Originally Posted by PaintHorseMares
It's really just because of the weight of the beasties. Try riding down a well traveled horse path and you'll see the same thing...you're riding in a valley!
No it is not the weight of the horse.
The holes develop as you clean the stall. Each time you remove the wet shavings, you take a bit more of the surface.
We have mats in our box stalls and tie stalls. Tie stalls are 100% and the mats in the box stalls are placed for the horse who lives in the stall. We line it up with their normal urination spot.
Even with the mats, we pull them once a year and level the stall - adding fill if needed and flip the mat.