I had one of those gravel falls once- also on my MTB. I went over a patch of gravel that was between a bike path and a park. Now it's paved. I just skidded on it like it was ice and blat! I tore a big piece out of me knee that bled all the way home, scraped my elbow, tore my gloves. I wasn't going very fast, though. Just one of those dumb falls. AFter that I made sure I kept some first aid stuff in my pouch.