That's probably where it's coming from, isn't it? On my old bike, at one point I looked down, and I think it was slash off the inside of the rear fender. I was going too fast yesterday to look down (darn new bike, it's fast!); but it makes sense that's mostly dry dirt being kicked up from the front wheel. That would be an interesting part to try to come up with--some sort of mini chain guard, just for that area.