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PLEASE NOTE: This guide is not yet complete (pictures, mileages and links are not finished yet). Please check back soon for a more finalized version!

A Virtual Field Trip along the Tuolumne Meadows Highway in Yosemite National Park

by Garry Hayes

Modesto Junior College

This trip is modified from a field trip conducted in September, 2006 by the Far West Association of the National Association of Geoscience Teachers. For the complete version of this trip as well as tours of Sonora Pass, the Mother Lode, and a visit to a restoration effort along the Tuolumne River, follow this link


This trip offers a cross section of the geology of the central part of the Sierra Nevada structural block, with observations of the easternmost part of the metamorphic terranes of the Mother Lode, the Tuolumne Intrusive Suite of the Sierra Nevada batholith, and exposures of a 220 million year old caldera complex in the vicinity of Tioga Pass. The route reveals outstanding examples of the erosional results of the Ice Age glaciers that scoured the range in Pleistocene time. Today’s trip is an great classroom for learning about one of the world’s spectacular mountain ranges! The field trip traverses Highway 120 through Tuolumne Meadows to the crest of the Sierra Nevada at Tioga Pass. A geologic map of the region covered by the trip is shown at the end of this page. The classification of plutonic (intrusive) igneous rocks is provided in Appendix 2.

This guide is designed for teachers and students who might not have an extensive background in geology, and I have tried to minimize the use of technical terminology in this guide. Terms in boldface are defined in the glossary at the end of this section. Please do not hesitate to ask questions as we travel.

0.4 53.6

Boundary of Yosemite National Park

0.7 54.3

Brief Stop: Yosemite National Park Entrance Station, elevation 4,872 feet (1,485 meters). Restrooms and park information available at parking lot on right.

4.2 58.5

Parking area for Merced Grove of Sequoia trees (see discussion of Sequoia trees at mile 0.5, the Tuolumne Grove parking lot). The small grove can be reached via a 1.5 mile long trail.

3.1 61.6

Turnoff to Crane Flat fire lookout. Make a sharp left turn onto road to lookout (if the access gate is closed, we will continue up the highway to Marker T4 at mile 4.8 on the next segment of the trip).

1.4 63.0

Stop #1: Crane Flat Fire Lookout (6,644 feet; 2,025 meters).

Park in the small lot near summit. Follow a short trail to the observation tower for an extensive view of the Yosemite region. Although the lookout is no longer staffed, the observation deck is accessible. The facilities nearby are used for emergency rescue training.

The rocks exposed along the trail are quartz-mica schist of probable early Paleozoic age. The unit is tentatively correlated to the Shoo Fly Complex, one of the earliest terranes of the Sierra Nevada metamorphic belt (Huber, Bateman, and Wahrhaftig, 1989).

The Crane Flat fire lookout lies on the divide between the Merced River drainage to the south and the Tuolumne River drainage to the north. The deep canyons are hidden from view, but a striking panorama of high peaks can be seen from the lookout, including the Clark Range to the south and east, and the high country around Hetch Hetchy Valley to the north. On clear days, the Great Valley and Coast Ranges can be seen to the west. The ridge tops have a decided western tilt that is clearly visible from the summit area.

Summary of Sierra Nevada Geology:

The Sierra Nevada is one of the world’s great mountain ranges. Some 400 miles long and 60 miles wide (600 by 100 kilometers), it is the largest single mountain range in the lower 48 states, and the highest, reaching an elevation of 14, 491 feet (4,417 meters) at Mt. Whitney on the eastern boundary of Sequoia National Park. The range takes the form of a huge tilted block into which deep canyons have been carved by water and ice (see Drive-by question #3 at mile 42.1). On the east, the steep slopes of the range end along a series of active fault zones. The more gentle western slopes disappear beneath the Tertiary and Quaternary sediments of the Great Valley (Fig. 2), except in the region to the south between the Kings and Tule Rivers.

