Final answer:
The exposed crescent-shaped rupture surface at the head of a slump is called the headwall.
Step-by-step explanation:
The exposed crescent-shaped rupture surface at the head of a slump is called the headwall. A slump refers to the downward sliding of a mass of rock or unconsolidated material along a curved surface, and the headwall is the uppermost part of the slump where the rupture occurs. It is characterized by a concave shape and can often be observed as a prominent feature in areas prone to mass wasting.