Glaciers and Glaciation - The Changing Earth: Exploring Geology and Evolution - page 367

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Figure 14.9 Roche Moutonnée A French term meaning “rock sheep,” a roche moutonnée is a
bedrock projection that was shaped by glacial abrasion and plucking.

a Origin of a roche moutonnée. A glacier abrades and
polishes the “upstream” side of a bedrock projection
and shapes its “downstream” side by plucking.

Flow direction

Plucking

Abrasion

b A roche moutonnée in Montana.
The glacier that formed this feature
moved from right to left.

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Figure 14.10 Glacial Striations, Polish, and Rock Flour

a Glacial polish and striations, the straight scratches, on basalt at
Devils Postpile National Monument in California.

b The water in this stream in Switzerland is discolored by rock fl our,
small particles produced by glacial abrasion.

broad, rather fl at valley fl oors; thus they exhibit a U-shaped
profi le (Figure 14.12b).
Many glacial troughs contain triangular-shaped
truncated spurs , which are cutoff or truncated ridges that

extend into the preglacial valley (Figure 14.12c). Another
common feature is a series of steps or rock basins in the
valley fl oor where the glacier eroded rocks of varying resis-
tance; many of the basins now contain small lakes.