Hypsometric forcing of stagnant ice margins: Pleistocene valley glaciers, San Juan Mountains, Colorado

Topographic and sedimentological evidence indicates that stagnant ice conditions prevailed during retreat of many Pleistocene valley glaciers in the San Juan Mountains, Colorado. I use topographic data from many of these valleys to test two conceptual models that predict the most likely position for a stagnant ice margin to develop during valley glacier retreat. In the first model, valley hypsometry controls the rate of accumulation area loss for a rise in equilibrium line altitude (ELA). The faster accumulation area is lost, the faster the glacier terminus must retreat, increasing the likelihood of ice stagnation. In the second model, a stagnant margin will develop if a topographic obstacle thins the glacier to a critical thickness, retarding internal deformation and pinching off a section of un-nourished ice. Comparison of modeling results with field evidence indicates that topographic obstacles do not force the development of stagnant ice margins in the San Juans. Instead, valley hypsometry, in particular the valley slope at the paleo-ELA, appears to be the primary control of ice stagnation. For a constant ELA rise rate, gentle valley slopes force ice stagnation (by increasing retreat rate) whereas steep slopes encourage active retreat. Ice stagnation is prevalent in the San Juan Volcanic Field because incompetent volcanic rocks are easily eroded to produce low valley gradients. This finding that the slope at the ELA is an important control on the development of a stagnant margin is supported by the abundance of stagnant ice deposits in continental settings where the slope at the ELA is very low.