Rectangular pits with rounded bottoms can be found very frequently in Minecraft worlds. In small bays, on cliffs, and on hillsides (basically wherever the larger mountains have been put in place), small inlets spawn. All of them are rather deep and are square. I have attached a picture from one I found in a bay for reference of what I am talking about. The ones on the cliffs are usually a bit smaller. I think this is an overall problem with symmetry because I see minor symmetry on the mountains too.

Thanks

-Sam