FORMATION OF LITTLE ROUND VALLEY, EASTERN SIERRA NEVADA, MONO COUNTY, CALIFORNIA

West of Tom’s Place on the slope above Crooked Creek (behind Forbes 40), Rinehart and Ross (1957) mapped a bedrock exposure of “diorite-gabbro”. Putnam (1960) mapped this location as a deposit of Sherwin till, but in the accompanying text said he was conflicted about its origin since the lithic contents were similar to nearby bedrock exposures of quartz diorite and hornblende gabbro. Bailey (1989) showed this “till” deposit on his map. He proposed that in the Bishop Ash eruption, ignimbrite banked up against and covered a tongue of the Rock Creek, Sherwin glacier. Volcanic tuff blanketing the ice was unwelded and easily eroded to form Little Round Valley.

A glacial origin is doubtful. The deposit lacks the mixture of rounded pebbles and cobbles expected in a glacial till. Boulders of similar composition cluster together as if spheroidally weathered in place from bedrock. Noticeably absent are boulders of Round Valley Peak granodiorite which predominates in Rock Creek Canyon drift.

At Rainbow Tarns Rd., the old Hwy 395 passes through a notch between the range front and a quartz monzonite inselberg. This notch can be explained as a windgap from a former course of Rock Creek. In the Bishop Ash eruption, deposits of ignimbrite banked against the foot of the range front. Afterwards, runoff from the impermeable granitic slopes eroded away the edge of the ashflow. Bishop Tuff blocked Rock Creek from its former northward course (Sharp 1968). A west flowing Rock Creek cut the “Crooked Creek Defile” through the Bishop Tuff to join the Owens River (Putnam 1960). Post-caldera, a west flowing Rock Creek would have followed a course at the base of the range front along the margin of the volcanic tuff. That course was through this Rainbow Tarns windgap and into what now is Little Round Valley. As Rock Creek eroded downward it encountered the granitic bedrock of the inselberg. Bishop Tuff east of the granitic spur was more easily eroded and eventually Rock Creek was defeated in its course and exploited a new route around to the east and north of the Rainbow Tarns inselberg (along the present highway 395), rejoining and widening its previous path to form the flat floored Little Round Valley at a base level determined by Long Valley Lake. Little Round Valley can be explained as the result of erosion by a west flowing Rock Creek, without the presence of a glacier.