Glacial Erosion
The two main ways that glaciers erode the underlying rock areabrasionandplucking. As
the thick layer of ice pushes against the underlying rock, it scrapes and polishes the rock
surface. As glaciers flow, they scratch the underlying bedrock with all the rocky material
they are carrying. These scratches make long, parallel grooves in the bedrock, calledglacial
striations,which show the direction the glacier moved. Also as the glacier slowly moves
over the rock, glacial meltwater seeps into cracks and fractures of the underlying rock. As
the water freezes, it pushes pieces of rock out of the underlying rock surface. These pieces
of rock get plucked out and carried away by the flowing ice of the moving glacier (Figure
10.36).
Figure 10.36: Iceberg Cirque in Glacier National Park was carved by glaciers. ( 18 )There are several erosional features that form as a glacier both scours the rock and pulls
piecesaway. Asrocksarepulledawayfromvalleywalls, asteepsided, bowlshapeddepression
forms at the top of a mountain, called acirque. The word comes from the French word
for circle. Once the ice melts away, a high altitude lake, called atarnoften forms from
meltwater trapped in the cirque. If several glaciers flow down in different directions from a
central mountain peak, these steep walled depressions can leave behind an angular, sharp
sided peak called ahorn. The Matterhorn in Switzerland is the most famous example of
this type of erosion (Figure10.37).
When two glaciers move down opposite sides of the mountain, the erosional landform that
is created where they meet is a sharp edged, steep sided ridge, called anarête.Sometimes
hiking trails follow along these narrow ridges, providing dramatic views in all directions
(Figure10.38).