In reply to James Malloch:
You have good old fashioned "grit rash" caused by the lichen on the grit getting into your wound. Usually clears up on its own but obviously seek medical advice if it is hot,itchy,pus comes out, or redness starts tracking from the wound site.
Do this a lot and you will get nice scarred, calloused skin which is perfect for those jamming cracks.
Wear your wound with pride