| (15.2) | Group B road log from Intersection of 395 and Line Street in Bishop. Proceed north from intersection, staying on 395.
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| 5.6 (20.8) | Ed Powers Road on left.
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Looking straight ahead, the prominent pyramidal peak at 11:15 is Mt. Tom (13,652'). The flat-topped peak to the left is Basin Mountain (13,240') and farther to the left the prominent sharp peak is Mt. Humphries (13,986'). The Wheeler Crest is to the right of Mt. Tom, north of Pine Creek Canyon. |
| 3.7 (24.5) | Enter Round Valley. On the west side of the valley, a view of the Pine Creek lateral moraines
(Plate 8).
The major fault fronting the Sierra at the base of the Wheeler Crest and Mt. Tom sweeps to the southwest, northwest of the Coyote Warp and toward upper Bishop Creek Canyon. |
| 1.5 (26.0) | Pine Creek Road. Turn left and proceed west. Wheeler Crest on right at 12-2:00. |
| 3.1 (29.1) | Town of Rovanna, the company town of the Pine Creek Mine.
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| 0.3 (29.4) | Entering the "canyon" flanked by Tahoe lateral moraines.
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| 0.6 (30.0) | Passing inset Tioga moraines. |
| 5.4 (35.4) | The contact of the Triassic Tungsten Hills quartz monzon-ite and late Paleozoic quartzite (dark brown) is on both sides of the road. The quartz monzonite can be seen as well on the far side of the pendant. |
| 0.2 (35.6) | Pine Creek Pack Station. Cross the bridge and proceed up the main road to the guard station. Park the vehicles in front of guard station along side of road. Notify the guard of your arrival. |
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GROUPS
A & B | From this point, we will use the cumulative mileage figures from Group A. Return to Rock Creek Road. |
| 9.6 (59.6) | Rock Creek Road (Paradise Road). Turn left. |
| 1.6 (61.2) | Mono County Line. Bishop Tuff exposed on right. |
| 1.4 (62.6) | Paradise Lodge. Bishop Tuff exposed on both sides of canyon. |
| 0.1 (62.7) | STOP 13. Stop on left side of road. Outcrops across the road are glacial outwash gravels lying on the Bishop Tuff. This is also a good place to look back to the south at Round Valley, Pine Creek and its morainal ridges, Mt. Tom, Bishop Creek, and the Coyote Warp. |
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Proceed north on Old Sherwin Grade Road. |
| 2.3 (65.0) | STOP 14, to view columnar jointing in Bishop Tuff on east side of Rock Creek gorge. The several small hillocks surmounting the surface of the Bishop Tuff are erosional remnants of fumaroles. These mounds have been left higher than the general level following erosional stripping of the uppermost part of the Bishop Tuff. |
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Later in the day, we will stop at excellent examples of both of the above features along the Owens River gorge. |
| 0.8 (65.8) | Entering the Inyo National Forest. |
| 1.2 (67.0) | Summit of road. Contact of Bishop Tuff on Sierran granites across the canyon to the right. Note the irregular granitic surface on which the tuff was deposited. The same contact is exposed just ahead in the roadcut on the left. |
| 0.8 (67.8) | Crossing Rock Creek. |
| 0.6 (68.4) | Highly weathered Sherwin till in roadcuts on right. Here, the typical Sherwin till overlies a highly weathered older till; the contact is marked by a red zone, interpreted by Sharp (1968) as the weathered zone separating Sherwin and the older McGee till. |
| 2.2 (70.6) | Junction of Lower Rock Creek Road and Highway 395. Turn left. The large exposure on the right, across 395, is the Big Pumice cut, which we will visit later today. |
| 0.3 (70.9) | On the skyline at 10-12:00, we get our first glimpse of the colorful rocks of the Mt. Morrison pendant. We will see more of these metasedimentary strata at Convict Lake at our next stop. |
| 0.6 (71.5) | Rock Creek Road on left. Here again, we are indebted to Steve Lipshie (1976) for the following: |
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"Rock Creek heads in the Sierra Nevada, flows north toward Toms Place, and then abruptly swings southeastward, eventually emptying into the Owens River east of Round Valley. Putnam (1960, p. 249) suggested that the creek originated as two separate streams, one flowing northward into Long Valley from the Sierra Nevada and one flowing southeastward down Sherwin Hill into Owens Valley. He believed that the south-flowing stream cut its way headward, eventually breaching the drainage divide between the two streams and capturing the north-flowing one." |
