noted that a small outcrop on the north side of the highway here is the terminus of the olivine basalt flow from the Tumac Mountain cone, about 6 mi (10 km) to the northwest. While some earlier researchers thought that the youthful-looking Tumac cinder cone was Holocene in age, Clayton (1983) carefully studied the evidence for glaciation at Tumac Mountain and concluded the volcano predated the most recent episode of alpine glaciation of about 22 to 15 ka.


20.3 32.5

Indian Creek. MP 159. The type locality of the Indian Creek Gneiss, one of the units in the Indian Creek complex, lies about 4 mi (~6.5 km) to the northwest. Those who want to further explore the rocks of the Indian Creek complex via foot trails and nearby roads can find information in Miller (1985) and Northwest Geological Society (1991).


20.7 33.1

Indian Creek Campground .


20.9 33.4

Silver Beach resort.


21.7 34.7

Heritage Marker for the Russell Ranch that was flooded by the reservoir.


22.3 35.7

Steeply-dipping beds of the Russell Ranch Formation are exposed north of the highway.


22.7 36.3

Rest area south of road (not marked). Chert beds north of the highway.


23.2 37.1

MP 162. Sheared sedimentary rocks north of the highway include turbidites of siltstone, sandstone. and shale. Faulted greenstone and cherts are below the turbidites. Radiolarian cherts in the Russell Ranch Formation were found near here by Miller and others (1993). They noted that the Radiolaria in the cherts are compatible with a Late Jurassic to Early Cretaceous age. For the next 1.5 mi (2.4 km) we are in sheared metasedimentary rockss and basalts of the Russell Ranch Formation.


23.3 37.3

An outcrop of sheared shale and greenstone with slickensides is north of the highway. The greenstones include remnants of pillow basalts.


23.7 37.9

Snug Harbor Resort.


25.0 40.0

The sheared intrusive rock exposed along the highway here is mapped as the “trond­hjem­ite” of the Indian Creek pluton of Jurassic age (Swanson and others, 1989). The name for this type of low-potassium granitoid rock originated at Trondhjem, Norway.


25.3 40.5

Rounded boulders of probable glacial till can be seen north of the road at MP 164. Note the slickensides in an area of sheared intrusive rocks near here.


26.3 42.1

MP 165.

Swanson and others (1989)  pointed out that there is a dark margin of hornfelsed Oligocene Wildcat Creek lapilli tuff in contact with the  Pliocene/Oligocene Westfall Rocks here. They described the Westfall R?ocks as a ‘micro’ diorite because of its small crystals. The heat of the Westfall Rocks intrusion along this contact has largely recrystallized the tuff along the dark margin.


26.6 42.6

Exit Tunnel. Tieton Dam was constructed between the masses of shallow intrusive diorite of Tertiary age (about 22 Ma) at Goose Egg Mountain (east) and Westfall Rocks (west) (Fig. [kloochman2]). The dam was built between 1917 and 1925, chiefly for irrigation purposes. In the next 0.1 mi (0.1 km), there are exposures of the Westfall Rocks diorite. Straight ahead in the distance is a Pleistocene lava flow of olivine basalt. This flow originated from a vent to the northwest and moved down Wildcat Creek (Campbell, 1975)


27.2 43.5

Rimrock Grocery store on our right. Gabbroic rock crops out on the left side of road, then we  cross Wildcat Creek.


27.3 43.7

MP 166.


27.4 43.8

Wildcat Creek Road is on the right, and FR 1306 is on the left


28.0 43.2

Light greenish, mostly fine grained volcanic sedimentary beds that crop out north of the road near here are the Wildcat Creek tuffs. Vance and others (1987) obtained dates on these rocks that range from about 34 to 30 Ma; Hammond (2005) recently revised the age to 34 to 29 m.y. from work reported in the 1987 source just mentioned and Landers and Swanson (1989). The beds include laharic deposits and tuff beds (Fig. [wildcatckseds6crop]) and might be correlative with the Ohanapecosh Formation. In fact, Vance and others (1987) interpreted these as the distal equivalent of beds in the upper part of the Ohanapecosh Formation. We’ll pass outcrops of tuffs for next 0.7 mi (1.1 km).


29.1 46.6

Junction of US 12 with Soup Creek Road (northwest of highway) and Tieton Road (east end of Tieton Reservoir Road, FR 1200).


29.5 47.2

Bethel Ridge Road (FR 1500) on the north. The forest road accesses the summit of Bethel Ridge, one of the westernmost folds of the Yakima fold belt in this area. Cash Prairie, a bedrock bench west of Bethel Ridge (not visible here), is composed of the Cash Prairie rhyodacite tuff erupted at about 25 Ma from the Mount Aix caldera (Hammond and others, 1994).


