THE MCGEE TILL; GLACIATION AND PALEOLANDSCAPE IN THE EAST CENTRAL SIERRA NEVADA, MONO COUNTY, CALIFORNIA

A paleochannel cut in black slate and calcareous quartz sandstone lies under 100 m of glacial debris in the saddle between Mt Aggie and McGee Mt. Above the McGee Creek elbow of capture, the SE channel x-section (approx base elevation of 3095 m) is outlined by bedrock, and a visible contrast between granodiorite till and that containing slate derived colluvium. The NW downstream side is a tree filled hollow with horizontally truncated exposures of slate at 3052 m, 73m in from the paleochannel wall. A slate bench in the wall, a possible strath surface, is accordant with the top of a nearby tor (3070m) in an adjacent tributary canyon originating between Mt Aggie and Mt Morrison. Beheading the paleo McGee Creek left the overly broad lower portion of this Aggie-Morrison Cyn to mark the former NNW flowing course of McGee Creek.

Associated with range front rifting, a 2.7 Ma basaltic andesite flow (Huber 81) partially filled a central drainage on McGee Mt. Andesite blocks in the outlet at 2985m and andesite exposures extending up canyon to the same elevation along McGee Mt's west flank, indicate that lava filled the former McGee Creek channel to near the base of the buried paleochannel segment in the Aggie-McGee saddle. Early McGee Creek Canyon glacial advances would’ve transitioned to a piedmont form upon encountering this flat stretch of andesite. This explains the extensive glacial deposits on McGee Mt, and along with range front faulting accounts for reconfiguration of McGee Creek's course.

Quartz sandstone of the siliceous Lower Convict Lake formation of Rinehart and Ross (1964) underlies cirques surrounding Mt Aggie, which are closer to McGee Mt than granodiorite based up canyon cirques. The front of the earliest ice advance would be laden with the sandstone. This correlates with the quartz sandstone deposits resting upon volcanics of the McGee Mt north bench.

Rock Creek drainage shows a similar geomorphic pattern. “Tamarack Bench” below Wheeler Crest, and Little Lakes Valley were headwaters of a drainage system which joined Hilton Creek, flowing out between Red Mt and N Mt Morgan in a paleovalley bounded on its SW side by the Hilton Creek fault. These early Rock-Hilton Creek and McGee Creek systems show a NNW drainage orientation that suggests a correlation with the Deadman Pass paleochannel for the Middle Fork of the San Joaquin River (Huber 81).