DOGAMI Field Trips
|Granite to Ukiah, Grant County, Oregon|
Note: The field trip shown below is an extended version of the U.S. Forest Service (USFS) field trip guide that starts at Ukiah and ends at Granite. The guide shown below starts at Granite and ends at Ukiah.
The basement also includes considerably younger granitic plutonic rocks. The field trip route skirts the western edge of the Bald Mountain Batholith, one of the larger granitic masses that intruded along faults during Late Jurassic and Early Cretaceous time, some 140 million years ago. This large body of magma was emplaced at shallow depths, perhaps as little as 5 km (8 miles) below the old land surface, before gradually cooling to form a crystalline mass of coarse-grained tonalite. As the batholith cooled, surrounding rock heated up and metamorphosed, with some minerals recrystallizing to form new minerals. Fractures and fissures in the surrounding rock opened up and became filled with circulating, gold-bearing fluids that eventually solidified to become veins.
The basement was gradually exposed as the region was uplifted and eroded over the next 100 million years. Little remains from this period of erosion. Deep soils and rare fossilized palm and tree ferns suggest a warm and humid climate. In places are found stream gravels with gold eroded from newly exposed veins. In late Eocene time, somewhere around 40 million years ago, the landscape began to be reshaped by volcanic activity. Streams were first clogged by volcanic debris from volcanoes just south of the preent location of the town of Granite. About 30 million years ago, eruptions began in the north. Explosive volcanic eruptions from the central vent at Tower Mountain culminated in a large, caldera-forming, ash-cloud eruption. Small andesite volcanoes flanking Tower Mountain were active until about 20 million years ago. The landscape changed dramatically about 17 million years ago, when the land surface formed by eroded basement rocks and younger volcanoes was overwhelmed by massive sheets of flood basalts. Some of the flows (including those generally known as the Grande Ronde Basalt) erupted from vents east of La Grande. Other flows (including those known as the Picture Gorge Basalt) erupted from vents to the south and west.
Although the high Elkhorns near Granite are in part "old," having escaped burial by Miocene flood basalts, much of today's modern topography is "youthful," having been carved by uplift, deformation, and erosion over the last 9 million years or so. Broken and tilted layers of flood basalts near Ukiah provide a record of deformation along faults and folds. Erosion has (and continues) to play a major role in shaping the modern landscape. The downstream section of the North Fork of the John Day River near Dale is a textbook example of mass wasting by landslide processes. The upstream (higher elevation) section of the North Fork shows classical evidence of mountain and valley glaciation.
Our field trip begins at the old historic gold mining camp of Granite and ends at the equally historic settlement of Ukiah. The route provides an opportunity to see how landscapes, topography, forest cover, and land use have changed over time, largely in response to geology. Granite itself owes its existence to the gold washed out from veins in the ancient basement rocks. Ukiah owes its existence to the intermixed open stands of Ponderosa Pine and grasslands atop a covering blanket of younger basalt lava flows and sedimentary rocks: the pine was logged and sheep and cattle grazed on the grasslands. Midway between the two, both in distance and time, lies Tower Mountain, a large, heavily forested rhyolite volcano.
Gold was first discovered at Granite on July 4th, 1862. The mining camp was originally known as Independence and was first located where Bull Run creek and Granite creek join. The first miners quickly discovered that much of the original camp was on gold-bearing gravels and moved the camp off the productive ground and onto its present location on the nearby bench.
Granite was originally a placer camp where miners depended upon the flow of water to separate the lighter sand and gravel from the heavier gold nuggets and smaller particles known as "gold dust." The gold in the gravels was eroded from veins further upstream that had been uncovered by erosion.
The tree-covered mounds of gravel along the road here are unreclaimed dredge tailings from a much younger period of mining. Porter and Company constructed a 5-cubic-foot floating bucketline dredge that was similar to the one at the Sumpter Dredge State Park. Much of the older 19th century workings were obliterated by the dredge, which operated 1938-1942 and 1946-1951. The dredge tailings are the relict of an older era with different values. Modern-day reclamation rules require restoring mined lands to beneficial use.
Incomplete records indicate that over 50,000 ounces of gold were produced from the dredge and older placer mines in the Granite area. Much of this gold came from the 15 miles of stream channels worked by the dredge.
Avé Lallemant, H. G., 1995, Pre-Cretaceous tectonic evolution of the Blue Mountains province, northeastern Oregon, chap. 7 of Vallier, T. L., and Brooks, H.C ., eds., Geology of the Blue Mountains region of Oregon, Idaho, and Washington: Petrology and tectonic evolution of pre-Tertiary rocks of the Blue Mountains region: U.S. Geological Survey Professional Paper 1438, p. 271-304.
