IS THERE A PALEOCHANNEL UNDER AGNEW PASS? EASTERN SIERRA NEVADA, MONO COUNTY, CALIFORNIA

Slopes above the south side of Gem Lake Reservoir (Rush Creek) appear to show an andesite filled paleochannel x-section. The western edge of this apparent channel can be traced southward across the horizontal erosional surface exposed around Clark Lakes, up to the edge of a talus and glacial debris covered south facing slope. Continuous exposures of mafic lavas to the southeast could allow for a buried paleochannel running east of and adjacent to Agnew Pass to join the course of the present day Middle Fork of the San Joaquin River, upstream from the Shadow Lake Trail river crossing. Here are andesite cliffs which appear to show an angled cut of the downstream paleochannel x-section. The right hand bank of the proposed paleochannel above its juncture with the Middle Fork is composed of "Rocks similar to the Cathedral Peak Granite (Kcp)", inked in pink on Huber and Rinehart's 1965 map.

The andesite fill of both upstream and downstream channel x-sections show columnar jointing, with talus blocks covering the bottom portion of each channel x-section. The downstream exposure shows entablature jointing along the base of the cliffs, which could indicate the interaction of water with the hot andesite within the channel. The base of the channel at Gem Lake appears to be around 9440 feet elevation (2877 meters), with the downstream Middle Fork junction paleochannel bottom at 8880 feet elevation (2706 meters), though both could be lower and hidden under talus. This proposed paleochannel extends 2.5 miles (4 kilometers), and for much of its course lies under the Pacific Crest "High" Trail. While the lower reaches of the tentative paleochannel are bounded on the right by bedrock, the mafic lava fill of the upper section below Agnew Pass lacks this western lateral bedrock confinement, possibly the result of a former juncture with the Pliocene Thousand Island Creek.

This proposed Agnew Pass paleochannel would extend the headwaters of the paleo NW branch of the Middle Fork of the San Joaquin River to encompass the present headwaters of the Rush Creek drainage, before volcanic and tectonic activity reshaped the landscape.