Field trip to White Pass and Naches (short 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 11.

*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. Stops include Riffe Lake viewpoint, Packwood, Palisades viewpoint, Knupenberg Lake, White Pass, Clear Creek Falls viewpoint, and more.

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.


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.

3.2 5.1  Roadcuts expose the clayey, reddish, deeply weathered top of the Logan Hill Formation. Weathering reaches depths > 50 ft (15 m).

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.

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.

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

23.3 37.3   STOP. Riffe Lake Viewpoint. Riffe Lake (a reservoir) is 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. Note vertical dikes of basaltic andesite


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. The smooth southwest-facing hillslope visible northwest of Morton is the dip slope of a limb of a northwest-trending anticline. Folding near

31.0 49.6  Junction of US 12 with SR 7 at Morton. We will continue east on US 12.

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.

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.

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. 

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.

41.4 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 3,500 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 IS THIS PUMICE SO THICK? 

43.0 68.8   Cliffs on the north side of road are Oligocene andesite flows.

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?

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 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.

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.

48.3 77.3 Randle and intersection of US 12 with SR 131..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.

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 were 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.

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).

64.1 102.6  MP 131, downtown Packwood. This small town may be located closer to more volcanoes than any other town in Washington.

64.3 103.5 Leave 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).

64.9 103.8  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.

67.2 107.5   Thin shale laminae under the sill are lake deposits that contain some leaf fossils.

68.7 109.9  The small outcrop of tuffaceous sedimentary rock on the right contains fossil wood.

69.6 111.4   Green to light-gray tuffs crop out to the right (southeast) of the road.

71.5 115.0  SKETCH AND LABEL THE VOLCANIC DEPOSITS HERE.  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


0.0 0.0  Junction of SR 123 and US 12. Sills and pyroclastic flows 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.

0.6 1.0   A poorly sorted deposit of 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

1.9 3.1   Ohanapecosh sedimentary rocks dip about 30 degrees to the southwest several hundred yards (meters) west of MP 141. 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  STOP 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.

4.2 6.7  MP 143.  Here we see 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.

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.

8.9 14.2     The pullout to the right has a good exposure of the Russell Ranch Formation. DESCRIBE THESE ROCKS!

9.1 14.6   Scenic vista points near here (MP 148) offer views of Goat Rocks to the south-southwest. Mount Rainier to the northwest and the shattered Russell Ranch Formation across the road to the north. The Clear Fork andesite flow is visible to the west.

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. 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.. Clayton estimated its age to be 30 to 20 ka.

12.3 19.7  White Pass, elevation 4470 ft (1363 m). Andesite on the north side of road, and Russell Ranch Formation on the south. Spiral Butte is ahead.

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. 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 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. Tom Sisson and Marvin Lanphere (USGS, written commun., 2004) report an 40Ar/39Ar age of about 102 ka for Spiral Butte lava.

14.8 23. STOP. 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.

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.

20.3 32.5  Indian Creek. MP 159. The type locality of the Indian Creek Gneiss, a unit in the Indian Creek complex, lies about 4 mi (~6.5 km) to the northwest.

20.9 33.4  Silver Beach resort.

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.

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.

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 Rocks 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.

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).

29.5 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.

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.

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.


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.