there is no scale supplied by that picture, but based upon the size of the grass blades those are some very large worm castings. by the looks of most of them they are subsoil type miners.
Those almost look like crawdad mounds they appear to be pretty huge. The only thing that makes me not think they could be crawdad mounds is the lack of a hole in the top. Must be pretty huge worms.
when it gets dry in the summer the crayfish here will close up their burrows. i first came across our native crayfish when i was digging a footing for the garden gate columns. i was rather surprised to dig down and find them living in their burrows. they do not break the surface at all in the flat areas but they do show up and leave piles of clay around their burrows when i am doing something like a trench or a ditch. interesting things happen underground all the time and we don't really know much about it yet.