Also on the ridge are outcrops of the Tieton volcano (Oligocene-Miocene). We will see many outcrops of  volcaniclastic deposits and lava flows related to Tieton volcano as we proceed on Highway 12.  Swanson described the Tieton volcano as a large stratovolcano having a basal shield more than 200 m (656 ft) thick and overlain by 1500 m (4922 ft) of tuff and breccia (Swanson, 1964). Many of the layers of fragmental debris have the steep dips that overall describe a composite volcano; Swanson and others (1989) described at least 300 m (984 ft) of gently dipping volcanic debris flow deposits, lava flows, and pyroclastic flow deposits “that can be traced nearly continuously into the cone.” These deposits likely formed the part of an apron at the foot of the volcano. The center of the Tieton volcano lies about 4 to 6 mi (6.5–10 km) east of the summit of Bethel Ridge. Shultz (1989) obtained an age of 26 to 25 Ma for the volcano. Hammond (unpub. data, 2003) confirmed the chemical similarity of its lavas to those of the older Fifes Peak Formation, whose source volcanoes, in order of decreasing age, are Fifes Peaks, Tieton volcano, Timber­wolf Mountain, and Edgar Rock.


Bethel Ridge may be cored by a fault. A ridge-top trench several meters in depth and evidence of recent landslide movement including tilted trees on its north and south slopes could be sackung. Not that the road to the top is rough, and four-wheel drive vehicles are advisable.



30.5 48.8

Outcrop of flow breccias of the Tieton volcano.  Blocks of andesite  as much as 30 in. (0.9 m) in diameter crop out in a buff-colored pumice matrix. 


31.0 49.6

An overhanging rock outcrop of flow breccia and andesite is slighty west of the Wildrose picnic area.



33.9 54.2

Enter Rimrock Retreat (elev. 2250 ft or 386 m). This is a popular put-in site for rafters on the Tieton River.


36.1 57.8

Diamict with angular rocks on the north side of the road likely is the toe of a large landslide from Bethel Ridge. Large landslides are mapped on both sides of the road near here, yet they go almost unnoticed by the casual viewer.

Many radial dikes of the Tieton volcano have intruded the rocks of this area. The dikes crop out on both sides of the valley here. Rounded river cobbles and gravels of Quaternary age sit atop the volcanoclastic rocks of the volcano.


37.0 59.2

Pillow basalts and basalt columns.


37.4 60.2

Another of the andesite or basaltic-andesite dikes of the Tieton volcano.


37.8 60.5

Cross the Tieton River.  The road cuts through a pillow palagonite breccia (Fig. [us12-pillowbreccia-wg]) that likely formed when part of a lava flow of the Columbia River Basalt Group flowed into impounded water—the flow probably dammed east-flowing streams. Slickensides on the south side of the highway show that the pillow breccia is cut by a fault. However, the fault has not been traced farther than this road cut (Swanson and others, 1989).


38.2 61.1

MP 177. We cross the Tieton River a second time (if eastbound).


 38.6 61.8

Windy Point Campground. Near here Swanson (1978) mapped 12 distinct flows of the Grande Ronde Basalt of the Columbia River Basalt Group. The lower three flows belong to magnetostratigraphic unit R2 and the upper nine flows are N1. (See the sidebar “Paleomagnetism—volcanoes as tape recorders” in Leg F in Pringle, 2002)


39.7 63.9

On the south side of the highway is a viaduct taking irrigation water from a diversion dam west of Rimrock Retreat to agricultural lands in the Yakima River valley. Big blocks on the north are probably part of a landslide deposit.


40.2 64.3

Eastern boundary of Wenatchee National Forest and the west boundary of the Oak Creek Wildlife area at MP 179.


41,2 65.9

Landslide deposit near MP 180. Exceptionally well developed jointing in the Tieton Andesite is visible at on the right (Fig. [tieton-andesite-columns]).


42.7 68.3

Stone stripes, a type of patterned ground, are draped on the hillside here. (See also Leg F, p. ??.)


43.3 69.3

About 0.4 mi (0.6 km) east of MP 182 there is a good view of the valley-filling Tieton Andesite lava flow overlying the Columbia River basalt.


43.4 69.8

A micaceous sandstone and siltstone layer between two N2 flows of Grand Ronde Basalt (Swanson and others, 1989). Swanson (1967) suggested that the metamorphic and plutonic minerals in these interbeds here indicate a northern source area for the stream that deposited them in Grande Ronde time. The mica in these sedimentary rocks, therefore, could be interpreted to be from the ancestral Columbia River or a major tributary. The location of the paleo-Columbia River channel here at the western edge of the Columbia Basin makes geologic sense in that previous researchers have suggested the river was pushed to the margin of the province by the basalt flows. About the sediments, Swanson and others (1989) also noted: “Mixed with the metamorphic and plutonic suite [of minerals] is a pyroclastic suite, including… glass shards, and glass-rimmed plagioclase and mafic minerals.” This suite provides clear evidence of explosive activity from Cascade volcanoes during Grande Ronde time 16.5–15.6 Ma (Campbell and Riedel, 1991; see Fig. [CRBGcorrelation], p. ?).


