caving in

A narrow ditch in the earth will widen by the caving-in of its sides.

And so it is with this cañon; the weathering or the caving-in of these walls goes ever on.

Cut narrow by the river, it has gradually widened by the caving-in of the walls.

He was of right blue blood; never caving-in before them, as Bill Whitney expressed it one day; he was only quiet and endured.