Robert Adam, Building How they Work, New York, Sterling Publishing Company, 1995, p. 14.

Stone protection

In some area near the sea or by rivers, stones are broken up by the movement of the water into just the right size for building.These boulders and slates can be used to make solid buildings. The caddis fly larva also finds smalls pebbles or other materials from the river bed and covers itself with them for protection.