INFLUENCE OF ROCK HARDNESS VARIABILITY ON CYCLOPEAN STEPS: NUMERICAL MODELING AND FIELD OBSERVATIONS FROM THE UINTA MOUNTAINS, UT

Some glaciated valleys exhibit cyclopean steps. Cyclopean steps consist of flat areas, often inhabited by an isolated lake, separated by short, steep slopes. There are several mechanism by which cyclopean steps can form, such as increases in ice discharge at tributary junctions and constrictions in the flow field, both causing increased sliding speed and erosion. We hypothesize that cyclopean steps can also reflect spatial variability in rock hardness due to lithology or structure. Areas of weaker rock type are proposed to be more susceptible to glacial erosion, resulting in overdeepenings.

We explore the influence of lithology and structure on the formation of these steps with a combination of numerical modeling and field mapping. The model is a flowline simulation of glacial mass balance, flow, and erosion. We model erosion by quarrying and abrasion under a stable, idealized climate with diurnal and seasonal temperature variations, as well as altitude dependent temperature and precipitation rates. We impose a layered bedrock pattern of variable hardness at different angles to the bed surface. The model predicts that distinct overdeepenings form when a weaker layer intersects the bed.

Remote sensing of glacial valleys in the western Uinta Mountains of Utah suggest preferential formation of cyclopean steps where bedrock layers intersect valley floors at high angles. This observation supports our hypothesis that variable hardness may be a formation mechanism for these features. We present field observations of bedrock type, bedding, and hardness as measured by Schmidt hammer which further test this hypothesis.