THE DISTRIBUTION, MORPHOLOGY, AND TEMPORAL SIGNATURE OF ROCK GLACIERS IN THE TOBACCO ROOT MOUNTAINS, MONTANA

A detailed analysis of nearly 250 periglacial deposits (mostly rock glaciers) in the Tobacco Root Mountains of southwestern Montana was performed using geospatial analysis techniques on high-resolution digital orthophotographs to better understand controls on their distribution and morphology. The geographic information system terrain analysis included the determination of morphometric attributes for the deposits (bounding geometry, thickness, surface area), and the extraction of spatial and topographic parameters such as slope, aspect, elevation, and incident clear-sky radiation. A field campaign was also undertaken, aimed at determining the relative ages of periglacial deposits and the nature of their contributing headwalls. A multivariate analysis of relative age data from 21 deposits revealed groupings that are consistent with at least four periods of periglacial activity. Of these, the Early Neoglacial was by far the most common. A morphometric analysis indicated that tongue-shaped rock glaciers have statistically lower surface slopes and larger maximum elevations, areas, thicknesses, and lengths than either lobate rock glaciers or protalus ramparts. Lobate rock glaciers also have statistically lower surface slopes and larger areas, thicknesses, and lengths than protalus ramparts. Most rock glaciers and protalus ramparts in the Tobacco Root Mountains emanate from topoclimatic niches that minimize incident solar radiation. The greatest difference in radiation reduction between morphologies occurs on the winter solstice, with tongue­­-shaped deposits receiving the least radiation and protalus ramparts receiving the most. Fracture orientation in the contributing headwall appears to influence the size of a given deposit, with the largest deposits having contributing headwalls with fracture planes parallel to compositional banding and dip-slope. Headwall retreat associated with deposit development ranges from 1-35m. Based on the inferred ages of deposits, headwall retreat rates in the Tobacco Root Mountains were calculated to range between 0.5 and 3.5 mm/yr during periods of active periglaciation.