CONSTRAINING THE AGE OF DEGLACIATION OF SOUTHEASTERN NEW ENGLAND

The absence of constraining radiocarbon ages and other accurate and precise dating techniques has left the chronology of initial deglaciation from the maximum position of the southeastern Laurentide Ice Sheet in New England only crudely estimated. This is in marked contrast to areas further from the terminal margin, where ice retreat is tied to abundant radiocarbon ages and a well-dated glacial varve chronology. A 265-year varve series from Glacial Lake Narragansett collected from the Providence River, Narragansett Bay, Rhode Island. was not correlated either with the North American Varve Chronology, or with other varve sequences from southern New England or southeastern New York. However, this uncorrelated sequence represents the minimum time of deposition within the northern segment of Glacial Lake Narragansett. Used in conjunction with the calibrated North American Varve Chronology and cosmogenic exposure ages from the Wolf Rocks and Congdon Hill recessional end moraines, minimum (> 19,400 yBP) and maximum (< 20,500 yBP) ages for Glacial Lake Narragansett can be established. The volume of sediment deposited and presence of overlapping lacustrine fans in Glacial Lake Narragansett indicate that the ice sheet was warm-based, or at least ‘warm poly-thermal’, with subglacial drainages that did not reorganize every year. The relatively rapid retreat of the ice sheet along with models of the landscape prior to the onset of isostatic rebound approximately 16,000 yBP, support the trend of a thinner, ‘warmer’ Laurentide Ice Sheet in southeastern New England. While many studies refer to the last glacial maximum occurring at 18,000 to 20,000 yBP, the constrained age of Glacial Lake Narragansett suggests that at least for the southeastern portion of the Laurentide Ice Sheet, deglaciation was well underway by this time. Significant questions remain regarding the initial deglaciation of southeastern New England, particularly the timing of ice retreat from the terminal moraine near Block Island, Rhode Island and across present-day Block Island and Rhode Island Sounds prior to the southern margin of the Laurentide retreating from the cosmogenically dated Charlestown moraine position around 21,300 yBP.