rimstone

[ rim-stohn ]
/ ˈrɪmˌstoʊn /

noun Geology.

a calcareous deposit forming a dam at the edge or outlet of an overflowing pool of water, as in a cavern.

Origin of rimstone

First recorded in 1925–30; rim + stone
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019