This report presents results of a high-resolution seismic-reflection survey conducted by the U.S. Geological Survey to determine the subbottom characteristics of Penobscot Bay, Maine. Penobscot Bay is one of the major embayments along the U.S. Atlantic coast (fig. 1), extending more than 50 km from the Gulf of Maine to the mouth of the Penobscot River. The bay constitutes an area of about 1,100 km" and is divided into three main passages by numerous islands.

The bay and surrounding coastal area have had a complex history of glaciation and sea-level change since late Wisconsinan time. The last ice sheet advanced across the area about 20,000 years ago, eroding the bedrock surface, removing most older sediments, and isostatically depressing the crust (Schafer and Hartshorn, 1965; Stuiver and Borns, 1975; Thompson, 1979). Deglaciation followed between about 12,700–13,500 years ago and was accompanied by a marine transgression (because of crustal depression), with ice and sea in contact (Stuiver and Borns, 1975; Thompson, 1979, 1982; Smith, 1982). Isostatic recovery of the crust then caused the coast to emerge during the immediate postglacial period, and emergence was completed by about 11,500 years ago (Stuiver and Borns, 1975; Thompson, 1979, 1982; Thompson and Smith, 1983). Finally, resubmergence of the Maine coast has accompanied the Holocene rise of sea level that began about 9,000 to 11,000 years ago (Bloom, 1963; Kaye and Barghoorn, 1964; Schmitker, 1974; Oldale and others, 1983).

In the maps presented here, we illustrate and briefly describe the sea-floor topography, bedrock morphology, and variable sediment thickness within Penobscot Bay that were either produced or modified during the period of geologic change since late Wisconsinan time.