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Shallow Geology, Seafloor Texture, and Physiographic Zones of the Inner Continental Shelf from Nahant to Northern Cape Cod Bay, Massachusetts

Figure 20. Map showing the elevation (from the North American Vertical Datum of 1988) of the late Wisconsinan regressive unconformity Ur, which identifies the truncated surface of Pleistocene glacial drift (Qd) and older adjacent units beneath the Massachusetts inner shelf between Nahant and northern Cape Cod Bay. The grey shaded area indicates where Ur was not identified and mapped due to insufficient seismic penetration. Ur represents a composite unconformity where it locally merges with the oldest fluvial unconformity (U1), Holocene transgressive unconformity (Ut), or both (fig. 7). The locations of geologic cross sections B-B' and C-C' (fog. 8) are indicated by cyan lines, and the locations of chirp seismic-reflection profiles A-A', D-D', E-E', and F-F' are indicated by black lines.