SURFICIAL GEOLOGY OF THE RICHMOND QUADRANGLE, MAINE

The surficial geology of the Richmond, Maine, 7.5-minute topographic quadrangle located in southwestern Maine was mapped in 2003 as part of the STATEMAP component of the USGS National Cooperative Geologic Mapping Program awarded to the Maine Geological Survey. Additional financial support by the Association of American State Geologists Field Mentoring Program provided for an educational and practical field research experience to an undergraduate geology major employed as a field assistant.

Striations and glacially streamlined features record the strongest glacial flow direction, 150º-174º; also, a younger late stage flow component is present, 180º-190º. At one site, relative striation age was determined: oldest to youngest 116º, 143º, 184º, 150º-174º. The trend of the older 184º set and the younger 180º-190º set is near parallel to local bedrock structure and topography, which may have controlled the advance and retreat of thinner ice, before and after the Late Wisconsinan glacial maximum.

During deglaciation, the sea submerged most of the quadrangle and the glacier margin was in tidewater conditions; maximum marine limit in the quadrangle is 83 m elevation. Features in the Kennebec River valley indicative of the tidewater margin include glacial-marine subaqueous fan deposits and related proximal to distal interbedded sand and marine clay. The fans represent ice- marginal discharge points for subglacial drainage, traced in the quadrangle by segmented eskers and fans, which are exploited for aggregate and provide a municipal water-supply source. Sequential east-northeast trending moraine ridges, near perpendicular to the esker trend, record an active northward retreating ice margin.

The most widespread surficial geologic deposit in the quadrangle is glacial-marine mud known as the Presumpscot Formation. Glacial-isostatic emergence of the land began before 12,200 radiocarbon years BP (-600 year marine reservoir correction factor) based on age analysis of Mya arenaria from nearshore deposits at 35 m elevation. Silty alluvium in broad areas adjacent to the Kennebec, Abagadasset, and Eastern Rivers was deposited during the later part of emergence and transition to a freshwater flood-plain environment.