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| 0.5 (72.0) | Bishop Tuff on right. |
| 0.4 (72.4) | Wheeler Crest quartz monzonite on left. |
| 1.0 (73.4) | Little Round Valley to the left. We are now travelling along the old abandoned course of Rock Creek, which drained into the Crowley Lake basin, ahead. At the north corner of Little Round Valley, we can see a narrow notch which is part of the abandoned Rock Creek stream course. |
| 1.2 (74.6) | Large terminal moraine of McGee Creek at 12:00 and the trace of the Hilton Creek fault at the base of the mountain. At the mouth of McGee Creek, an escarpment about 50 feet high cuts the moraine. The scarp extends for some distance to the north as well. The Hilton Creek fault was the locale of the several sharp earthquakes (6.0-6.1) that were felt in the Mammoth Lakes area in May, 1980.
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| 1.0 (75.6) | Lake Crowley at 3:00, and the Benton Range to the east
of the lake. The Benton Range is a large island of Paleozoic metasedimentary rocks and Mesozoic granitics surrounded by the Bishop Tuff (see Mariposa Map Sheet).
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| | Again, borrowing from Lipshie (1976): |
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"Although a large lake once filled Long Valley during Pleistocene time, the present Lake Crowley dates only from 1941, when the LADWP built Long Valley Dam at the head of Owens River Gorge.
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"During part of the Pleistocene, ... Long Valley Lake filled much of the Long Valley caldera. Shoreline terrace deposits can be found in the central part of the caldera at elevations as high as 7600 ft ... Bailey and others (1976, p. 736) believe that the lake at its highest level reached an elevation of 7800 ft ... and that this occurred sometime before 0.63 m.y. ago. On the basis of K-Ar dating of Long Valley volcanics associated with terrace deposits, they infer that the level of Long Valley Lake was near 7500 ft ... between 0.51 and 0.47 m.y. ago and 7300 ft ... after 0.47 m.y. ago, dropping below 7000 ft ... by 0.1 m.y. ago. The Pleistocene lake disappeared sometime in the last 100,000 years when the outlet at Owens River Gorge was cut deeply enough to drain the lake.
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"The mountain mass to the north, across the lake, is Glass Mountain Ridge, which forms the northeastern boundary of the Long Valley caldera. The forested area southeast and east of the lake is the Bishop Tuff surface that extends southward to form the Volcanic Tableland north of Bishop."
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| 4.9 (80.5) | Rounding the bend to the left, we get a view of Mammoth Mountain (11,034'), the Minarets and Mt. Banner and Mt. Ritter on the skyline. Mt. Morrison (12,243') is at 9:00. |
| | The large sage-covered northeast trending ridge at 9-7:00 is a Tahoe latarel moraine that has been truncated by younger moraines from the Convict Lake basin. The flat-topped mountain to the left (south) of this large lateral moraine ridge is McGee Mountain. Again, from Lipshie (1976): |
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>"On top of McGee Mountain is an old erosion surface cut across Ordovician metasedimentary and Pliocene volcanic rocks (Rinehart and Ross, 1964) ... The Pliocene volcanic unit ... was K-Ar dated at 2.6 m.y., ... which establishes a maximum age for the erosion surface. The summit erosion surface is mantled with a bouldery deposit ... [called the] McGee till. ... [It] occurs at elevations ranging from 9720 ft ... to 10,800 ft ... on McGee Mountain. ... Boulders and cobbles in the till consist mostly of Round Valley Peak Granodiorite, with lesser amounts of metamorphic clasts derived from formations not cropping out on McGee Mountain ... The ... granitic bounders range up to 30 ft (10 m) in maximum dimension; ... the nearest exposure from which they could have been derived is 3 miles ... to the south (Rinehart and Ross, p. 67). McGee Creek has cut a canyon 2500 ft ... deep between the till and its source area, which indicates that much of the local range-front relief developed after deposition of the till ... From geomorphic evidence, Putnam (1960) ... postulated vertical displacement along range-front faults at McGee Mountain amounting to 3000 ft ... between the McGee and Sherwin glaciations and another 1000 ft ... between the Sherwin and Tahoe glaciations ... Thus, the unusual perched glacial deposit atop McGee Mountain provides evidence for Plio-Pleistocene uplift along the eastern escarpment of the Sierra Nevada."
