Superimposed drift sheets on Block Island provide evidence of two advances of the late Pleistocene glacial margin. The lower Montauk Drift crops out as two till units separated by a deformed stratified unit or till over outwash. It closely resembles the drift at Montauk Point, Long Island. This complex unit was probably deposited by a glacial lobe that moved across the Narragansett Bay region. The upper New Shoreham Drift forms the morainal topography of the island and was probably derived from a glacial lobe that crossed the Connecticut region.
The stratigraphy indicates that an oscillating early Wisconsinan ice sheet deposited the Montauk Drift and subsequently receded from its terminal position. Laminated silt, sand, and clay (varves?) were deposited in a proglacial lake, and irregularly stratified silt was deposited as alluvium adjacent to the end moraine. With the advance of the late Wisconsinan glacier, the New Shoreham Drift was deposited. This ice overrode and deformed existing drift and created a drumlin field and prominent morainal topography.