It used to be rubber & sand, but gets topped up with rubber every few years and not sand, so its nearly all rubber. Its really nice to ride on and very comfy if you fall off
Both our menages have about 2 inches of sand underneath and then about an inch of rubber on top. Our jumping one is on a slight slope and can get a little deep if it isn't harrowed properly but our flat schooling menage is lovely (both have underneath watering systems)
Pics of jumping menage (please excuse me on B - that was taken 4 years ago!)
(link to a vid if you want to see how it rides - Vid)