Drift is divided into two types of material. One is till deposited directly by
glacial ice and shows little or no sorting or stratification (layering) as though
dumped haphazardly.The other is stratified drift, which is a well-sorted, layered
material transported and deposited by glacial meltwater. Streams flowing from
melting glaciers r ew ork part of the glacial material, some of which is carried
into standing bodies of water, forming banded deposits called glacial varves.
Long, sinuous sand deposits called eskers (Fig. 203) formed out of glacial
debris from outwash streams. They are winding, steep-walled ridges that can
extend up to 500 miles in length but seldom exceed more than 1,000 to 2,000
feet wide and 150 feet high. Eskers appear to have been created by streams
running through tunnels beneath the ice sheet.When the ice melted, the old
stream deposits were left standing as a ridge. Eskers appear to have been
deposited in channels beneath or within slow-moving or stagnant glacial ice.
Their general orientation runs at right angles to the glacial edge. At the mar-
gin of a glacial lake, they might form river deltas.
Kames are mounds composed chiefly of stratified sand and gravel
formed at or near the snout of an ice sheet or deposited at the margin of a
melting glacier.They are found in areas where large quantities of coarse mate-
rial are available during the slow melting of glacial ice. Meltwater is present in
sufficient quantities to redistribute the debris and deposit the sediments at the
margins of the decaying ice mass. Most kames are low, irregularly conical
mounds of roughly layered glacial sand and gravel that often occur in clusters.
They accompany the terminal moraine region of valley and continental glac-Figure 203An esker in
Wisconsin.
(Photo by W. C. Alden,
courtesy USGS)
Historical Geology