In this manner the ice sheet acts like sandpaper because the entrained material grinds more rock from the bedrock. Chances are that the rocks, dirt, sand and other debris that you saw were the product of this grinding process. However, this is, indeed, a form of mass wasting. As an avid snowboarder, I have heard and read about several stories that involved back country snowboarding on glaciers and how there are many snowboarders who have fallen into crevasses, and some of whom have died. It would seem that they are unable to see it, or else I would assume that there would far fewer stories involving injury and even death because of a fall into a crevasse.
Crevasses often form in glaciers where the ice flows over an irregular surface.
Such a surface causes the ice sheet to bend and snap or crack at the top much like a candy cane would snap if you were to bend it too much. Crevasses often get covered with snow bridges which look solid but are really quit weak. One can walk onto a snow bridge without realizing that there is a crevasse is underneath. If the bridge is weak the unlucky person takes a fall which is often very dangerous. You explained that air products are studied to see how glaciers worked.