The glacially derived sediments covered much of the landscape, burying older
rocks under thick layers of till. Glacial till is nonstratified or mixed material
comprising clay and boulders deposited directly by glacial ice. Basal till at the
base of a glacier was usually laid down under it.Ablation till on or near the
surface of a glacier was deposited when the ice melted. Tillites are a consoli-
dated mixture of boulders and pebbles in a clay matrix. They were deposited
by glacial ice and areknown to exist on every continent.
In some areas, older deposits were buried under thick deposits of glacial
till, forming elongated hillocks aligned in the same direction called drumlins
(Fig. 201). Drumlins are tall and narrow at the upstream end of the glacier and
slope to a low, broad tail. The hills appear in concentrated fields in North
America, Scandinavia, Britain, and other areas once covered by ice. Drumlin
fields might contain as many as 10,000 knolls, looking like r ows of eggs lying
on their sides. Drumlins are the least understood of all glacial landforms. How
they attained their characteristic oval shape remains a mystery. Perhaps the
extensive drumlin fields of North America were created during cataclysmic
flood processes during melting of the vast ice sheets.
Similar to a drumlin is roche moutonnée, from French meaning “fleecy
rock.” The term was applied to glaciated outcrops because they resemble the
backs of sheep, thus prompting the name sheepback rock. Roche moutonnée
is a glaciated bedrock surface with asymmetrical mounds of varying shapes.
The up-glacier side has been glacially scoured and smoothly abraded. The
down-glacier side has steeper, jagged slopes, resulting from glacial plucking.Figure 201A drumlin
in Middlesex County,
Massachusetts.
(Photo by J. C. Russell,
courtesy USGS)
Historical Geology