THE INTERNAL STRUCTURE OF ROCK GLACIERS AND GEOMORPHOLOGIC INTERPRETATIONS: YANKEE BOY BASIN, CO, USA AND HIORTHFJELLET AND PRINS KARLS FORLAND, SVALBARD

For years geomorphologists have had the luxury of indulging in idle speculation regarding the internal structure of rock glaciers because of the difficulty in obtaining detailed physical data. That time has come to an end. We recently "viewed" the internal structure of rock glaciers located in the San Juan Mountains of southwest Colorado and Hiortfjellet and Prins Karls Forland, western Svalbard using ground penetrating radar (GPR). Longitudinal profiles (f = 25 MHz and f = 50 MHz) oriented along the central axis of the rock glaciers show moderate to strong coherent reflection horizons or layers that can be recognized clearly to depths ranging from 15-35 m. We interpret the layers to represent ice-supersaturated sediments and coarse blocky rockslide debris that are the result of flow, perhaps generated by seasonal snow pack and snowdrifts covered by episodic mass movement events or high-magnitude discharges of talus. Profiles collected at f = 50 MHz indicate that in the upper 20 m thickness of the rock glaciers, many of these layers are laterally continuous. The bedrock interface was visible on several of the rock glaciers. At Yankee Boy Basin, the total depth of penetration (~40 m at f = 25 MHz) was sufficient to detect the cirque floor, which is composed of underlying moraine. Several prominent reflection events that subdivide the profile into 10-15 m thick layers represent contacts between major depositional units. These units are believed to be individual flow lobes that were initiated at various cirque headwall locations. We interpret these rock glaciers to be composite features that formed by a process involving the development and subsequent overlap of discrete flow lobes that have overridden older features and protalus rampart materials. The latter materials have been incorporated into the present flow structure of these rock glaciers.