DRIFT, or GLACIAL DRIFT, a general name given to the deposits made by the ice sheets of the glacial period, or by the water which came from them. By some geologists the term is restricted to the unsorted deposits left by the ice itself, the deposits worked over by water being called stratified or modified drift. In this restricted sense drift includes (1) lateral and terminal moraines, and (2) till boulder clay or °hardpan," the sheet of clay that frequently covers rock surfaces in the glaciated regions. Drift is sometimes more or less sandy and always contains angular boulders with polished and often striated surfaces, the boulders being scattered through the clay without order. Usually the boulders are derived from rocks nearby, but fragments of tough hard rocks were sometimes transported long distances before be ing ground up by the action of the ice. Thus boulders from the peridotite at Cumberland Hill, R. I., have been found in the till of Martha's
Vineyard, Mass., over 30 miles distant. The exact method by which till was formed is in dis pute among geologists, though it is generally regarded as the ground moraine of the ice sheet, in spite of the fact that present glaciers are not forming a similar deposit. The boulders in the till in New Zealand are sometimes as much as 20 feet in diameter and in some places the till is so full of boulders as to render the soil formed from it unfit for agriculture. In north west Ohio the till in many places is over 100 feet deep, and as it includes the detritus from softer rocks such as limestones, in that region boulders are fewer than in New Zealand and the result ing soil is very fertile. Englacial drift is the rock detritus carried along in the body of a glacier. See GLACIER; GLACIAL PERIOD; Son..