Figure 12. Chirp seismic-reflection profile C–C' with seismic stratigraphic interpretation. This profile illustrates the stratigraphy beneath northwestern Nantucket Sound in the vicinity of L'Hommedieu Shoal. See figure 9 for descriptions of stratigraphic units and major unconformities (indicated by red lines). Ice-contact material (Qdt) with a steep northern margin is covered by relatively thick, ice-proxal outwash (Qdf) that grades ice-distally and southward to terraced outwash plains (Qdf). Fluvial and estuarine fills (Qfe) are restricted to the topographic lows of outwash channels in the regressive unconformity U_r, and sandy Holocene sediments, reworked from the underlying drift, form relatively thick draping shoals and mobile sand waves (Qmn) across the ice-marginal core locally. See figures 14 and 21 for profile location. A constant sound velocity of 1,500 meters per second was used to convert two-way traveltime to depth, in meters.