On Tuesday, February 5, 2019 at 4:43:08 PM UTC-5, Joerg wrote:
Snipped
We have lots of decomposed granite, some of it in the shape of almost a
powder. The heavier a trail is traveled the more powder there is. The
last remaining bits are crushed by horses. Decomposed granite mixed with
mud is an almost ideal abrasive paste. It is a horrible sound similar to
holding a piece of aluminum onto a belt sander.

--
Regards, Joerg

http://www.analogconsultants.com/

Where I tour on dirt roads in Northern Ontario Canada there's a real mix of surface conditions from small gravel to gravel, sand and rocks to deep loose sand. Here's an image of a fairly smooth road I toured on. Still haven't worn out a rim by using rim brakes.

https://www.flickr.com/photos/738325...n/photostream/

Cheers