Figure 2: The Sierra Nevada as a tilted fault block (from Matthes, 1930)

The range is composed mostly of plutonic rocks of the Sierra Nevada batholith, although about of the quarter of the exposed bedrock is pre-batholithic metamorphic rock. Many slopes, especially north of the San Joaquin River, are covered by Tertiary volcanic flows and lahar deposits.

The geological history of the Sierra Nevada can be summarized rather quickly, but the details of the story have been controversial and unsettled for more than a century. The oldest rocks, ranging in age from latest Proterozoic to Triassic-Jurassic in age, were intruded by a series of granitic plutons in Jurassic and Cretaceous time. The ancestral mountain range formed by this tectonic activity eroded to a relatively low range of hills in early Cenozoic time. In the last part of the Cenozoic era, uplift was renewed and west-flowing rivers eroded deep canyons. In the last two million years, glaciers repeatedly scoured the high country, forming the familiar alpine topography seen today. Post-glacial changes include mass wasting, and the arrival of humans and their logging, mining and tourism activity.

The pre-batholithic metamorphic rocks of the Sierra Nevada consist of a series of terranes. These fault-bound blocks of crust were docked to the North American continent as the floor of the Pacific Ocean was swept into a subduction zone over a period of some 200 million years in late Paleozoic and Mesozoic time. Our trip begins with the marbles of the Calaveras Complex, and culminates in exposures of Triassic metamorphic rocks at the crest of the range at Tioga Pass.

The Sierra Nevada batholith is also related to subduction along the west coast of North America, being part of a series of batholiths extending from British Columbia to Mexico. The magma is generated as the sinking slab of ocean crust heats up, releasing superheated water into the overlying continental crust, which leads to melting and formation of the plutons. The intrusions range in age from latest Triassic to late Cretaceous, but the greatest number were intruded between about 114 to 85 Ma (Bateman, 1992).

A long tectonic quiescence allowed the Ancestral Sierra Nevada to be deeply eroded, exposing the batholithic rocks, and apparently decreasing the overall relief of the range. By Eocene time, the range was a series of low hills, with a number of sizable rivers flowing across the crest from sources at least as far away as Nevada. Sometime between 10 and 3 Ma, the Sierra block began to rise (Wakabayashi and Stock, 2003). Uplift of the range may have been related to a mantle delamination event, in which the dense mountain root of the Sierra Nevada broke away and sank into the mantle (Ducea and Saleeby, 1998). Freed of this mass of rock, the overlying continental crust buoyed upwards.

By the time the worldwide glacial ice ages began, around 2 Ma ago, the mountains were high enough in places to support a thick ice cap and extensive valley glaciers. Although evidence is difficult to come by, numerous individual glacial stages have taken place (Burke, 2003).

Glaciation had a profound effect on the appearance of the high country of the Sierra Nevada. Formerly muted hillsides were sculpted into today’s characteristic alpine topography with rugged horns, arêtes and cirques. Deep river-cut canyons were widened and straightened by the flowing ice. Tremendous amounts of sediment filled the canyons below the glaciers and spread out into the Great Valley.

Return to Junction, and turn left onto Tioga Pass Road toward Crane Flat.

1.4 64.4

Junction with Tioga Pass Road. Turn left.

Tuolumne Meadows and reset odometer to 0.0.

0.0 0.0

Crane Flats area, elevation 6,192 ft (1,905 meters). The meadows and flats are underlain by a small outlier of the granodiorite of Arch Rock surrounded on three sides by early Paleozoic metasedimentary rocks, primarily quartz-mica schist (Huber, Bateman, and Wahrhaftig, 1989). Contrary to prevailing perception, not all Sierran meadows were glacial lakes that slowly filled with sediments. Several large meadows occur in non-glaciated terrains. Their origin is related to changes in climate and groundwater levels, since high levels suffocate tree roots. Crane Flats includes a meadow of this type.