44.2 71.1 

On the left at a tight (dangerous) curve to the left, upper entabulature  and well-developed lower colonnade (columns) of Tieton Andesite sit on rubbly, cobble-rich fluvial deposits of the ancestral Tieton River (Fig. [tieton-and-fluvgravwg]).


44.6 71.4

Entrance to Oak Creek State Wildlife Area. Note that the exposure on the northwest side of the highway shows the Tieton Andesite only a few meters above river level. Swanson and others (1989) suggested that this implies the base level has not changed much in the approximately 1 m.y. since the andesite was erupted, so little or no downcutting took place—except through the lava flow.


45.6 73.4

We can see well-developed columns of Tieton Andesite to the south.



46.4 74.2

Cross the Naches River. This is slightly upstream of its confluence with the Tieton River.


46.6 74.6

Junction of SR 410 and US 12. From here we can drive east or west on Leg F. We join this leg about 16 mi (26 km) west of where it begins. Reset odometers if proceeding on Leg F.


References (some on hold or in the collection in Kirk Library=*    …or available online)

Campbell, N. P., 1975, A geologic road log over Chinook, White Pass, and Ellensburg to Yakima highways: Washington Division of Geology and Earth Resources Information Circular 54, 82 p

Dethier, David P., 1988, The soil chronosequence along the Cowlitz River, Washington: U.S. Geological Survey Bulletin 1590-F, 47 p

Hammond, P. E.; Pringle, P. T., 2008, June 18th field trip guide: Selected geologic sites along US 12 between Yakima and the Cowlitz River valley: NAGT 2008 fieldtrip: IN 2008 Annual meeting Pacific Northwest Section of the National Association of Geoscience Teachers, p. 31-38. [Accessed May 31, 2013 at   ]

Hoblitt, R. P.; Walder, J. S.; Driedger, C. L.; Scott, K. M.; Pringle, P. T.; Vallance, J. W., 1998, Volcano hazards from Mount Rainier, Washington, revised 1998: U.S. Geological Survey Open-File Report 98-428, 11 p., 2 plates. [Accessed Feb. 11, 2002 at]

Mullineaux, Donal R., 1996, Pre-1980 tephra-fall deposits erupted from Mount St. Helens, Washington: U.S. Geological Survey Professional Paper 1563, 99 p. [accessed Feb. 12, 2002 at     ]

*Pringle, Patrick T., 2002, Roadside geology of Mount St. Helens National Volcanic Monument and vicinity; rev. ed.: Washington Division of Geology and Earth Resources Information Circular 88, 122 p. [accessed on May 1, 2011 on p. 15 of ]

Pringle, Patrick T., 2008, Roadside geology of Mount Rainier National Park and vicinity: Washington Division of Geology and Earth Resources Information Circular 107, 191 p. [Accessed on June 22, 2008 at ]

Scott, K. M.; Vallance, J. W.; Pringle, P. T., 1995, Sedimentology, behavior, and hazards of debris flows at Mount Rainier, Washington: U.S. Geological Survey Professional Paper 1547, 56 p., 1 plate. [accessed on May 21, 2006 at ]

Walsh, Timothy J.; Korosec, Michael A.; Phillips, William M.; Logan, Robert L.; Schasse, Henry W., compilers; Meagher, Karen L.; Haugerud, Ralph A., digitizers, 1999, Geologic map of Washington--Southwest quadrant (digital edition): U.S. Geological Survey Open-File Report 99-382, 15 p. [accessed Sept. 7, 2000 at]


The distant rounded hills on the north side of the valley near Naches are composed of the Ellensburg Formation; the type section is exposed there. Note the color and shape of hills determined by the softer, lighter-colored sediments. The Ellensburg Formation is made up of coarse lahar runouts, conglomerates, and interbedded pumiceous and ashy, stream‑worked deposits that occurred between approximately 7 and 13 Ma. Smith (1988a) reported ages of 7.4 to 13.3 Ma. These deposits are the distal or downvalley facies of the lahars produced by Miocene volcanoes. Although the volcanoes themselves have since been eroded away, these deposits have provided important evidence of the past eruptions.

If we have time, we’ll take the Allan Road/Naches–Wenas Road to volcanic deposits. For an optional side trip to see laharic and fluvial deposits of volcanic origin, turn right onto Allan Road, pass under the old aqueduct, and proceed for several miles along the type section for the Ellensburg Formation. We are traveling “up section”, through successively younger deposits, as we drive up the grade. The Ellensburg Formation is noteworthy for its assemblages of volcanic deposits including lahars and lahar runouts whose main lithic component is a hornblende-rich dacite. These deposits were generated by explosive volcanism in the Cascades about 12 Ma, but they are one of the few relicts of this volcanism aside from some recently discovered vent areas. The volcanoes that produced these slurries and volcanic floods have largely been eroded away. Notice the many classic sedimentary features in the Ellensburg deposits. Make sure to find a safe location to turn around for the return trip to Naches.