In Beaver World by Enos A. Mills

Millions of beaver ponds graced America's wild gardens at the time the first settlers came. These ragged and poetic ponds varied in length from a few feet to one mile, and in area they were from one hundred acres down to a miniature pond that half a dozen merry children might encircle. These ponds were formed by dams built by beaver, and the dams varied greatly in size and were made of poles variously combined with sticks, stones, trash, rushes, and earth.

In the Bad Lands of Dakota I saw two dams that were made of chunks of coal. This material had caved from a near-by bluff. I have noticed a few that were constructed of cobble-stones. The water-front of these dams was filled and covered with clay, and they were the work of "grass beavers," -- beaver that subsist chiefly on grass, and that live in localities almost destitute of trees.