0.5 0.5

Parking area for Tuolumne Grove of Sequoia Trees. Although we will not have time to stop at the grove on this trip, it is a potentially valuable stop for students, and a discussion of the sequoia trees is therefore included here (adapted from Hayes, 2005):

The study of the Giant Sequoia (Sequoiadendron giganteum) of the Sierra Nevada offers some data concerning landscape evolution and climate change in the region over the last few million years. The trees occur in isolated groves along the western slope of the range, in a belt extending from the Kern River drainage north to Calaveras Big Trees State Park (one very small grove of six mature trees also is found in Plumas County). The greatest concentration of trees is found in Sequoia and Kings Canyon National Parks, and in Giant Sequoia National Monument. Three groves are found in Yosemite National Park: the Mariposa Grove at the south end of the park, and the Tuolumne and Merced Groves in the western central part of the park. They are related to the Coast Redwood (Sequoia sempervirens) and the Dawn Redwood (Metasequoia glyptostroboides). Their limited distribution of the present day contrasts strongly with a fossil record that shows they were once widely dispersed across North America, Europe and Asia (Engbeck, 1988).

The trees can approach 300 feet (90 meters) in height, and with their wide girth are the largest living organisms on the planet (the related Coast Redwoods are taller, but thinner). They are practically impervious to attack by pests and fire, and may live for thousands of years. On the other hand, the species requires rather specific climate and soil conditions for germination and reproductive success. Fire suppression in the groves over the last hundred years has largely prevented successful germination, but the use of prescribed burning is resulting in a rejuvenation of some of the groves.

The trees seem to favor warm summers and mild winters, conditions that are mostly found between about 4,000 and 6,000 feet (1,200 to 1,800 meters) in the Sierra Nevada. During the ice ages, the trees may have may have expanded their range down the western slope of the Sierra, but in the drier, warmer conditions of the early Holocene, the trees declined, nearly to the point of extinction (Stephenson, in Stock, 2003).

Discussions of the evolution of the Sierra landscape center on the isolated nature of the groves, high on divides between major river drainages. Were the trees once more widely dispersed, but separated by the erosion of the deep canyons, or did they migrate down individual canyon divides as the Sierra rose in the last few million years? The question remains open (Engbeck, 1988).

The Tuolumne Grove is located about a mile west of the parking lot, with access provided by a now blocked paved road that was once the main access road to the Big Oak Flat Park Entrance. The rocks exposed in the grove are quartz-mica schist similar to those seen at the Crane Flat lookout station (Stop #1).

3.3 3.8

Turnoff to Tamarack Flat Road. Until 1945, when it was blocked by landslides, this was the main access to Yosemite Valley from the north. The remaining three mile section of road provides access to a small campground along Tamarack Creek. Outcrops along this narrow winding road are primarily of El Capitan Granite, with an age of 102 Ma (Huber, Bateman, and Wahrhaftig, 1989).

1.0 4.8

Marker T4. Overlook into Tuolumne River drainage (possible substitute stop if Crane Flat Fire Lookout is not accessible). Outcrops in the area are of the tonalite of the Gateway dating to about 114 Ma (Huber, Bateman, and Wahrhaftig, 1989).

5.1 9.9

Drive-by question #4: Note the barren expanse of granitic rock on the left side of the road (Fig. 4). Why is this dome here? What shaped it? The slope along the road is composed of Sentinel Granodiorite, intruded about 93 Ma (Huber, Bateman, and Wahrhaftig, 1989). Domes in the Sierra Nevada form in one of two ways. They may be sculpted by ice (see stops 3 and 3A), or they may result from exfoliation. Granitic rocks form at great depth, and as erosion removes the overburden, pressure is released, causing the rocks to expand and fracture. The resulting joints often form parallel to the surface, which tends to remove corners and edges, and results in a dome-like shape (Fig. 3).



Figure 3: Exfoliation acts to remove corners and edges from homogeneous rock like granitic intrusives (from Huber, 1987)

Figure 4: Dome surface at mile 9.9 (photo by author)

2.5 12.4

8,000 foot elevation marker

1.4 13.8

Small Siesta Lake is visible through the trees on the right. Rocks on the slope above the lake are in Sentinel granodiorite (Huber, Bateman, and Wahrhaftig, 1989). Drive-by Question #5: Why is this lake here? This will be a stop later in day.

