Mokelumne Wilderness Geology

The volcanic rock covered in snow.

The geologic history of the area now known as the Mokelumne Wilderness has been long and complex, and what is visible today reveals only a fraction of this. The area is a prime example of crest zone geology in the Sierra Nevada with its prominent volcanic peaks and ridges overlaying massive intrusive granite features. The granitic valleys and highlands of the Mokelumne are part of the massive Sierra batholith, the combined masses of granitic rock that have been uplifted by tectonic action to form the 400 mile long Sierra Nevada range. The bedrock geology of the area consists of metamorphosed sedimentary and volcanic rocks, intruded by the granites, or granodiorites, of the Sierra batholith, all of which are capped by the remnants of volcanic flows.

The Bedrock - Originally sand, mud, and volcanic rock were deposited on the ocean floor between 230 and 100 million years ago. Heat and pressure from deep burial metamorphosed them. Today these metamorphic rocks are seen covering only small areas such as the prominent Mokelumne Peak.

Intrusions and Gold - Between 150 and 80 million years ago magma intruded up beneath the metamorphics and cooled in several pulses. This phenomenon, related to the movement of continental plates, formed much of the granite or, more correctly, granodiorite seen today in the wilderness area. Heat associated with this intrusion of magma further metamorphosed the original ocean deposits, and deposits of minerals like gold and copper formed as a by-product.

A mountain range is born - About 70 million years ago the area began to be uplifted, although the most dramatic uplifting occurred in the last ten million years. As a result of uplifting, erosion has stripped away much of the original sequence of metamorphic rocks and has exposed the granitic rocks. This process created the Sierra Nevada Mountains; the range, over this vast time frame, has been lifted over eight vertical miles.

Volcanoes - Volcanic activity between 20 and 4 million years ago buried the area with lava flows, mudflows, and ash deposits. The vents from which these materials erupted now form some of the higher peaks, such as Elephant Back, Round Top, and Thimble Peak. Later erosion has since removed much of the volcanic rock, leaving the volcanics along the peaks and ridges.

Ice Carving - In the Pleistocene epoch (the last million years) glacial action and erosion have shaped the Mokelumne Wilderness. Ice sheets over 1,000 feet deep covered the western slopes and all but the highest peaks of the Sierra Crest during the last ice age, one of four ice ages that have affected the region in the period. The sheer mass and grinding action of the ice packs scoured bowl shaped cirques and U - shaped valleys in much of the Mokelumne. Tributary forks like Underwood Valley, Lake Valley and Upper Summit City Canyon were left as hanging valleys. Glacially polished rock surfaces, moraines, and erratic boulders left behind when the glaciers receded are further evidence of the ice. Glaciers as recently as 10,000 years ago flowed down Summit City Creek, Pleasant Valley, Thornburg Canyon, and other Mokelumne River tributaries.

Glacial activity has had a profound affect on the soils currently found in the area. Glaciers have stripped much of the area to bare rock or shallow coarse soils associated with rock outcrops. Deeper, coarse textured soils are found on moraines and pockets of glacial outwash. Residual coarse textured soils are found on volcanic ridges. Plant and animal life in the area are based on a complex mosaic pattern originally defined by the soil, the geology and climate.