Nantucket Island is a Pleistocene terminal moraine which encompasses a bay approximately 7 miles long

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and 1.5 miles wide. The bay is bordered on the northwest by post-glacial marine deposits that have been modified into cuspate projections. Remnants of kames and moraines form the border on the east and south. Most of the bay is not more than 12 feet deep, but 3 basins range in depth from 24 to 30 feet.

Sediments are composed of gravel, sand, silt, clay, and shell debris. Excess current energy of incoming over outgoing tides causes shell debris to accumulate where otherwise only fine sediments might be expected. In some areas, calcium carbonate content of fine sediments is high due to large foraminiferal populations. Therefore, calcium carbonate percentage is high in both coarse and fine sediments, whereas organic carbon and nitrogen are high only in fine sediments. Quartz is the major constituent of the sands; larger grains are characteristically well rounded and frosted.

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