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| 0.8 (81.3) | The hills on the right at 3:00 are the Moat rhyolites,
about which we will learn more enroute from Convict Lake to Hot Creek. |
| 0.8 (82.1) | Convict Lake Road. Turn left. The irregular hills on the right are a Tioga terminal moraine which we can see truncates the Tahoe lateral at 10:00. |
| | The high peak at 11:00 is Mount Morrison (12,243'). Laurel Mountain is the highest peak to the west of the Convict Lake basin, which is an excellent example of a U-shaped glacial valley. |
| 2.2 (84.3) | Here we cross through the Tioga recessional moraine which provided the original natural dam at the east end of Convict Lake. Cross over the bridge spanning Convict Creek and park in parking lot ahead. |
| 0.2 (84.5) | STOP 15. Convict Lake. |
| | Here at Convict Lake, we will take some time to enjoy the lovely Alpine scenery and to look at the glacial features and to examine, from a distance, the rocks of the Mount Morrison pendant. |
| | Yet again, from Lipshie (1976): |
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"The rocks exposed in the cliffs of Laurel Mountain, west of the lake, and Mount Morrison, south of the lake, are Ordovician and Silurian slate, marble, siliceous hornfels, and sandstone (Rinehart and Ross, 1964) - Bedding, which strikes slightly west of north and dips eastward, is overturned with tops to the west. The rocks exposed in Sevehah Cliff on the east flank of Laurel Mountain are Ordovician and Silurian (?) calcareous quartz sandstone, siliceous hornfels, and metachert that appear to be complexly folded. However, most of the 'folds' are purely illusory, resulting from the effects of topographic irregularities upon steeply dipping beds. ...
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"The metasediments of the Mount Morrison pendant [more than 30,000 feet of eugeoclinal strata (Rinehart, Ross & Huber, 1959)] are [among] the oldest rocks recognized so far in the Sierra Nevada; [they range in age from Ordovician to Permian]. ...
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"As you might expect, Convict Lake owes its unusual name to an interesting historical incident. The canyon in which the lake is situated was formerly called Monte Diablo Canyon. The events that led to its renaming began with a prison break in 1871 at Carson City. Twenty-nine convicts escaped from the Nevada State Penitentiary and scattered in all directions. Six convicts headed south-westward into California, where they killed a pony express rider from Aurora, a mining town northeast of Mono Lake. Posses from Aurora and Benton joined the pursuit and caught up with the convicts near Monte Diablo Creek (now called Convict Creek). The pursuers spent the night at the McGee ranch and next morning cornered the convicts in Monte Diablo Canyon. The convicts put up a fierce fight in which two posse members were killed: Robert Morrison, a Benton merchant and Wells Fargo agent, and Mono Jim, a Paiute Indian. The posse retreated, and the convicts headed southeast toward Round Valley. There a local posse led by James Sherwin captured two of the desperados, a third being captured a couple of days later along Pine Creek. A few days later, when the three prisoners were being taken back to Carson City from Bishop, a band of local vigilantes seized the convicts at Bishop. The hapless convicts were 'tried' by an impromptu jury, and two of them were lynched near Bishop. A group of big trees on the north side of Highway 395 near Brockman Lane west of Bishop is reputed to be the site of the hangings. Ultimately, all but two of the 29 convicts were recaptured. As a result of his bad luck, Morrison got his name attached to the mountain peak just south of Convict Lake. Mono Jim, although equally unlucky, was not equally commemorated."
