The composition of the cliffs of Étretat can be analyzed through the fallen blocks. Instead of chalk, some seem encased in biocalcarenite, an accumulation of marine fossils such as bryozoans, bivalves, and sea urchins. That would explain how the cliffs of Aval, Amont, and La Manneporte were pierced with arches. Subterranean rivers probably dug the rock by eroding the limestone. Even today, an underground river lets freshwater pass between the pebbles.