Dickenson, W. R., 1979, Mesozoic fore-arc basin in central Oregon: Geology, v. 7, no. 4, p. 166-170.
Ferns, M. L., and Brooks, H. C., 1995, The Bourne and Greenhorn subterranes of the Baker terrane, northeastern Oregon: Implications for the evolution of the Blue Mountains island-arc system, chap. 9 of Vallier, T. L., and Brooks, H. C., eds., Geology of the Blue Mountains region of Oregon, Idaho, and Washington: Petrology and tectonic evolution of pre-Tertiary rocks of the Blue Mountains region: U.S. Geological Survey Professional Paper 1438, p. 331-358.
0.7 Outcrops of basalt to left. Basalt flows such as this one are commonly covered by open stands of Ponderosa Pine.
0.8 STOP 1: Confluence of Granite and Boulder creeks
Parking area to right. Confluence of Granite and Boulder creeks. Stream channel has been mined at least three different times: first, by white miners shortly after the discovery of gold in 1862; second, by Chinese placer miners during the latter quarter of the nineteenth century; and third, by mechanized dredges during the middle of the twentieth century. The Chinese companies commonly worked ground that was either "mined out" or low in grade. Intricately stacked mounds of boulders are characteristic of the painstaking work done by Chinese companies. Remains of a hand-stacked "Chinese Wall" are visible in this area.
2.0 Road to the west leads to the old Cougar Mine. The Cougar was one of the larger lode mines in the Granite District. The Cougar was somewhat new discovery, not coming into production until 1900. The mine was most active 1938-1942, producing 19,126 ounces of gold and 10,976 ounces of silver.
2.1 Road continues through younger volcanic rocks, crossing a fault from dark black basalt to lighter colored dacite. The basalt flows contain green olivine pheoncrysts, while the dacite contains black hornblende phenocrysts. West of here, the flows are interbedded with volcanic breccias and gravels that in places contain well preserved leaf fossils. Because these rock types are very similar to those found in the Clarno Formation of central Oregon, these rocks are generally considered to be the "Eastern Clarno." Radiometric age dates (approximately 34 Ma) show the Granite Creek exposures to be somewhat younger than the type Clarno.
Granite Creek here runs parallel to a mineralized belt about 2 miles long and 3/4 mile wide that trends northeasterly, parallel to the margin of the Bald Mountain Batholith. Within this zone lie a number of quartz veins from which gold and silver have been mined. Chief mines were the Buffalo, Cougar, and Independence.
2.3 Cross fault between dacite and older basement rocks. Exposures of Elkhorn Ridge Argillite. Sea floor muds over 200 million years old. Badly fractured and disrupted. Part of the Baker Terrane.
2.9 Road to the right leads to the New York Mine. Like the nearby Cougar and Independence mines, New York was re-opened in the 1980s for exploration. The high price of gold at that time awakened interest in the lode mines of the Granite district.
3.1 Optional Stop: Independence Mill Overlook
Stamp mills were mostly used to crush ore into fine flour. "Free-milling" ores contained gold that could be recovered without further processing. Complex ores required further treatment to separate the gold. Most of the Granite District lode mines mined complex ores. Successful mines such as the Cougar and Buffalo used the floatation process to concentrate gold- and silver-bearing sulfides. The sulfide concentrates were then shipped to smelters where gold, silver, and copper were extracted.
3.7 Road crosses into granitic rocks of the Bald Mountain Batholith. The Bald Mountain Batholith is one of the largest intrusions in northeastern Oregon, covering about 240 square miles. The batholith is a composite intrusion, made up of smaller individual intrusions. Individual intrusions include silicic (granite), intermediate (tonalite), and mafic (gabbro) rocks. Most of Bald Mountain Batholith is tonalite and granodiorite, which are comparatively silicic in composition. Many of the higher mountain ranges in northeast Oregon, including the Wallowas and the Elkhorns, are cored by comparatively silicic batholiths that are lower in density than the surrounding rocks.
3.9 STOP 2: Contact between Bald Mountain Batholith and Elkhorn Ridge Argillite
Fragment of metamorphosed serpentenite is caught up along the contact. Because the batholith was originally a body of molten rock and quite hot, the "country" rock into which it was emplaced was heated up and recrystallized (this process is called contact metamorphism). Intrusive contacts can be complex, with country rock being invaded by dikes and lenses of granitic rock. Geologists believe that the batholith was emplaced at a depth of between 5 and 15 km.
