alcove
A recessed, arched opening in a cliff face, carved by water seeping through porous sandstone and undermining a harder caprock layer above. Alcoves in canyon country can be enormous — roofed chambers large enough to hold entire Ancestral Puebloan villages, as at Mesa Verde and Canyon de Chelly. The word applies to architecture too, but the geological version came first: the rock makes the room.
Etymology
Spanish alcoba, from Arabic al-qubba, the vault or dome. A vaulted recess — the shape is the same whether carved by water or by hands.
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