Well, perhaps a larger animal left these mounds behind. But upon closer inspection, they appear to be made out of wet dirt. They certainly don't smell like scat. The mounds are made of little scoops of moist sand stacked one upon the other to form a sandcastle. I'd need a melon-ball scooper to build something like this.
So I finally turned to Google and typed in "mounds of wet sand in my yard." And guess what came up?
Crawdads, also known as crayfish, crawfish, fresh-water lobsters and mudbugs, burrow down into moist sand near standing water, ditches, rivers and lakes and leave their diggings behind in the shape of a mound or chimney. At night they come out and creep about in search of dead leaves to munch on. During times of drought, they plug the holes to their burrows to keep the water from drying out because they breathe through gills like fish.
But now that I've solved the mystery, I can't get the image out of my head of all those crawdads crawling out of their mounds at night and creeping through my yard, lobster-pinchers clacking like something out of a Roger Corman movie. I can see it now: Attack of the 50-Foot Crawdads--Coming to a Theater Near You.