Stumbled upon an ant nursery, and safely replaced the rock to exactly where it was after taking a picture.
I think. Mound about 2' wide and 8" tall. Hundreds of ants coming out of holes on the sides of the mound, and on the ground nearby, attacking my boots if I stayed too long in one spot. Powerline ROW habitat with a lot of lowbush blueberry. Never seen these here before!