A composite quartz vein is also exposed in the road cut. The vein is a quartz-filled shear zone that contains pyrite. The Ajax and Magnolia mines lie to the west; the Buffalo Mine is situated on the hillside to the northeast.
4.0 Baldy Creek Trail takes off to the east, following road USFS 7345 up Granite Creek. This trail rovides access to the top of Mt. Ireland and the heart of the Bald Mountain Batholith.
4.2 Road crosses divide between Granite and Crane creeks. Bedrock here is a mixture of basalt lava flows and volcanic gravels.
4.5 The lower workings to the Buffalo Mine are visible in the draw up the hill to the right. The Buffalo was the largest and most productive mine in this part of the Granite District. The Buffalo was discovered in 1873 and was first known as a silver mine. Most of the recorded production, 33,142 ounces of gold and 252,893 ounces of silver, took place between 1900 and 1965. Concentrates of the complex sulfide ore were processed through a floatation plant and shipped to a smelter.
4.6 Road crosses onto "Clarno" volcanic gravels and basalt lava flows.
6.7 STOP 3: Crane Flat Overlook
6.9 Volcanic conglomerate exposed in road bed. The rounded landforms in this area are largely volcanic gravels that have been disturbed and partially buried by glacial debris. During the Ice Ages, a valley glacier flowed down the North Fork of the John Day River and blocked Onion Creek, forming a lake that backed up into Crane Flat.
7.2 The previous extent of the North Fork glacier can be determined by the distribution of large granite blocks such as those exposed along the road to the east.
8.6 Onion Creek has been extensively worked for gold. Glacial deposits are typically not good places to find gold. Glaciers tend to plow up the stream beds and mix the gold and alluvium together. Terminal moraines can be productive, becuase light material is washed off and heavier gold remains.
8.7 Tailings from Klopp Placer to left. The Klopp Placer was a hydraulic mine. Gold-bearing gravels on the hillside to the left extend to an elevation of more than 130 feet above the road. The Klopp Mine was a hydraulic mine that in the summer months worked 24 hours a day. An early flood light that was used to illuminate the workings is on display at the Eastern Oregon Regional Museum in Baker City.
8.8 Crossing of the North Fork of the John Day River. Continue left on USFS Road 52 at the intersection with USFS Road 73. Restroom facilities are available at the campground to the left. USFS Road 73 continues east to the Anthony Lakes ski area and Baker Valley.
9.2 Road crosses terminal moraine. The moraine represents one of the last glacial advances of the North Fork glacier. Small landslides are present where the road has cut the end of the moraine.
10.0 Road crosses Trail Creek.
10.2 Exposures of amphibolite on left. Rock is badly deformed, folded, brecciated, and broken. Deformation is related to collision of arc and ocean floor material before the Bald Mountain Batholith was intruded.
10.5 Continue left on USFS Road 52 at the intersection with USFS Road 51. Proceed on USFS Road 52 toward Ukiah. USFS Road 51 leads north to Starkey and La Grande.
11.1 Road crosses onto a flat underlain by gravel that contains pumice cobbles.
12.0 Tower Mountain comes into view to the northwest. Here the road passes along the south side of Trout Creek Meadows. Geology here is uncertain; Tertiary deposits sit on deeply weathered bedrock. There may be older gravels at the contact.
13.1 Deeply weathered gabbro in road cut. Interpreted in part as an old soil zone.
13.8 Road crosses Trout Creek. Low knob in foreground to left (west) is Trout Meadows Butte, a small andesite plug. Part of an old volcano; possibly an andesite vent. Similarly aged vents and plugs are visible to east at Chicken Hill. The High Elkhorns are visible further to the east.
15.7 Platy andesite flows along the road are believed to be about 20 million years old. They erupted from nearby vents and flowed over an apron of silicic lava flows, volcanic debris-flows, and ashes that erupted earlier from the large Tower Mountain caldera.
20.1 Dacite lava flows associated with the Tower Mountain caldera are crystal-rich and tend to weather to shades of purple, giving outcrops a "speckled" appearance.
21.4 Here the volcanic debris flows are interbedded with darker colored basalt lava flows.
22.5 STOP 4: Volcanic debris flows at White Creek
25.7 Road crosses Big Creek and enters the Tower Mountain burn. Mountainous region to the north is part of the resurgent core of the Tower Mountain caldera. USFS road 5226, to the right, climbs to the top of Tower Mountain but is somewhat narrow and steep and not really suitable for passenger cars.
