COMPARISON OF ESKER MORPHOLOGY AND SEDIMENTOLOGY WITH FORMER ICE-SURFACE TOPOGRAPHY, BURROUGHS GLACIER, ALASKA

Topographic maps of the Burroughs Glacier ice surface and the surround ing land surface from 1948 to 1990 were used to generate subglacial hy draulic head maps and compare esker paths to hydraulic gradients. Hydr aulic gradient at the glacier bed is controlled predominantly by ice-s urface slope, but it is also influenced by the slope of the glacier be d. Eskers at Burroughs Glacier have been observed forming in subglacia l and englacial tunnels. Most of the eskers formed in subglacial strea m tunnels oriented parallel to the calculated hydraulic gradient. Wher e the former ice-surface slope and slope of the bed do not coincide, t he relative influence of these factors on esker paths is analyzed. One esker is oriented parallel to the land-surface contours and perpendic ular to hydraulic head contours, implying an esker path controlled by the ice-surface slope. In another area, a set of subglacially engorged eskers trends perpendicular to the land contours (directly down the s lope of the former glacier bed) and parallels the calculated hydraulic head contours. These eskers formed beneath thin ice in tunnels that w ere air-filled much of the time, and thus the slope of the glacier bed controlled the esker paths. In a third area where the hydraulic gradi ent and glacier bed slope in the same direction, a subglacially engorg ed esker trends down the land slope and the calculated hydraulic gradi ent.Most of the eskers at Burroughs Glacier are <6 m high, are sharp c rested, commonly cross small hills, and contain poorly sorted, poorly stratified gravel and sand. The lack of sedimentary structures implies high sediment influx rates to the ice tunnel during formation and/or rapid deposition late during esker formation. ''Anticlinal'' bedding i s not common. An esker observed melting out of the ice was initially c apped by flat fluvial terraces exhibiting a braided channel pattern. A sharp crest developed as the sediment slumped to either side of the r idge.