Glaciation of the Sierra Nevada

By late Tertiary time, the crest of the Sierra Nevada was high enough to support a winter mantle of snow. As worldwide temperatures began to fluctuate about 2 or 3 Ma, glaciers began to form in the high country.

The commonly used term "Ice Age" is somewhat of a misnomer, since the Quaternary Period has been characterized by instability and rapid changes from hot to very cold conditions. Numerous major cooling periods during that time have been documented from deep-sea core studies (Shackleton and Opdyke, 1973). These were probably accompanied by glacial advances on land. Unfortunately, the later larger glacial episodes erased the evidence of earlier, smaller glaciations. Consequently, the record of glaciations in the Yosemite Valley and Sierra Nevada is controversial for all but three of these presumed glacial stages. Nevertheless, glaciers had a profound effect on the high country of the Sierra Nevada and even in the Great Valley, though the glaciers themselves never reached that far.

In the high country, the glaciers quarried and plucked at the crest of the Sierra Nevada, forming numerous cirques, horns, aretes, and glacial tarns. Nearly all the lakes in the Sierra Nevada are glacial in origin, although a few, like Mirror Lake in Yosemite Valley, result from rock falls and mass wasting. As the trunk glaciers scoured deep U-shaped troughs, tributary glaciers could not keep pace and were left stranded as hanging valleys high about the main valley floor. Most of the spectacular waterfalls in the high country fall from the lips of these hanging valleys.

(Illustrations from Huber, 1987)

In the lower reaches of the Sierra Nevada, glaciers could no longer carve deep canyons, and instead dropped their loads of boulders and debris as glacial tills. Moraines formed around the margins of the ice. When not removed by subsequent erosion, the terminal, lateral and recessional moraines provide one of the best means for distinguishing between the different advances of the glaciers (Fig. 2). The evidence and criteria for distinguishing between glacial stages, especially on the temperate west slope of the Sierra, has been a source of controversy. Most researchers agree that at least three major advances, or stages, have taken place: the Tioga, the Tahoe, and a much older Pre-Tahoe (possibly the Sherwin). The timing of each stage is still a topic of spirited debate (Bursik and Gillespie, 1993).

Yosemite Valley Glaciers

Yosemite Valley is practically devoid of tills and moraines that would clearly reveal the correct chronology of all the glaciations that have effected the region. Evidence concerning the timing of the stages shown in Table 1 was most often found on the east side of the Sierra Nevada, where the climate is drier and weathering occurs at a slower rate. Canyons on the western slopes of the Sierra Nevada are longer and deeper than those found on the east, and in the steep terrain most of the evidence of glacial activity has been washed away.

Matthes (1930) described evidence for three glaciations, including a Wisconsin stage and two older glaciations, the El Portal and Glacier Point stages. The Wisconsin glaciation has since been divided into the Tioga and Tahoe stages, as noted above. The evidence for the Glacier Point stage is tenuous, but the El Portal is probably related to the well documented Sherwin glaciation of the eastern Sierra Nevada (Huber, 1987). The evidence for this older glaciation in the Yosemite Valley area is limited to scattered erratic boulders and formless bodies of glacial till on the highlands above Yosemite Valley. Because of the uncertainties of correlation, the older glaciations are referred to simply as "pre-Tahoe". One of these tills can be seen in a roadcut just east of Cascade Creek on Big Oak Flat Road where it leaves the valley (page 46, mile 24.6), and erratic boulders can be seen on Turtle Dome, just west of the valley. The pre-Tahoe glaciers filled Yosemite Valley to the rim, and played the biggest part in shaping the valley as we know it today.

The Tahoe and Tioga glaciers filled the valley in part, but only extended to the base of El Capitan and Bridalveil Falls. The Tioga glaciers left a series of prominent moraines that can be seen along the road near El Capitan and Bridalveil Meadows

In most valleys of the western Sierra Nevada, and on the east side as well, the Tahoe glaciers extended farther down their valleys than those of the more recent Tioga glaciation. Yosemite Valley is an exception. The lateral moraines of the older Tahoe Stage dive beneath the later Tioga moraines in the canyons above Yosemite Valley, and the only moraines visible on the valley floor are Tioga in age (Huber, 1987).

The floor of Yosemite Valley was scoured out by pre-Tahoe glaciations into a deep lake basin, which was filled by subsequent glacial episodes. The most recent glaciers lacked the thickness and erosive power to scour the basin, and simply slid over lake deposits and tills already present. A shallow lake remained in the wake of the Tioga glaciation, but it quickly filled with sediment carried by the Merced River and Tenaya Creek.

*Sierra glaciations recognized in Yosemite Valley are shown in boldface. Direct correlation to Sherwin has not yet been fully established, so older glaciations are generally described only as Pre-Tahoe.

Bibliography

Written by Garry Hayesİ