26.2 After leaving Big Creek, the road follows the southern margin of the ring fracture rhyolite domes. In map pattern, the domes form an arcuate belt or "smiley face" that wraps around the central core of the Tower Mountain caldera. Older basement rocks of the Baker Terrane are exposed to the south (left).
27.2 STOP 5: Gray perlite and black obsidian can be found in the dark vertical band visible in roadcut to the right. The ditch along side the outcrop has numerous rounded black beads. If the beads were translucent, they would be make nice "Apache tears." Narrow seams of opaline quartz occur in the white rhyolite. Precious opal has been found elsewhere in the ring fracture domes.
32.5 Road continues through the burn and crosses the head of Cable Creek. Interbedded dacite and dacite lahars are exposed along both sides of the road to the west.
32.8 STOP 6: Overlook of the west edge of the Tower Mountain Burn and headwaters of Cable Creek
Dacite porphyry lava flow overlies lahars at the Cable Creek Overlook. The dacite flow contains large plagioclase crystals that are easily seen by the naked eye. This overlook provides a good view of the west margin of the Tower Mountain caldera and the 1995 burn. Although the core to the caldera is heavily mantled by soil and rock, more than 130 m (400 ft) of caldera-fill ash-flow was exposed in a gully that formed during a severe thunderstorm in 1996. Soil and rock on the steep, burnt-over hillsides were stripped by intense rainfall following the fire, forming debris flows that clogged stream channels downstream. On hillsides where the rainfall was especially intense, debris flows became channelized, stripping the hillside down to bedrock. U.S. Forest Service and Oregon State University scientists are studying the Cable Creek drainage to determine how an undisturbed forest is eroded following an intensive fire.
37.1 STOP 7: Texas Bar Overlook
To the south lies the deep canyon cut by the North Fork of the John Day River into Columbia River Basalt lava flows. One of the youngest Grande Ronde Basalt flows caps a thick section of Picture Gorge Basalt flows. The "layer cake" appearance of the canyonlands to the west results from the stacking of one lava flow atop another. Lava flows spread out and filled pre-existing lowlands, as eruption followed eruption, gradually burying all but the highest points in a sea of lava. Contacts between individual lava flows are sometimes marked by bands of trees. Resistant cores to thicker flows are marked by cliffs and grassy slopes. In the distance to the west lies Madison Butte, a mound of ersoion-resistant sandstone that formed one of the high points that the flood lavas flowed around. Across the North Fork to the south lie the Greenhorn Mountains. The Greenhorns, like the Elkhorns, are made up of Baker Terrane rocks that have been intruded by Late Jurassic-Early Cretaceous granitic intrusions. Like the Elkhorns, the Greenhorns are a gold-bearing. The Baker Terrane rocks in the Greenhorns are different, however, in that they are composed of serpentinite-matrix melange rather than deep-ocean chert and argillite. The melange is generally considered to be part of a subduction zone complex.
The Aldritch Mountains form the ridge line farthest to the south. The Aldritch Mountains are part of a large, east-west trending, fault block along which the main stem of the John Day River flows. Eroded volcanoes of the Strawberry Volcanics sit atop the east edge of the fault block. The fault block's west edge, which culminates in the Aldritch Mountains, is part of the much older island arc that was being fed by subduction of the Baker Terrane.
37.7 Intersection with Texas Bar Road. (This is the starting point for the U.S. Forest Service Field Trip Guide brochure [PDF].)
42.4 Drift Fence Campground and restroom facilities are available.
45.0 Landforms to the north and west result from regional tilting and faulting. As the geology and gemorphology changes, so does the vegetation. Columbia River Basalt flows are marked by comparatively less heavily forested benches and tablelands. Open stands of Ponderosa Pine are more common, and dense thickets of lodgepole pine are scarce. The upper Grande Ronde Basalt flow here marks a once-flat tableland that has since been tilted and broken up by faults. Here the tablelands dip to the west.
45.7 STOP 8: Bridge Creek Flats.
Unmarked gravel road to left; paved road curves right. The topography at the top of the hill is formed on Columbia River Basalt lava flows. The grassy meadowlands are used for cattle grazing in summer and are winter habitat for approximately 2000 elk. The slopes between the flats support open Ponderosa Pine forests.
