Stage 1:

A stream cuts a steep valley into the rock.

Stage 2:

Part of the valley wall collapses or falls into the valley.

Stage 3:

The stream continues to cut downward, forming a space between the fallen rock and the stream. More collapse adds roof material to the cave and weathering forms soil on the fallen rock. Trees, grass, and other plants grow in the soil above the cave and the stream continues to cut deeper into the bedrock.