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| | On to Hot Creek. Return the 2.4 miles to the intersection of 395 and Convict Lake Road. |
| 2.4 (86.9) | Highway 395. Cross the highway and continue east on the Hot Creek Road. Going east on the dusty Hot Creek Road is a good time to digest the lore of the Long Valley caldera, courtesy of Steve Lipshie (1976): |
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GEOLOGICAL OVERVIEW
OF THE LONG VALLEY CALDERA | |
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"The margin of the Long Valley caldera is poorly defined by physiographic features in the vicinity of Lake Crowley. We crossed the caldera boundary somewhere around Tobacco Flat before we turned off of U.S. Highway 395 to go to Convict Lake. Now that we are in the caldera, it is appropriate to summarize the geologic history of the volcano-tectonic depression of Long Valley ... This summary is intended to provide a reference frame for understanding the relationships of the various geologic features of the caldera ... The overview of the caldera is based on detailed mapping by Roy Bailey ... and K-Ar geochronology by Brent Dalrymple and Marvin Lanphere, as described by Bailey and others (1976). |
| "The oldest rocks that are associated with the Long Valley magma chamber are the rhyolite flows of Glass Mountain, which were extruded along the northeast part of the caldera ring-fracture system. Bailey and others ... interpret Glass Mountain as having been formed over a period of 1 m.y., based on two K-Ar dates, one of 0.9 m.y. and one of 1.9 m.y. |
| "The next major event in the formation of the caldera was the extrusion of the Bishop Tuff ; about 0.7 m.y. ago. About 125 cu mi ... of ash flows were emplaced over a very short time span, perhaps only a few hours or days ... After eruption of the Bishop Tuff partially emptied the magma chamber, its roof collapsed and formed the caldera, an elliptical depression measuring 10 miles ... from north to south and 18 miles ... from east to west ... Caldera subsidence totals 2 miles ... of which one third is reflected in present topographic relief and the other two thirds are represented by post-caldera basin fill. |
| "Shortly after collapse of the caldera, the central part underwent resurgent doming during which rhyolite flows ... and rhyolitic tuff breccia were extruded. The rhyolite was emplaced from at least 12 vents during an interval of perhaps 50,000 years, between 680,000 and 630,000 years ago (Bailey and others, p. 730). Doming and rhyolite extrusion in the central region was probably completed by 0.5 m.y. ago, thus producing what [has been called] ... a 'resurgent cauldron' with an uplifted central dome surrounded by a 'moat.' |
| "The next phase of volcanism involved the emplacement of marginal (moat) rhyolites in three discrete episodes: 0.5, 0.3, and 0.1 m.y. ago. The moat rhyolites ..., which were emplaced on the periphery of the central dome, are probably related to ring fractures around the resurgent dome. A later stage of volcanism produced rim rhyodacites ... from at least 10 vents in the western part of the caldera. The main mass of these hornblende-biotite rhyo-dacite flows is Mammoth Mountain ... which consists of flows ranging in age from 180,000 to 50,000 years. |
| "During late Pleistocene time, basalt and andesite flows ... were extruded in the west moat of the Long Valley caldera. The K-Ar ages of the flows range from 222,000 to 62,000 years. Near the town of Mammoth Lakes, basalt flows are interbedded with pre-Wisconsin glacial deposits. |
| "The most recent eruptive activity in the caldera was Holocene rhyolitic volcanism that formed the Inyo Craters and domes in the northwest quadrant of the Long Valley depression. These young volcanic features appear to be aligned on a north-trending fissure between the Long Valley caldera and the Mono Craters ring fracture zone. One of the Inyo domes has an age of 720 ± 90 years, based on radiocarbon dating ... The widespread geothermal activity that continues today in the caldera indicates that the Long Valley magma chamber should still be considered capable of generating volcanic activity, despite its present quiescence." |
