Anyway, I planned it all out and it's going to cost me around $1,000 with the sand. I just got some advice today on how to keep the rock from coming up through the sand (I'd appreciate any more advice on that). I have a friend who has a rocky, rocky arena because he didn't put down anything between the ground and the sand. The advice today was to use AB (road bed gravel) as the first layer, maybe 3-4 inches, then to add the 4 inches of sand on top of that. I was told that the AB will pack and become solid, thereby blocking the rocks from coming up. My neighbor had to dig out all his sand because of rocks and lay AB, then sand again.
So I'm building a 60-foot round pen. I used 8' long 6" round posts and I put them 3 feet into the ground. I will end up with 5-foot high walls. They are 10 feet apart and the pen will have two 10-foot gates, one on either side. I plan to rototil the dirt and dig it down to about 8 inches, then lay 4" of AB and 4" of sand. I'm putting on a top rail, a middle rail and a lower rail, with the lower rail being higher that my horses knee. That was recommended by my trainer, so Bella can't get a foot caught. So the bottom rail will be 2.5' or so off the ground with no barrier at ground level other than the hard-packed dirt around the outside.
If you have an opinion I'd love to hear it. I've gotten all the post holes dug and I've set about half the posts at a 5 degree angle, angled out. I'm going to set the rest of the posts tomorrow, but I haven't bought the rail wood yet, so I can still change my plan on rails.