What causes this?

[beslagsmed]

Had a friend send this photo to me asking what it was. I haven't seen it before, but was wondering if sole pressure causes this type of trauma.

http://www.horseshoes.com/forums/picture.php?albumid=316&pictureid=2668

[Julie Plaster]

Too much sole pressure from shoe? Maybe? Since it looks spotty, too much sole relief and tiny rocks got trapped and caused it?