These are mole holes. They have a tunnel underneath. Moles, snakes, frogs, use the tunnels. When there is rain, the dirt separating the holes on the surface falls in and then there is the beginning of a "wash." I am not against the moles because they eat insects in the ground, but I am seriously against the tunnels. I've stepped injudiciously before and went in up to my ankle. This is especially apt to happen when we've had rain. We've had rain.