EXTENT OF THE MIDDLESEX READVANCE IN THE WINOOSKI RIVER BASIN, NORTHERN VERMONT

The Middlesex Readvance of the Laurentide Ice Sheet in northern Vermont occurred approximately 14,000 years ago during or immediately following a 100 year-long period of colder temperatures as recorded on the Greenland Ice Sheet (Gl-1d, Ridge et al., 2012). Prior to the readvance, the ice sheet had retreated sufficiently north and west to allow glacial lakes in the Winooski river drainage system to form and expand as the ice sheet retreated down-valley. Readvance of the ice sheet in these valleys produced a distinctive suite of deformational structures in the overridden and overcompacted lacustrine sediments and an overlying till that frequently has a matrix dominated by lacustrine sediment (e.g. many of the "Lake bottom deposits with ice-rafted boulders mapped by Stewart and MacClintock, 1970). However, with poor age control it's difficult to know whether different sections of deformed lacustrine sediments were deformed during the same readvance, particularly if those sections are widely separated. This paper compiles locations of field sites in the Winooski River drainage basin showing evidence of readvance and, based on these observations, attempts to outline the maximum extent of the ice sheet in its readvanced position in northern Vermont. To date, the evidence suggests that the readvancing ice extended up the different tributary valleys of the Winooski River drainage no more than 20 km south and east of Montpelier. If moraines were produced at these advanced positions, they remain unrecognized or are buried beneath younger lacustrine sediments. Few constraints presently exist on the rate at which the ice sheet advanced up the Winooski River valley. If the westernmost observed sites of deformed sediments near West Bolton mark a potential maximum distance the ice sheet had retreated prior to the readvance, then the ice sheet advanced approximately 55 km during the 100 year-long cold period, an average advance rate of 0.55 km/yr. If the advance period was closer to 200 years, the average advance rate would be 0.27 km/yr.