IS TILL THE ONLY EVIDENCE OF ICE ADVANCE? WHAT 15 YEARS OF POST-SURGE RETREAT HAVE REVEALED BENEATH BERING GLACIER, ALASKA

Bering Glacier surged in 1993-95. Subsequently, the area below the surge margin has been revealed following ice retreat. In one location both basal and ablation till can be distinguished, but commonly only a thin diamict represents the multiple advances and surges during and following the Little Ice Age and throughout the 20^th century. In all cases the till occupies only the uppermost stratigraphic unit.

In many cases, no till can be found beneath areas known to be covered by ice in 1993-95. The sediment package reveals other evidence of ice advance including faulted sedimentary beds, clastic dikes, and transported subglacial organics.

Faulted lacustrine and sandy outwash deposits appear to represent drag or shear beneath the moving ice. Clastic dikes propagate from surface till through sandy outwash to Tertiary sandstone bedrock. Peat bogs are ripped up, deformed, and transported. The Ancient Forest was exposed prior to the surge event, then over run by ice which sheared tree trunks and transported wood fragments. Imbricated peat blocks derived from the forest floor were transported during a subglacial flood event. Thrust blocks of sand and mud are found on peninsulas where ice rode up from the deep lake basin onto land. Blocks contain deformed sediments and shells of marine invertebrates dating 5-13,000 BP. Tertiary bedrock containing in situ boring clams were dropped as striated erratics across the deglaciated surfaces.