Seeing your pile of sand(?) brought back a funny memory of Will, the first horse I had as an adult. He was in training at HIgh Prairie in Denver (now the Colorado Horse Park) until I broke my humerus ball coming off him at a jump.
I ultimately ended up having surgery with hardware and a bone graft and was going to be out of commission for a long time, so I pulled him from HP and found a woman with a very casual backyard boarding barn where Will could have 12-14 hours turnout every day in nice pasture. He became a little bit of a pet to her daughters, who let him wander free around the barn with a similar setup to yours here (he was a very sensible TB and seemed to enjoy it, so I crossed my fingers and let it continue).
Anyway, they had a big pile of sand/dirt and Will loved climbing on it and into it. Many times I showed up to find him perched at the top like a mountain goat! The was pre-smart phones and I wish I had been able to capture on film, because I lost him to e. coli a few years later at a different barn.