Great to hear you found a "nice" bit of thick mud, 'cause most of us (except Jacket) think mud is crap. lol. All you can do is take the wheels off and gerni the calipers and disc to clean the rubbish out and save some money for the brake overhaul you will eventually need.
The good thing about disc brakes is it's easy to remove the calipers and pads if you have to clean them properly.