One way they form is through plucking. Different bedrock types may be more susceptible to plucking. If a highly jointed layer of bedrock is on the surface, large portions of it will be picked up by the glacier and deposited later as a glacial erratic. The valley formed in this fashion may have a steep wall at its head caused by a change in the bedrock type. The stronger bedrock will remain in the form of a riser at the end of a hanging valley.