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 glacier s in the San Juan Mountains, Colorado. I use topographic data from man y of these valleys to test two conceptual models that predict the most likely position for a stagnant ice margin to develop during valley gl acier retreat. In the first model, valley hypsometry controls the rate of accumulation area loss for a rise in equilibrium line altitude (EL A). The faster accumulation area is lost, the faster the glacier termi nus 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 deforma tion and pinching off a section of un-nourished ice. Comparison of mod eling 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 pa leo-ELA, appears to be the primary control of ice stagnation. For a co nstant ELA rise rate, gentle valley slopes force ice stagnation (by in creasing retreat rate) whereas steep slopes encourage active retreat. Ice stagnation is prevalent in the San Juan Volcanic Field because inc ompetent volcanic rocks are easily eroded to produce low valley gradie nts. 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.