To reach Stop 9 (below), follow FR 52 (Camas Street) into Ukiah. Turn left on Highway 244 (Main Street). Drive 1.3 miles; pass the Ranger Station (and public restroom). Cross Highway 395, go another 0.1 miles, and U-turn into turnout on left side of road. This is stop 9.
48.0 Road turns north and descends into the Ukiah valley, crossing one of the fault scarps that make up the Ukiah fault zone. At the base of the hill, Grande Ronde Basalt flows are separated from Picture Gorge Basalt flows by a lignite coal seam.
49.2 Miocene sedimentary rocks are preserved in the down-dropped fault block on which the town of Ukiah rests. Livestock and lumber were the foundation to the community of Ukiah. Open meadows atop the sediments and open stands of large Ponderosa pine atop Grande Ronde Basalt flows combined to make good grazing and timber lands.
50.1 Center of Ukiah. The USFS field trip ends here at Stop 9 (below). To see additional geologic sights, including an excellent exposure of one of the faults that formed the Ukiah valley, turn east on Highway 244 toward La Grande and proceed to the optional stop at milepost 52.1.
51.4 STOP 9: Camas Valley Viewpoint
Tower Mountain is visible 20 miles to the east. Open meadows in the valley contrast with the forested benches and mountains. Clay derived from weathered volcanic ash forms soil lenses in the meadows. Run-off water perches on these lenses and supports vigorous grass and sedge growth. This is the end of the USFS field trip. Commercial services are available in the town of Ukiah.
52.1 Optional Stop: Fault
(On Highway 244 toward La Grande; see milepost 50.1 text.) A northeast-trending fault is exposed in outcrop along the north side of Highway 244. The lava flow belongs to the Sentinel Bluffs member and is one of the last of the Grande Ronde Basalt flows to erupt. Geochemical analyses and magnetic polarity measurements are used to define members in the Grande Ronde Basalt. Normal magnetic polarity and high amounts of magnesium in this flow are the reasons that this particular flow can be correlated with the Sentinel Bluffs member. Not surprisingly, this last of the Grande Ronde Basalt flows is overlain by a younger tuffaceous conglomerate. The conglomerate forms the base to the thick section of sedimentary rocks that underlie the Ukiah valley.
It is believed that Sentinel Bluff flows formed a vast lava field that, on a regional scale, was a nearly horizontal, flat-lying tableland. Much of today's modern topography results from uplift, faulting, and erosion. Faults, such as the small northwest-trending fault exposed here, broke the lava field into a series of blocks. Down-dropped blocks, such as the one that forms the Ukiah valley, were in some cases filled with sedimentary rocks. Up-thrown blocks were extensively eroded to form canyon lands. This fault is part of the larger Fly Valley fault zone, which here forms the northeast margin to the Ukiah valley.
52.1 Turn around and proceed west on Highway 244, back through Ukiah.
54.1 Ukiah is located in a fault-bounded depression that was partially filled by fine- to medium-grained sedimentary rocks. Coarser-grained conglomerates occur at both the base and top of the unit. Many of the rounded rocks in the conglomerate are rhyolite and tuff eroded from the Tower Mountain Caldera.
54.9 Poorly cemented siltstone, sandstone, and pebbly conglomerate are exposed along Highway 244 west of Ukiah. The sediments form low-lying hills that are in many places capped by coarse, uncemented gravel. Although no fossils have yet been found in these sediments, similar-appearing sedimentary rocks that are exposed near Pendleton, at McKay Reservoir, have Miocene vertebrate fossils.
55.4 Intersection of Highway 224 and Highway 395. For overview of the Ukiah Valley from the north, turn and proceed north on Highway 395 on a second side excursion to the milepost 58.1 optional stop.
58.1 Optional Stop: Overlook of the Ukiah Valley
Crushed rock at the site is a Sentinel Bluffs flow. Some of the Sentinel Bluffs eruptions were immense. These flows can be traced back to the northeast, to a north-trending, linear chain of that vents east of Joseph, Oregon. The same flows can be traced across the Columbia Plateau all the way to the Pacific Ocean, covering more than 10,000 square miles, making these flows among some of the largest single basalt flows in the world.
The Ukiah valley is a much smaller version of the much-larger Baker and Grande Ronde valleys. Vertical offsets along the Ukiah and Fly valley fault zones are probably less than 500 feet. In comparison, vertical offset along the west Grande Ronde Valley fault zone is more than 3,000 feet. This difference causes one to wonder: are the Ukiah faults older and comparatively inactive, or are they younger and just getting started?
Return to Highway 395, turn right and proceed south, toward John Day.
