Glaciers erode tremendous volumes of rock.
For anyone who has observed the terminus of an alpine glacier,
the evidence of its erosive force is clear.
You can witness firsthand the release of rock fragments of various sizes from the ice as it melts.
All signs lead to the conclusion that the ice has scraped, scoured, and torn rock debris from the floor and walls of the valley and carried it downvalley.
It should also be pointed out that in mountainous regions mass-wasting processes also make substantial contributions to the sediment load of a glacier.
The chapter opening photo (p. 156) provides on excellent example.