this is a chipmunk hole.
i don't know this because of my superior naturalist knowledge of burrow entrances of small animals; i know this because it is located in the middle of a mountain bike racecourse and as i came up to this stretch of course a few days ago, the chipmunk was perched paralyzed halfway out and no way to turn and go back in.
"oh, honey.", i said. "that is a very bad place to put your entrance."
i have seen other similarly placed burrow entrances and i have to wonder WHAT about a frequently used mountain bike trail makes a groundsquirrel decide to construct a door? certainly it's not seclusion, and probably not superior protection from predation. firmness of ceiling? access to travelled routes?
all of you groundsquirrels feel free to weigh in.