SURFICIAL GEOLOGY OF THE PORTLAND 1:100,000 QUADRANGLE, MAINE

The Maine Geological Survey (MGS) has compiled a digital map of the surficial geology of the Portland 1:100,000 quadrangle in southwestern Maine. This map synthesizes information from 31 detailed 1:24,000 surficial quadrangle maps completed under STATEMAP cooperatives between MGS and the U. S. Geological Survey. The compilation provides an overview of regional surficial geology that is more detailed and up-to-date than earlier Maine surficial maps at smaller scales. It offers a perspective that is informative to both the academic user and land-use planner. The colored map units represent late Pleistocene glacial, glaciomarine, and glaciolacustrine sediments, as well as major Holocene deposits. Digital elevation data were used to add a shaded-relief effect that helps in visualizing relations between terrain, glacial processes, and deposition of water-laid sediments. In the coastal lowland, fine-grained glaciomarine sediments of the Presumpscot Formation are differentiated from the coarser marine deposits. The latter include deltas, submarine fans, regressive sand plains graded to falling late-glacial sea level, and sediments deposited in various nearshore environments. Inland from the marine limit, the locations and sequences of glacial lakes are seen in regional perspective for the first time. The original 1:24,000 map data have also been used to produce derivative 1:100,000 digital maps to accompany the surficial geology compilation. These maps show glacial striations and other ice-flow directional indicators. They also depict features related to the recession of the last ice sheet, including end moraines, radiocarbon-dated fossil localities, meltwater channels, and ice-margin positions inferred from heads of outwash.