To make the generalization that ‘glaciers don’t erode mountains’ seems absurd to me. However, the bottom (or bed) of a glacier may be frozen to the rock or it may slide over the surface or a patchwork combination of the two. If you glue sandpaper to a piece of wood, the wood will not be worn down (similar to where the glacier is frozen to the bed). These frozen areas will appear old and untouched by glaciers, even if there was a glacier over them and in this sense, some areas may be protected by glaciers from erosion. The U-shaped valleys and erosional features, which are typical of glacier landscapes, occur where the bed of the glacier is sliding past the bedrock, lubricated sometimes by water or till. You don’t have to look very hard to see the erosive power of glaciers.
- Glaciers protect Alps mountain peaks (bbc.co.uk)
- Glaciers Protect Alpine Peaks From Erosion (news.slashdot.org)