I found this relatively large beetle inside a large rotten log while searching for snails in Great Falls Park last weekend. It appears to be an Odontotaenius disjunctus. Rotting logs are indeed their normal abodes.
They have a small horn on top of their heads that is difficult to notice. In fact, I didn't see it in the field. Only after I read that one of their vernacular names was horned passalus did I examine the pictures carefully and then noticed the horn. It is marked with an arrow in the next picture that shows the beetle crawling at the tips of my fingers.