0.9 14.7

Turnoff to White Wolf, a small resort area north of the highway. In the next few miles, views emerge of the glaciated upper drainage of Yosemite Creek and the Mt. Hoffman area.

5.3 20.0

Bridge over Yosemite Creek

1.2 21.2

Exfoliating granitic rocks

4.1 25.3

Porcupine Flat. This area marks the western edge of the Tuolumne Intrusive Suite, which will be discussed at Stop #2 on Tioga Pass. The rocks present include a narrow exposure of the granodiorite of Kuna Crest, followed within the next mile by exposures of the Half Dome Granodiorite.

2.1 27.4

Turnoff to May Lake trailhead. A challenging three-mile hike allows the hiker to attain the summit of Mt. Hoffman (10,850 feet; 3,307 meters) for a sweeping view of much of the region encompassed by Yosemite National Park. Huber (1987) provides a detailed description of the features seen from the summit.

0.6 28.0

Roadcut reveals Tioga stage tills (Dutton and McHenry, 1989). The Tioga was the last of the major ice advances, between about 25-14 ka (see discussion at stop #4).

0.3 28.3

Turnout provides a striking view of Clouds Rest (9,926 feet; 3,025 meters) to the south

1.4 29.7

Olmsted Point. Drive-by Question #6: Note the large boulders scattered across the surfaces around the viewpoint (Fig. 5). How did they get here? This will be a stop later in the day. East and north from the parking lot, Tenaya Lake appears in the distance, with Mt. Conness as a backdrop. The rock here is the Half Dome Granodiorite.

Figure 5: Boulders at Olmsted Point (photo by author)

1.6 31.3

South end of Tenaya Lake. Drive-by Question #7: How did this lake form (Fig. 6)? A large tongue of ice emerged from the Tuolumne Meadows icecap (See Stop #3) and scoured the canyon of Tenaya Creek, forming Tenaya Lake. The glacier continued down the canyon to the base of Half Dome and into Yosemite Valley. The modern river drainage of the lake is much smaller, and so the creeks that enter and leave Tenaya Lake are inconsequential. In periods of severe drought, the lake may not drain at all, and evidence has been found that extreme droughts have affected this area around 1100 AD and 1350 AD (Stine, 2001). The evidence for drought takes the form of the stumps of trees rooted in the bottom of the lake at depths as great as 70 feet (21 meters). The maturity of the trees indicates that the droughts lasted for as long as a century.

Figure 6: Tenaya Lake and Mt. Conness from near Olmsted Point (photo by author)

6.3 37.8

Pothole Dome parking lot (possible stop later in the day). Panoramic views of the extensive Tuolumne Meadows and surrounding mountains dominate the next few miles of highway.

1.1 38.9

Tuolumne Meadows Visitor Center on right

1.0 39.9

Tuolumne Store and Gas Station

0.2 40.1

Bridge over the Tuolumne River

0.1 40.2

Lembert Dome parking area. Drive-by #8: How did Lembert Dome come to be shaped the way it is? This will be a stop later in day.

1.5 41.7

Outcrop of fresh glacial tills of Tioga glaciation.

0.1 41.8

9,000 foot elevation marker

4.9 46.7

Old avalanche scar. Note downed trees in meadow below the road.

0.5 47.2

Stop #2: Parking lot adjacent to Tioga Pass Entrance Station, elevation 9,945 feet (3,031 meters). Be aware of the high elevation, and that we have not had time to acclimatize ourselves, so take it easy while hiking or walking about. This will be our lunch stop and includes an opportunity to see a variety of geological features.

Tioga Pass is a col (Fig. 7), a u-shaped pass formed when a ridge of rock between two cirques (an arête) was removed as ice plucked and carried the rock away. Such passes are uncommon along the crest of the Sierra Nevada, and as a result were preferentially utilized as transportation corridors from the earliest times of human habitation. The small lakes scattered around the summit of the pass are kettle lakes, formed as chunks of stagnating ice melted, leaving depressions that later filled with groundwater.

