Geological Tour of the Eastern Sierra
As you drive south along 395 get ready to experience spectacular mountains, valleys, lakes, streams, and volcanic mountain chains. The dramatic display of well preserved geologic features covers volcanic, glacial, erosive, and structural processes (i.e. faulting, uplifting, and folding).
After leaving Walker Pass you will enter Bridgeport Valley, then drive through another pass called Conway Summit. Descending Conway Summit there is a great view of Mono Lake - a salt water lake fed by streams. Just south of Mono Lake on the eastside of 395 is the Volcanic domes. From north to south these include Panum Crater, Mono Crater Chain, Inyo Crater.
Mammoth Mountain is part of the northern edge of the largest caldera in the united States - Long Valley Caldera. This caldera is approximately 20 miles east to west and 10 miles north to South. The boundary of the Long Valley Caldera is the Sierra Nevada range to the west, the Benton range to the east, and Crowley Lake makes up the southern boundary.
Descending Sherwin Summit is difficult with out using your brakes because you are now entering the deepest valley known as Owens Valley. This valley is a graben valley meaning that it has dropped a few feet to a tens of feet at a time as the Eastern Sierras and White-Inyo Mountains rose. As you drive down Sherwin grade look for grayish steep marks on the Sierra mountain sides. These grayish marks are called truncated spurs by the Geologists and they indicate fault movement. Furthermore, you may notice mound of rock and soil creating a huge berm around the mouths of Pine Creek, Convict Lake, and the canyon west of Crowley Lake. These are glacial moraines (approximately 710,000 years old), which are material deposited in front of the moving glacier as it proceeded out of the canyons. Glaciers have the ability to push vast amounts of material in front of them and when the glaciers melted these lateral moraines were left behind. Glaciers also have the ability to carve U-shaped valleys, cirque lakes, and sharp peaks called horns and aretes.