Hi Nigel. Here's a link.

http://en.wikipedia.org/wiki/Muskeg

A horse with northern mountain experience can usually tell if where he is about to step is muskeg. If you sink a horse in it, you have to wait until he is stops flailing, unpack him, and then get him out. Crossing lots of muskeg can take a long time, especially with a pack train, and you do your best not to cross it on the way out if possible. On one trip, George had to jump in with the horse and lift each front leg out as there was a branch stuck across the horses chest preventing it from moving at all. It is not a fun time.