A glacier erodes in three ways:

Plucking

As the glacier moves, friction causes the bottom of the glacier to melt this water freezes into joints in the rock. When the glacier moves again the rock is pulled away or ‘plucked’ from the base of the valley.

Abrasion

Rock pieces that have been plucked away and carried by the glacier (moraine) act like sandpaper scraping away at the valley bottom and sides.

Freeze – thaw or frost shattering

Water enters into cracks in the rock over time it freezes and repeatedly thaws putting large amounts of pressure on the rock – eventually this will force the rock to break apart or shatter.