The data indicates that an ice sheet, probably centered in McMurdo Sound, transported materials from the north to the south of Ross Island towards the Ross Ice Shelf, and to the west of Ross Island into Taylor Valley. That some of this activity probably took place in about the last 500,000 years is indicated by a potassium-argon date of 0.68 +/- 0.14 million years for the anorthoclase trachyte at Cape Royds.