Round pens

We are finally getting to preparing a site to set up the round pen that we bought 1.5 years ago. We are just busy and it wasn't high on the priority list until recently. I'll post pictures later of our work. Right now, the base grade is done and the surrounding dirt leveled off and smoothed. Now, I need to get the fill in. I can't decide what I want to use for fill, so I thought I'd throw it out there and ask what others have used in their arenas/roundpens. The previous round pen that I used when I lived in Idaho had basalt gravel screenings (very small diameter gravel). I loved that base because it had give and cushion, but also a lot of support without being hard, and it wasn't as cushy as sand. I think that sand can be a little hard on their tendons if it is deep sand. But I don't have access to basalt gravel screenings and the gravel that we have here (limestone gravel) will most likely shed dust as it is used and eventually pack firm (much like the gravel roads around here do). So I've entertained the thought of mixing limestone gravel screenings and pea gravel (which will not pack)...or perhaps looking into granite gravel. I'm concerned about cost, of course, but I also don't want to have to redo the footing. Sand would be free (we can get it from our neighbor's sand pit if we haul it ourselves), but I'm concerned about it being too soft and too much stress on tendons. We plan to put 6 inches deep...probably if it wasn't so deep it wouldn't be as cushy. I just don't want my horses to be bogged down when I work with them. My neighbor has sand in his round pen and it is not too cushy, but I think it also isn't super deep. I don't know. Coming up with the right footing is stressful though!