THE ROLE OF LITHOLOGY IN GLACIAL VALLEY CROSS PROFILE FORMATION

It is commonly inferred that glaciers transform fluvial valleys from V to U-shaped. It has been suggested that under certain circumstances, glaciers may be unable to modify V-shaped valleys and that lithology may play an important role in a glacier’s ability to effectively erode bedrock. To test this hypothesis, glaciated valleys with both U and V-shaped cross-sections were chosen in the Sierra Nevada (CA) to examine how lithological properties might affect patterns of glacial erosion. Six valleys were chosen as sites which represented these conditions: Tenaya Canyon, Tuolomne River, Little Yosemite Valley, the San Joaquin River, the north fork of the King River and Deadman Canyon. Valley profiles were fitted with the power law, y = ax^b, where the exponent b represents the shape of the valley cross-section. Rock mass strength (RMS) values for these sites were determined by field inspection. We found no clear relationship between RMS and valley shape. In addition, rose diagrams of measured joint orientations were plotted and compared to glacial flow direction for each valley. A weak association was exhibited between the two in Little Yosemite Valley, the north fork of the King River and the San Joaquin River valleys, but glacial flow direction appears independent of joint orientation in Tenaya Creek, Tuolomne River and Deadman Canyon. These results suggest that in the Sierra Nevada granitic batholith patterns of glacial erosion may not be dependent on rock mass strength (as typically measured) while joint orientation may play a minor role.