THE ORIGINS OF THE NORRIDGEWOCK SAND PLAIN, SOMERSET COUNTY, CENTRAL MAINE

The Norridgewock sand plain of central Maine is a broad, gently sloping feature covering ca. 15 km² in southern Somerset County, Maine, in the Norridgewock and Hinkley, ME, USGS 7.5-minute quadrangles. A study of the geomorphology and probable origin of the sand plain was conducted by detailed topographic map and aerial photograph analysis, study of outcrop characterisics, test auguring for thickness, evaluation of unpublished borehole data, and analysis of sediment samples for grain size & mineralogy. Sands of the plain have a maximum thickness of at least 15 m (50 feet) and pinch out both to the south and southeast, where they show clear oscillation ripple structure. The sand plain unit is predominantly composed of a moderately well-sorted subrounded fine to medium sand of lithic fragments, quartz, and feldspar. To the north, the sand plain is bordered by an esker sequence; to the west is a higher plain and subaqueous fan complex, and to the south and the east are morainal complexes and bedrock ridges. Along the margins of the plain, there are dunes mineralogically identical to the main sand plain; slip face dips indicate winds from the NW and SW. Surficial geology of the area is dominated by recessional glacial tills and glaciofluvial units, overlain by postglacial marine transgressive and fluvial regressive sedimentary units. Basal Quaternary sediment is a late Wisconsinan till, overlain by postglacial marine Presumpscot Formation silts and fine sands; uppermost Presumpscot sediments have yielded marine fauna indicative of shallow, at least seasonally brackish waters. Maximum thickness of Quaternary deposits is locally as much as 51 m (170 feet). The likely source for the sand plain material is sediment from the esker complex to the immediate north, though reworking of Embden Formation sediments, a more poorly sorted unit directly overlying the Presumpscot Formation in the Kennebec River valley to the north, cannot be completely ruled out. Of proposed mechanisms for the origin of this unit, a stillstand during late-stage isostatic rebound and marine transgressional/regressional sequence seems most likely, as the sand unit was deposited in a shallow water, low-energy marine environment; pending radiocarbon analyses are expected to indicate this occurred about 12,500 ¹⁴C years b.p.