I used to fish a couple of east Tennessee TVA lakes that had large earthen dams that were completely covered in the same size rock your describing. It wasn't uncommon to see muskrats come crawling out of the cracks.

.10 surface acre pond, 10.5 foot deep. SW LA. The epitome of a mutt pond. BG, LMB, GSF, RES, BH, Warmouth, Longear Sunfish, Gambusia,Mud Minnows, Crappie, and now shiners!!...I subscribe!!