AN EPHEMERAL ICE STREAMING EVENT IN THE SOUTHERN CHAMPLAIN VALLEY

Ice streams are restricted areas of very fast flowing ice around the margins of ice sheets. In both the Greenland and Antarctic Ice Sheets ice streams are the principle avenues by which ice is channeled to the ice sheet margin. Numerous paleo-ice streams have been identified around the periphery of the Laurentide Ice Sheet and their distribution suggests that ice streaming was an on-going process during the retreat of the ice sheet. Several observations suggest that ice streaming also occurred in the southern Champlain Valley for a limited period of time during the retreat of the Laurentide Ice Sheet. Ice flow patterns around modern ice streams show ice flow in the surrounding ice sheet converging into these ice streams. Converging ice flow into the Champlain Valley is indicated by a set of NE-SW striations along a 65 km reach of the Green Mountain ridge line between Pico Peak and Appalachian Gap that distinctly cross-cut older striations. Most current ice streams are also located in topographic troughs. The Champlain and northern Hudson River valleys form a substantial north-south trough separating the Green and Taconic Mountains to the east from the Adirondack Mountains to the west. Abundant, wide-spread N-S striations in the Champlain Valley cross-cut all older striations indicating that ice flow was funneled parallel to the valley. The low surface slopes of modern ice streams suggest very low basal shear stresses facilitated by high basal water pressure and/or weak substrates. Both the Champlain and Hudson River valleys are underlain by relatively weak Cambrian and Ordovician shales and carbonate rocks. Furthermore, by analogy to the modern setting, these valleys were partially filled with Illinoisan-age glacial lake sediments deposited during the retreat of the previous ice sheet which would have been incorporated into the basal till in these valleys. Highly elongate streamlined bed-forms (e.g. drumlins, flutes, crag and tail bed forms) have been imaged beneath modern ice streams and have been utilized by many workers to identify paleo-ice streams. Newly available Lidar imagery clearly shows many of these streamlined bed forms in the Champlain Valley. The limited areal extent of striations indicative ice flow converging into the Champlain Valley indicates that this period of fast ice flow was relatively short-lived.