FIELD EVIDENCE INDICATES SUBSTANTIAL SUBGLACIAL WATER EROSION ON AN OUTWASH SUBSTRATE, BERING GLACIER, ALASKA

Outwash terrain overridden during the 1993-95 surge on the eastern ice front of the Bering piedmont lobe is now being uncovered by rapid retreat. Two recently exposed subglacial basins that did not exist prior to the surge, the largest being 0.75 km2, project a minimum of 15 m downward into the pre-surge landscape. They are located immediately upglacier from the 1995 surge-limit moraine and in the vicinity of outburst sites and extensive sandar on Weeping Peat Island. Topographic information gathered prior to the surge is compared with post-surge landforms, with specific focus on formative processes leading to an assessment of the relative significance of erosion by ice versus water. The dominant results of overriding ice include the formation of fluted drumlinoid hills, deposition of a new till, and construction of a semi-continuous push moraine. Substrate glacial erosion appears limited to the truncation of outwash bedding beneath streamlined slopes. Basins cut in outwash do not appear gouged by ice. Annual field observations indicate that ice marginal streams were not a factor in basin formation. Instead, the basins appear to have been scoured by pressurized, subglacial meltwater moving through conduits toward outburst sites. The erosional effects of this process working on an unconsolidated substrate appear to far exceed that of the overriding ice.