Yeah, terrain - I should have mentioned that... sand dunes.
I guess the dangers are more from impact than scraping off skin, and no roost stones in my face, pretty smooth on the kidneys and internal stuff. But there's cresting over and jumps, and landing on the bike is pretty bad, though not as bad as the bike landing on you! Quite possible to topple over, even head over heels. But no trees. I don't do long high jumps (yet!
Helmet, definitely, and I've got that.