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| 1.6 (88.5) | Turn left into Hot Creek Parking area. STOP 16. Although we are here in part to partake of the beneficial effects of the hot spring waters, some of us may wish to look at the geological features as well. For this purpose, I again borrow freely from Lipshie (1976): |
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"Walk down the asphalt path to the creek. As you walk along the path, note the steam rising from fumaroles and hot springs to your right along the creek. There are also some submerged hot springs in the creek itself near the wooden footbridge. The mingling of hot spring water and cold stream water produces a broad gradation of water temperatures ranging from uncomfortably cool to unpleasantly hot. The wide variety of water temperatures has made this stretch of Hot Creek a popular swimming area, so the Forest Service has built restrooms and shelters for changing clothes and has also paved the parking area. |
| "As you walk around, note the alteration of the rhyolite in the Hot Creek gorge. Thermal activity has produced an extensive zone of nearly white, leached rock in which older rhyolite ... has been kaolinized and opalized. Perlitized flow-banded obsidian can be seen along the asphalt path. |
| "The Forest Service has built a wooden walkway overlooking the hot springs and steam vents about 400 ft ... downstream from the footbridge. This cluster of hot springs suddenly appeared without warning on the night of August 24-25, 1973. At least five hot springs formed, and the two largest started as geysers that spouted water about 10 ft ... into the air. When I first visited the locality, six days after the new springs appeared, the geyser activity had diminished to spouts about 3 ft ... high about every half minute. Water in the new springs was superheated to 96°C (Fred Wilson, U.S.G.S., oral communication, 1973); the boiling point is 93°C at this elevation (6950')... |
| "The cause of these new hot springs remains unclear, but a hypothesis has been advanced to explain their sudden appearance. On the afternoon of August 25, 1973, after the new springs had appeared, a mild earthquake occurred northwest of Bishop. The tremor, which had a magnitude of 3.5, was centered about 25 miles ... southeast of here ... Bailey and others (1976, p. 738) suggest that the quake and hot spring activity are related and that seismic activity altered the subterranean plumbing that feeds the thermal springs. In support of this hypothesis, they cite a similar event that occurred a couple of months later, when new springs appeared along Hot Creek within hours of another small quake." |
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| | After indulging ourselves, we must return to reality and return to Highway 395. |
| 1.6 (90.1) | Intersection of 395 and Hot Creek Road. Turn left. We will return directly to the intersection of 395 and the Lower Rock Creek Road, of course recalling all the geological detail absorbed on our way north. |
| 11.5 (101.6) | Junction of 395 and Lower Rock Creek Road. Immediately beyond the junction, park on right side of Highway 395. STOP 17.
Big Pumice Cut.
Here again we will take advantage of the labors of Lipshie (1976) and borrow freely from his superb guidebook: |
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"This roadcut, named the 'Big Pumice Cut' by Sharp (1968, p. 352), was made in 1957 when the present highway was constructed to replace the steep, winding road that follows Rock Creek along the Sherwin Grade. The Big Pumice Cut is famed in story and song and revered by geologists throughout the land because it finally laid to rest the controversy regarding the relative ages of the Bishop Tuff and Sherwin till. ... This till is one of the oldest Pleistocene glacial deposits yet recognized in the Sierra Nevada, being exceeded in age only by the McGee and Dead-man Pass tills ... Putnam (1938, p. 82) reported that in the Mono Craters Tunnel, then being constructed as a link in the Los Angeles aqueduct system, the tuff overlies a pre-Tahoe glacial deposit, which Gilbert (1938, p. 1860) interpreted as being Sherwin till. |
