COSMOGENIC EXPOSURE DATING OF AN ICE-MARGINAL FLOOD SCOUR AND BOULDER BAR NEAR THE NEW YORK/QUEBEC BOARDER

Altona Flat Rock, in northeastern New York, is one of a group of sandstone surfaces exposed during the break-out flood of Glacial Lake Iroquois at Covey Hill, Quebec. The flood event, estimated at around 90,000 m³/s, scoured the sandstone along the Laurentide Ice Sheet (LIS) margin, and deposited a large boulder bar (Cobblestone Hill) where the flow entered Glacial Lake Vermont in the Champlain Valley. Radiocarbon ages constrain this event to between 13,400 and 12,800 calibrated years B.P. Samples from the Cobblestone Hill boulder bar (n=6, diameters 2-4 m) and from an adjacent scoured sandstone surface (n=1) were dated by ¹⁰Be exposure in order to test the feasibility of using this technique to date similar LIS flood events. Given the relatively close ¹⁴C constraints for the age of Cobblestone Hill and the scoured bedrock surface, we will address the accuracy of ¹⁰Be production rates in this region, as applications of ¹⁰Be dating in the northeastern US and eastern Canada are increasing.