LATE PLEISTOCENE LACUSTRINE, MARINE AND FLUVIAL TERRACES IN THE AUSABLE RIVER VALLEY, NORTHEASTERN NEW YORK

Recession of the Late Pleistocene Laurentide Ice Sheet in the Ausable River Valley, northeastern New York is recorded by deltaic and fluvial terraces deposited in or graded to a succession of falling proglacial lakes and marine water bodies. The chronology of these features is generally known based on conventional mapping at scales of 1:24,000 or smaller. The highest terraces are small deltaic sandplains that are graded to six proglacial lakes listed here from oldest to youngest with the present day elevation (in meters above sea level) of the outlet thresholds in parentheses; Boreas Pond (615), Chapel Pond (505), South Gulf (425), The Gulf (345), Colby Mtn. Lakes (335-295) and Mud Brook (265). These lakes were confined to the upper portions of the Ausable Valley. Lower terraces include fluvial and deltaic terraces graded to regional proglacial lakes Coveville (195) and Fort Ann (155) and the Champlain Sea (115-50) in the Champlain Valley, which inundated the lower portions of the AuSable Valley. In this case the values in parentheses indicate the deltaic surface elevations above sea level. Modern stream terraces are often graded to knickpoints that emerged during post glacial incision.

In this study we used high-resolution LiDAR elevation and hillshade models to identify and map terrace surfaces to update the extents of proglacial lakes in the valley and improve estimates of post-glacial isostatic uplift in the region. We have identified more than 180 terraces in the valleys of the East Branch and main stem of the AuSable River. Work on the West Branch terraces is ongoing. Visual interpretation of LiDAR models facilitate the identification and correlation of terrace surfaces, especially for the large fluvial and deltaic sandplains associated with proglacial regional water bodies in the Champlain Valley. At their maximum extents the fluvial sandplain filled the East Branch more than 40 km upstream from the distal margins of their associated deltas.