| "... [Some] workers [have] questioned whether the till underlying the Bishop Tuff is really Sherwin till, because of the relatively great antiquity of the tuff compared to the Pleistocene glacial epochs (Rinehart and Ross, 1964, p. 79). Basal pumice ash collected about 3 ft (1 m) above the till-tuff contact in the Big Pumice Cut was K-Ar dated and found to have an age of about 0.71 m.y. ... This age was thought to be too great for the Sherwin glaciation, which was generally considered to be equivalent to the Kansan and/or Illinoian glacial stages of midcontinental
North America ... To settle the controversy conclusively, Sharp (1968) undertook a comprehensive study of till-tuff relationships in the vicinity of the Sherwin type locality. He ... convincingly demonstrated that the Sherwin till is older than the tuff and that the till exposed in the Big Pumice Cut is almost certainly Sherwin till. Because the till apparently is over 0.7 m.y. old, the Sherwin glaciation more likely correlates with the Kansan glacial stage than with the Illinoian stage. |
| "... [N]ote the well defined contact between the boulder-laden till and the overlying pumice-ash tuff. Boulders in the till are highly weathered and have largely decomposed to grus. The basal 15 ft (5 m) of the tuff consists of air-fall ash and lapilli, whereas the overlying material is an ash-flow deposit that presumably was emplaced as a nuee ardente ... Layering in the tuff is parallel to the till surface for the air-fall unit and roughly horizontal for the ash-flow unit. The tuff in this road-cut is much less indurated than the more typical Bishop Tuff ... |
| "The tuff here is cut by a conspicuous series of clastic dikes that are nearly perpendicular to layering and extend into the till. The dikes, which were derived from boulder gravel deposited on the tuff at the top of the cut, consist of unconsolidated material that becomes finer-grained downward. ... |
| "Around 1971, the California Division of Highways (CDH) decided to reroute Highway 395 to eliminate some of the curves east of Toms Place. As part of its so-called beautification program, the U.S. Forest Service insisted that the CDH fill in the 'unsightly' old roadcut when it was abandoned. Fortunately, one of the CDH employees, a graduate of UCLA in geology, realized the significance of the roadcut and alerted various geologists to the proposal to obliterate the cut. Thereupon, a number of geological luminaries ... wrote letters to the Forest Service pointing out the unique geological significance of this cut. As a result, the Forest Service and CDH agreed to preserve the roadcut for the edification of future generations of geologists."
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| 0.9 (102.5) | Near Sherwin Summit (7,000'), turn left onto dirt road. |
| 0.3 (102.8) | At first wide spot in road on right, STOP 18. The profile ahead of us (looking at 12:00) shows a craggy cliff of Bishop Tuff on the left; to the right is a long gentle north-sloping surface underlain by Sherwin Till. Before the making of the Big Pumice Cut and any of the drilling for the Los Angeles water system, such relations as we see before us were just about the sum total of what could be said about the Sherwin-Bishop Tuff age relations. As you can see, it is quite equivocal. The Sherwin might easily be resting atop a broad basin in the Bishop Tuff, or it might just as well extend northward beneath the tuff. |
| | Proceed east on dirt road. |
| 0.5 (103.3) | Granite knob surrounded by Bishop Tuff where road begins to swing to the left (north). For you terminology buffs, this is a special kind of steptoe (old bedrock surrounded by young volcanic rocks). |
| 1.7 (105.0) | Turn left at the Surge Tank. |
| 0.1 (105.1) |
Owens River Gorge overlook.
