Determining the Direction of Ice Advance Forming the Roanoke Point Moraine From a Survey of Hartford Basin Erratics

Introduction Glacial erratic boulders are plucked from bedrock by flowing ice, transported in the direction of ice flow, and deposited at the terminus of the glacier. If derived from a localized, distinctive region of bedrock, glacial erratics may be used as indicator stones to map the direction of ice flow. Furthermore, glacial erratics evolve during transport, becoming smaller and more rounded with increasing distance from their source. Thus the size and roundness of a population of erratics can record information about the bedrock geology over which the glacier flowed (Pacholik and Hanson, 2001). Erratic boulders are also pulverized during transport so that most large erratics are derived from nearby bedrock and few boulders survive more than 18 to 21 miles of transport (Goldthwait, 1968; Pacholik and Hanson, 2001). In Connecticut, the Hartford Basin forms a narrow corridor of Upper Triassic to Lower Jurassic sedimentary redbeds (sandstone, siltstone, and arkose) situated between the metamorphic bedrock of eastern and western Connecticut (Figure 1). Hartford Basin (HB) erratics are ideal indicator stones because they are distinctive, easily identified in the field, and have a limited source region. By identifying the area of maximum density of HB erratics along the north shore of eastern Long Island, we can connect this point to the southern end of the Hartford Basin (most proximal to Long Island) and determine an approximate direction of ice flow for the glacier that deposited the Roanoke Point Moraine. In addition, by comparing the size and roundness of HB erratics to metamorphic erratics, we can determine if some of the HB erratics were derived from a more proximal extension of the Hartford Basin beneath Long Island Sound. In this study, we surveyed glacial erratics at eight localities along the Roanoke Point Moraine on the north shore of eastern Long Island to determine the size, roundness, and relative abundance of erratics derived from the Hartford Basin in Connecticut. Analysis of these data indicate a N-NW to S-SE direction of ice flow accompanied deposition of the Roanoke Point Moraine. Furthermore, we find no evidence in our data for an extension of the Hartford Basin south beneath Long Island Sound.