Figure 15. Map showing the elevation (referenced to the North American Vertical Datum of 1988) of the late Wisconsinan to early Holocene age regressive unconformity U_r, which identifies the eroded surface of Pleistocene glacial drift beneath Vineyard and western Nantucket Sounds. U_r represents a composite unconformity where it locally merges with the fluvial unconformity U₁ at the base of Pleistocene sediments or is truncated by the Holocene transgressive unconformity (U_t). The surface illustrates the topographic morphologies of prominent glacial features in the shallow subsurface, including high-relief bouldery moraines (M) and ice-contact deposits (IC), moderate-relief ice-marginal drift ridge cores of Lucas (L), Middle Ground (MG), L'Hommedieu (LH), Hedge Fence (HF), and Squash Meadow (SM) shoals, fluvial drainage outlets cut through ice-marginal sills (O), relatively low-relief outwash plains (OP), and linear-to-sinuous depressions of meltwater drainage channels (MD). SV, sapping valleys; CH, Cuttyhunk; A, Aquinnah; MPP, Mashpee Pitted Plain; MC, Muskeget Channel.