OBSERVATIONS OF GLACIER COMPLEXES IN ROCKY MOUNTAIN NATIONAL PARK, COLORADO

Mappable perennial ice masses occur above treeline in cirques and hollows of most north- and east-facing drainages in the park. These exposures occur most commonly either as steep, concave ice masses against cirque walls and in couloirs or as convex longitudinal drifts. Layering and crevasse patterns provide evidence of active creep. Many drainages exhibit multiple ice exposures with intervening sections surfaced with rock debris. Debris rived from cirque walls above the ice exposures bypasses steep ice surfaces and accumulates on low angle ice surfaces or in contact with older debris. Debris accumulating within this zone eventually reaches sufficient thickness to insulate underlying ice. Crevasses, stream-cut channels and other exposures of the ice core beneath the debris cover show that these debris layers are typically 1-2 meters thick and are underlain by glacier ice that is continuous with the massive ice exposures. In many cases, the insulated surface rises abruptly down-valley from the accumulation zone, forming a spoon-shaped depression at the head of the rock glacier. In the past, these rises were commonly mapped as terminal moraines. Glacier monitoring work begun in the 1920s was later abandoned when field observations were inconsistent with the moraine interpretation and they were unable to determine the down-valley extent of ice beneath the debris. In this study, the topographic rise is hypothesized to be the glacial equivalent of a hydraulic jump with ice continuing beneath the debris cover. Up-valley of the depressions, the ice masses exhibit characteristics of cirque and drift glaciers while the down-valley continuations of these ice masses have the surface characteristics of rock glaciers. Continuous masses of glacier ice both with and without surface debris are referred to in this study as glacier complexes. Within each glacier complex, the area of debris-covered ice is commonly an order of magnitude greater than the area of exposed-ice portions, though the ratios vary widely. In this interpretation, the glacier toe sought by previous workers is the toe of the debris-covered section.