Every once in a while you run across little patches of darker soil, especially in the flood plains, but for the most part it's sand. But the upside is when it rains. Instead of turning into swampy slippery mud, wet sand drains well and becomes super super tacky, the traction at a sugar sand track after rain is amazing. And sand is
very forgiving when you crash, because it's soft. The only time sand sucks to ride in is during our relatively short dry season in the winter. Deep powdery dry sand is very hard to ride in because it moves under your wheels constantly and dramatically. And if you lose even the slightest bit of momentum in turns the front wheel will plow in and you'll crash. It definitely teaches you to maintain your speed.
A lot of the tracks down here ship in different kinds of dirt, sawdust, etc. to mix with the sand to give it a more robust consistency.