STOP 19. |
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At the Gorge overlook, we again make use of Lipshie's guidebook:
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"... Four lithologic units are exposed in the wall of the gorge at this locality. The oldest rock, which comprises the lower half of the wall, is Wheeler Crest Quartz Monzonite, which is thought to be of Cretaceous age ... Outcrops of this unit are scattered along the eastern flank of the Sierra Nevada from Pine Creek near Bishop northward to Lee Vining Canyon near Mono Lake. Overlying the quartz monzonite in the gorge are basalt flows that have an age of 3.2 ± 0.1 m.y., as determined by K-Ar daring (Dalrymple, 1963, p. 380). The basalt is overlain by a thin, discontinuous layer of glacial debris, which can be recognized from here by its light-colored cobbles and boulders of granodiorite. This material is Sherwin till outwash (Sharp, 1968, p. 355) derived from glacial deposits west of the gorge. |
| "The stratigraphically highest unit exposed in the gorge is the Bishop Tuff, which here is about 200 ft (60 m) thick and consists of rhyolitic ash deposits that grade downward into welded tuff. The unit has a basal zone of non-welded pumice ash which is mostly concealed by talus in the gorge but which can be found locally ... The non-welded zone includes both the basal air-fall ash and the lower part of the ash-flow sequence. The more highly welded tuff zones form cliffs, whereas the unwelded or partially welded parts of the unit, although indurated by vapor-phase crystallization and agglutination, form slopes on the gorge walls. |
| "Putnam (1960, p 245) studied the origin of the Owens River Gorge and concluded that it formed by headward cutting of the Owens River across the Volcanic Tableland. He ... argued that headward growth of the south-flowing river captured the Long Valley drainage and provided an outlet for the Pleistocene lake that once filled the valley. Thus, according to Putnam, the gorge was entrenched prior to draining of the Pleistocene lake. But Wahrhaftig (1965) advanced several objections to Putnam's hypothesis: (1) the present rim of Owens Gorge is the lowest point on the Long Valley hydrographic basin, and one would expect overflow to occur at that location; (2) the lower part of the gorge follows an antecedent channel across the warped surface of the Volcanic Tableland, which suggests that the river course predates warping; (3) only streams that drain areas outside the Bishop Tuff have incised deep canyons across the tuff, which suggests that gorge cutting by headward erosion of a stream within the tuff is unlikely; and (4) the gorge has incised meanders reaching nearly to the canyon rims and having a wavelength and amplitude appropriate for the stream's present flow, rather than for that of a much smaller stream. From this evidence Wahrhaftig concluded that the Owens River is an antecedent stream that was dammed at the Long Valley outlet by warping of the Bishop Tuff. Both he and Rinehart and Ross (1957) believe that Long Valley Lake was drained by overflow into and downcutting of the Owens River Gorge." |
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| | Return to the Surge Tank and proceed on road directly to the south. |
| 2.3 (107.4) | Turn sharply to left at "Y". |
| 0.2 (107.6) | Stop on right side of road, just before locked gate. STOP 20. Walk about 150 feet down the road beyond the locked gate. |
| | I have run out of ways to introduce items from the guidebook of Lipshie (1976), but here it is: |
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"At this locality, the Owens River Gorge is carved entirely in Bishop Tuff. The lower, more highly welded tuff is generally massive with irregularly developed jointing. The upper part of the tuff, however, has strikingly unusual columnar jointing that commonly forms radiating sets ...
[Plate 11].
Column diameters typically range between 3 and 5 ft (1 and 1.5 m), and columns have diverse orientations ranging from horizontal to vertical (Gilbert, 1938, p. 1836). If you want to photograph the rosette jointing, visit this locality in the afternoon. |
| "The Bishop Tuff in the roadcut at this stop ... consists of very light gray to pale pink, agglutinated but only slightly welded, vitric pumice ash that weathers grayish orange pink ... The tuff contains abundant pumice fragments , as well as plentiful phenocrysts of sanidine, quartz, and plagioclase - The lower, more highly welded tuff can be seen by walking farther down the road into the gorge. It is pale red purple ... to medium gray on fresh surfaces, and it is noticeably denser than the sillar by the locked gate. Pumice clasts in the denser tuff are considerably flattened in the plane of bedding. Granitic xenoliths can be found in the lower part of the tuff. |