A great variety of rocks can be found in the immediate vicinity of the pass (if you intend to collect any samples, be sure you are outside the park boundary). Immediately to the east of the pass are the metamorphic rocks that had their origin in a caldera eruption some 222 million years ago. The Tioga Pass caldera developed within a magmatic arc during the earliest evolution of a subduction zone on the west coast of North America in Triassic time (Lahren and Schweickert, 1999).

Figure 7: Looking southeast towards the Tioga Pass col from Mt. Gaylor ridge.

Mt. Dana (13,053 feet; 3,967 meters) is the high peak on the left (photo by author).

On the other side of the pass lies the eastern margin of the Tuolumne Intrusive Suite. It is one of the most-studied plutonic intrusions in the Sierra Nevada, and has provided a great deal of information about the mechanics and timing of pluton emplacement (Bateman and Chappell, 1979; Glazner et al., 2004). The following discussion from Huber (1987) provides an excellent introduction (see Figs. 8 and 9):

“…The Tuolumne Intrusive Suite, one of the best studied groups of granitic rocks in the Sierra, consists of four bodies of plutonic rock, sequentially emplaced and partly nested one within the other. The suite is well exposed in the area centered on Tuolumne Meadows, and the Tioga Road (Route 120) provides access to many conspicuous outcrops of the suite's components.

The oldest and darkest plutonic rock generally forms the margin of the suite, and the youngest rock is in its core. The rocks are, from oldest to youngest: the granodiorite of Kuna Crest (about 91 million years old), the Half Dome Granodiorite, the Cathedral Peak Granodiorite (about 86 million years old), and the Johnson Granite Porphyry. Field relations indicate that the Johnson Granite Porphyry is the youngest granitic rock in the park, although a radiometric age has not yet been determined. The granodiorite of Kuna Crest normally occupies the margin of the suite, but on much of the perimeter the Half Dome Granodiorite and the Cathedral Peak Granodiorite have broken through the granodiorite of Kuna Crest to form the marginal units.

The overall concentric zonation of rock bodies within the suite, as well as the overall chemical similarities among the rocks, suggests that these rocks originated from the same magma chamber. This inferred common parentage provides the rationale for grouping these rocks into an intrusive suite. The composition of the magma, however, changed over time: the older, hornblende- and biotite-rich rocks at the margins give way to quartz- and potassium feldspar-rich rocks toward the center. Hornblende and biotite crystallize at higher temperatures than quartz and feldspar, and so during cooling of a magma, these dark minerals generally crystallize earlier than the light-colored ones. This relation suggests that cooling of the magma started at the margins and progressed inward over time. North of the Tioga Pass Entrance Station, the trail to Gaylor Lakes crosses over the granodiorite of Kuna Crest, the oldest and darkest rock in the Tuolumne Intrusive Suite. This trail weaves back and forth near the contact between the granodiorite and the metamorphic rocks that it intruded. The granodiorite also contains many disc-shaped inclusions that are oriented parallel to its contact with the older metamorphic rocks. These inclusions were probably stretched and oriented by movement within the magma during intrusion and cooling.

The Half Dome Granodiorite, the next youngest rock in the suite, is in contact with the granodiorite of Kuna Crest to the west along the ridge crossed by the Gaylor Lakes Trail. The best exposures of the Half Dome, however, are surrounding the turnout at Olmsted Point west of Tenaya Lake. Fresh, clean outcrops of the rock abound at and across from the turnout. Half Dome Granodiorite makes up much of the southwestern part of the Tuolumne Intrusive Suite and in several areas is the marginal rock.

Heading east toward Tuolumne Meadows, the Tioga Road crosses the contact between the Half Dome Granodiorite and the Cathedral Peak Granodiorite just east of Tenaya Lake. The contact is obscure, however, because here the Half Dome contains nearly as many potassium feldspar phenocrysts as does the younger Cathedral Peak. Pothole and Lembert Domes, both marginal to the meadows, are composed entirely of Cathedral Peak Granodiorite. The rock of these domes clearly displays potassium feldspar phenocrysts, commonly as much as 2 to 3 in. long. These impressive crystals stand out against a medium-grained background. The Cathedral Peak Granodiorite forms the largest pluton of the Tuolumne Intrusive Suite, extending long distances to the north and south of Tuolumne Meadows.

