The Sankaty Head cliff of Nantucket Island, Massachusetts, exposes drift of at least two glaciations and interglacial marine deposits. Radiocarbon, amino-acid-racemization, and uranium-thorium analyses were used to determine the absolute ages of the beds. The results indicate that (1) the Sankaty Sand correlates with oxygen-isotope stage 5 (Sangamonian), (2) the underlying drift is older than stage 5 (Illinoian or older), and (3) the overlying drift is Wisconsinan in age. Ostracodes and molluscs within the Sankaty Sand indicate that the marine climate during deposition of the lower part was somewhat warmer than the present climate off Sankaty Head and that the marine climate during the deposition of the upper part was as cold as or somewhat colder than the present climate. The paleoenvironmental data support a stage 5 (Sangamonian) age for the marine deposits.