| "Putnam (1960, p. 236) and Sheridan (1970, p. 862) observed that radiating columnar joints commonly occur beneath fumarolic mounds. Sheridan (p. 862-863) notes the similarity of the radial jointing pattern to the heat flow pattern during cooling around a gas vent - the heat would flow radially outward and upward from the vent. This similarity suggests that jointing formed normal to isothermal surfaces around vents, and Sheridan (p. 863) has used this relationship to reconstruct thermal regimes in the tuff during cooling." |
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| | Return to vehicles and return to "main road." |
| 0.2 (107.8) | At the Y junction, turn left and proceed south. |
| 5.4 (113.2) | STOP 20a (not on route map). Here we will look at a road-cut section cut through one of the fumarolic mounds on the surface of the Bishop Tuff. And here is the last of the borrowings from Lipshie (1976). Steve can now rest easy that he will no longer be misquoted: |
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"Roadcut through a fumarolic mound. The white, altered tuff in the vapor-phase zone of the mound consists largely of cristobalite, tridymite, and alkali feldspar ... |
| "Note the small fumarolic mounds about 50 to 100 ft ... high that dot the surface of the Volcanic Tableland on both sides of the road. The low, rounded mounds on the Bishop Tuff surface are believed to be the sites of formerly active gas vents. The temperature increase due to the escaping hot gases enhanced vapor-phase crystallization near the vent (Sheridan, 1970, p. 865). The tuff near fumaroles thus became more highly indurated and less susceptible to weathering. Subsequent erosion has stripped 50 to 200 ft ... of the surface of the Volcanic Tableland ..., leaving the more resistant rocks at sites of former gas vents as low mounds. Sheridan (1970, p. 860) reports that fumaroles were aligned along early joints in the tuff, which indicates that jointing occurred after compaction but before the completion of cooling."
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| | Proceed to south. |
| 0.6 (108.2) | Turn right at T intersection. |
| 0.7 (108.9) | Intersection of Gorge Road and Highway 395. Turn left (south). Proceed south toward Bishop on 395. |
| 10.8 (119.7) | Intersection of Highway 395 and Line Street in Bishop. As we travel the 15 miles to Big Pine, you can renew your acquaintance with the Coyote Warp on our right. |
| 4.7 (124.4) | Collins Road. Marble Canyon is on our left at 9:00, in the White Mountains. The section
(Plate 12)
exposed on the face of Black Mountain (9,083'), south of Marble Canyon, extends from the uppermost Wyman Formation (not really discernible from the highway) through the Reed dolomite (massive, non-bedded carbonates), the Deep Spring Formation (well-bedded carbonates), to the Campito Formation (black) capping the ridge. We will see more of this Precambrian-Cambrian succession on the third day. |
| | Beyond, from 10-11:00, the Waucobi embayment (the large recess in the front of the White Inyo Range), the Pleistocene Waucobi Lake beds (white), and the Caltech radio telescopes on the floor of the valley. |
| 1.7 (126.1) | Keough's Hot Spring Road. The hot springs, at 3:00, are located along a frontal fault at the front of the Sierra. Even in the area of the Coyote Warp, frontal faulting is of local significance. |
| 5.7 (131.8) | As we approach Reynolds Road (on right), we get a good view of Crater Mountain at 1:30, and for the geomorphology buffs-two steptoes-granite beckrock hills not quite buried in younger basalt flows. |
| 2.1 (133.9) | Highway 395 and Crocker Street in Big Pine. Turn right and proceed toward Camp Inyo. |
| 1.0 (134.9) | Split Mountain (14,051') lies straight ahead on the skyline; Birch Mountain (13,655') to the right; Mt. Sill (14,162') is seen on the skyline farther to the right
(Plate 6)
|
| 0.7 (135.6) | Sugar Loaf Road - do not turn into Camp Inyo, but continue to what Wayne Sawka believes is the most exciting part of the trip. |
| 1.5 (137.1) | Turn left onto McMurray Meadows Road. Continue to the left and stay on the main road. |
| 1.1 (138.2) | A small felsic intrusive lies directly ahead. Notice the
contrast in weathering styles. |
| 0.8 (139.0) | STOP 21. Park on the side of the road and walk up the
hill to the right of the road to the large boulders with the overhang. Here we will examine schlieren in the Tinemaha granodiorite, and Wayne Sawka will no doubt have something to say, perhaps even more than what is contained in the text and in Figures 13-16. |