The youngest, smallest, and most central rock body of the suite is composed of the Johnson Granite Porphyry. In a porphyry, the conspicuous phenocrysts are set in a finer grained matrix than in such porphyritic rocks as the Cathedral Peak Granodiorite, and so individual mineral grains in the matrix are difficult to identify without a microscope. Low outcrops of the porphyry can be seen in Tuolumne Meadows along the Tuolumne River, across from the store, and east of Soda Springs on the north side of the river. The rock is very light colored, with only a few scattered potassium feldspar phenocrysts within a fine-grained matrix. Dikes of Johnson Granite Porphyry intrude Cathedral Peak Granodiorite, and the porphyry itself is cut by light, fine-grained aplite dikes.

The fine-grained matrix of a porphyry requires that partially crystallized magma be quenched or cooled relatively quickly. Such conditions would result from a sudden release of pressure, as would occur if some of the magma were erupted at the Earth's surface. Thus, volcanic eruptions probably accompanied final emplacement of the Tuolumne Intrusive Suite--a volcanic caldera may once have existed far above what is

now Johnson Peak.”

We have the opportunity to view all four phases of the intrusion at this stop and at stops 3 and 4.


Turnaround Point: Reset odometer to 0.0

Figure 8: Stages in the formation of the Tuolumne Intrusive Suite (from Huber, 1987)

Figure 9: Summary of geochronologic data for the Tuolumne Intrusive Suite, modified from Coleman et al. (2004) . Ages are from concordant U-Pb zircon data. Ages for units are arranged in sequence from outermost to innermost (Kse— Sentinel Granodiorite; Kga-Kkc—tonalite of Glen Aulin–Kuna Crest Granodiorite; Khd—Half Dome Granodiorite; Kcp—Cathedral Peak Granodiorite; Kjg—Johnson Granite porphyry).

7.0 7.0

Stop #3: Lembert Dome

Park in lot on right side of highway. The Tuolumne Meadows region was the source area for the Sierra Nevada’s largest single ice field during the Pleistocene glaciations. Ice accumulated here to depths as great as 2,000 feet (610 meters), and spilled over into several drainages, including Lee Vining Creek, Tenaya Creek canyon, and the Grand Canyon of the Tuolumne River. The ice stream in the Tuolumne Canyon was the longest glacier in the Sierra, with a length of 60 miles (96 km). Only a few of the highest peaks in the area (including Cathedral, Unicorn and Johnson Peaks, visible from the base of Lembert Dome) stood out above the ice. Such peaks are called nunateks.

Lembert Dome is a classic roche moutonnée (Figs. 10 and 11) a glacially sculpted asymmetrical “dome” with a gentle eastern (“stoss”) slope where ice abraded, and a steep western (“lee”) slope where ice plucked and removed boulders (Drive-by question #8). The dome is composed of Cathedral Peak Granodiorite of the Tuolumne Intrusive Suite, and glacially polished surfaces reveal a variety of plutonic textures including large potassium feldspar phenocrysts, schlieren, silica-rich dikes and veins. A short stroll west on the dirt road towards the Glen Aulin trailhead or across the bridge towards the store reveals exposures of the Johnson Granite Porphyry, the final intrusion of the Tuolumne Intrusive Suite. It has a fine-grained equigranular texture punctuated by sparse phenocrysts of potassium feldspar (Huber, Bateman and Wahrhaftig, 1989).

Figure 10: Formation of a roche moutonnée (from Huber, 1987)

Figure 11: Lembert Dome from the south (photo by author)

2.4 9.4

Possible Stop #3A: Pothole Dome

Like Lembert Dome, Pothole Dome is a roche moutonnée composed of Cathedral Peak Granodiorite, albeit much smaller (Fig. 12). It displays the same glacial features, with the addition of evidence for subglacial water erosion in the form of potholes and flutes. The fluted surfaces are polished, but in this case, the polishing agent was water, not ice (Huber, 1987; Konigsmark, 2002).

