deeply furrowed rocks, and great heaps of rocks marked the extent of former
glaciers. Many of the northern lands are dotted with glacial lakes developed
from deep pits excavated by roving glaciers.
Most evidence for extensive glaciation is found in moraines, tillites, and
other glacially deposited rocks. Moraines (Fig. 200) are composed of rock and
material carried by a glacier and deposited in a regular, linear pattern.They are
named according to their position in relation to the glacier.A ground moraine
is an irregular carpet of till mainly composed of clay, silt, and sand deposited
under a glacier. It is the most prevalent type of continental glacial deposit. A
terminal moraine is a ridge of erosional debris deposited by the melting for-
ward margin of a glacier. It is a ridgelike mass of glacial debris formed by the
foremost glacial snout and deposited at the outermost edge of glacial advance.
Many parts of the upper midwestern and northeastern United States
were eroded down to the granite bedrock.The debris was left in great heaps.
Figure 200Mount
Powell from a remnant of
preglacial erosion surface
with glacial moraine at the
top of Dora Mountain,
Summit County,
Colorado.
(Photo by O.Twito,
courtesy USGS)QUATERNARY GLACIATION