61.7 Highway 395 begins crossing the Ukiah fault zone, entering a deep canyon cut by Camas Creek. Grande Ronde Basalt flows are exposed along both sides of the highway. As Highway 395 continues deeper into the canyon, it continues to cross south-side-up faults. The Ukiah fault zone, like many in northeast Oregon, is not a single fault but a series of paralleling strands, none of which can be traced any great distance. Collectively, the strands combine to form structures that can be traced for 50 or more miles. Vertical offsets that may be on the order of mere tens of feet along individual fault stands reach as much as 500 feet when displacements along all of the strands faults arecombined.
62.9 Several fault zones are exposed in the road cut to the right. The faults cut hackly jointed Grande Ronde flows that underlie the Sentinel Bluffs member. These flows have lower amounts of magnesium than do the Sentinel Bluffs and typically form narrow columnar joints that break apart to form fist-sized blocks. Small, fist-sized blocks tend to make very good crushed aggregate.
65.7 The underlying Picture Gorge Basalt flows tend to form thinner flows that weather to form wider, more blocky columnar joints. Most Picture Gorge Basalt flows contain much higher amounts of magnesium and tend to weather to shades of orange and yellow.
71.9 Confluence of Camas Creek and John Day River. The thick flow exposed at the confluence is one of the lower units in the Picture Gorge Basalt. Picture Gorge flows often have large plagioclase crystals as well as smaller olivine crystals that can be seen easily with the naked eye. In contrast Grande Ronde Basalt flows seldom have recognizable crystals.
75.6 Turn left off Highway 395 and onto USFS Road. Proceed upriver on the gravel road toward Texas Bar.
75.9 Piles of boulders across the river to the south are remnants of a placer mine. This is about the farthest downstream the North Fork of the John Day River has been mined by placer operations. Upstream of here, small-scale placer mining, including some small-scale dredging continued into the early 1950s. Although the amount of gold recovered was probably not great, the placers of the North Fork of the John Day River were noteworthy in that they were some of the few placer mines in northeast Oregon where platinum group metals were recovered along with the gold.
76.3 Although the road continues physically upstream, the westward tilt of the geology is such that the river has carved deeper into the geologic section further upstream. Older units gradually become exposed at river level as the road makes its way upstream. Here, on the left, rounded, light-colored outcrops appear beneath the Picture Gorge Basalt. This is the top of a very thick welded ash-flow tuff that is now referred to as the Dale ash flow.
77.7 Optional Stop: Dale ash-flow tuff
Black, columnar -ointed Picture Gorge Basalt lava flows crop out above the ash-flow, making for a striking color contrast between the black basalt and white rhyolitic ash-flow tuff.
80.2 Turn left onto USFS Road 55 and proceed up Texas Bar Creek. The road continues through a large landslide complex that is in various stages of development. This is a very active geologic landscape in which competent, cliff-forming rock units, such as the Picture Gorge Basalt and the welded part of the Dale ash-flow, collapse when underlying softer strata are exposed. The massive landslide complex on Texas Bar Creek has two slip surfaces: a basal slip surface where the ash-flow rests on eroded basement rocks or tuffaceous sediments, and an upper surface where Picture Gorge Basalt flows overlie tuffaceous sediments. The walls come down when weak units are exposed by down-cutting of streams. Although this entire landscape is one large landslide, not all of the slide is active at the same time.
82.7 Although in a geologic sense the canyon walls here are in a constant state of collapse, mass wasting due to landslide processes is retarded by vegetation. Landslide surfaces are often rejuvenated when the covering vegetation is removed by processes such as fire. The Tower Mountain Fire provided an excellent opportunity to study some of the triggering causes for landslide and debris flows.
84.3 The small rock slide to the left originates in a down-faulted block of Picture Gorge Basalt. Raised levees on the most recently active part of the slide suggest that part of the mass moved as a debris-flow. Bedrock here is a down-faulted block of Picture Gorge Basalt. Here the Picture Gorge flow overlies bedded, coarse-grained volcanic conglomerates and sandstones that are less competent than the basalt flows.
86.0 Road crosses one of the faults that dropped the Picture Gorge Basalt down against underlying volcanic conglomerates. The contact between the basalt flows and underlying volcanic conglomerates is in many places unstable, resulting in landslides. Conglomerates here weather to form hoodoo topography.
87.5 Junction of USFS 55 and USFS 52 and the end of field trip. Turn left (west) to reach Ukiah, Pendleton, or John Day. Turn right (east) to reach Granite, Sumpter, or La Grande.
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