Existing comment:
Mechanical Weathering:
Water enters the cracks, freezes and expands, relieving inherent stresses in the rock. Gravity pulls out loosened pieces of rock, enlarging the opening.
Chemical Weathering:
Slightly acidic rainwater saturates sand that accumulates between fins. The calcium carbonate "glue" that holds sandstone together is dissolved, rocks fall, and fins get thinner. Eventually, an opening forms. |