Figure 12: Pothole Dome from Tioga Pass Road (photo by author)

7.7 17.1

Stop #4: Olmsted Point

From this famous vantage point, Half Dome and Clouds Rest are visible to the south, and Tenaya Lake and the high country of the Sierra Crest near Mt. Conness can be seen to the north. Outcrops of Half Dome Granodiorite display jointing and exfoliation, and offer the best opportunity to observe this particular phase of the Tuolumne Intrusive Suite (Huber, 1987).

This stop is one of the better places in the Yosemite region to see the results of the Pleistocene glaciations which had so much influence in the shape and appearance of the canyon. Numerous examples of glacial activity can be seen from the viewpoint: polish, grooves, glacial erratics (note Drive-by question #6), roche moutonnées, u-shaped valleys, glacial tarns (Tenaya Lake), and in the distance, horns, arêtes and cirques (Fig. 13). More than a century of study has thrown much light on the intensity and pattern of glaciations, with new studies highlighting the complexity of the Pleistocene and Holocene climate changes (Clark et al., 2003).

Figure 13: Features produced by glacial erosion (from Huber, 1987)

At best, the western slope of the Sierra Nevada has been an ambiguous source of data on Pleistocene glaciations. The glaciers ended within deep canyons instead of spilling out into adjacent flat valley floors, as did many on the steep eastern slope of the Sierra. Lateral and terminal moraines, which could reveal a great deal of information about the sequence and timing of the glacial advances, have been eroded away by vigorous river erosion, or covered by extensive mass wasting deposits. The few deposits that remain are often covered by thick forests, and weathering of tills in the moist climate is more intense, sometimes obscuring important relationships (an important exception will be seen at Stop #5).

The Tuolumne River is a prime example of these west-draining canyons. The Grand Canyon of the Tuolumne is some 5,000 feet (1,525 meters) deep, and, as mentioned earlier, once contained the longest glacier of the Sierra Nevada, at some 60 miles (96 km). The passage of the ice is recorded in many places by straight U-shaped valleys, hanging valleys, recessional moraines, erratic boulders, and widely scattered till bodies and outwash deposits. Specific data on the sequence and timing of the glacial advances is ambiguous or absent. Attempts to relate glacial stages between the east and west slopes have met with limited success (Birman, 1964; Burke, 2003).

Studies of ocean floor sediments and arctic ice cores have suggested that numerous “ice ages” have occurred during the last two million years, the culmination of a global cooling event in the late Cenozoic era. Though the issue is far from settled, many researchers agree that at least six glacial episodes are represented by tills and other evidence in the eastern Sierra Nevada. They include the Tioga (~ 25-14 ka), Tenaya (~30 ka), Tahoe (~140-50 ka), Mono Basin, Sherwin (~820 ka), and pre-Sherwin (Burke, 2003; Clark et al, 2003). The picture is less clear in the western Sierra Nevada: the Tioga, Tahoe, and pre-Tahoe are the least controversial divisions (the preceding discussion is adapted from Hayes, 2005).

16.3 33.4

Stop #5: Siesta Lake

Siesta Lake is a small glacial lake dammed by moraines (Drive-by question #5). The glacier that formed the lake was limited in size, covering an area of only about 1 square mile, and did not form a prominent cirque, although features of glacial erosion can be seen on the slopes south of the lake. Glacial tills have been abundant along the trip route between here and Tioga pass, but well developed moraines have not. The terminal moraines (figure 14) at Siesta Lake provide one of the best places on this trip to compare and contrast tills from the younger Tioga (Fig. 15) and older Tahoe glaciations (Fig. 16). Pre-Tahoe tills are also accessible several hundred yards to the north.

Figure 14: Features produced by glacial deposition (from Huber, 1987)

Figure 15: Tioga stage moraine at Siesta Lake (photo by author)

Figure 16: Tahoe stage moraine at Siesta Lake (photo by author)

Geological Map of Yosemite National Park, from Huber (1989), with stops for this trip shown.



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