Sorry if I made the wrong impression, because it wasn't muddy. (Then again, I didn't say it was muddy, I said it was dirty, but oh well)
I took a picture of my shoes before I cleaned them off, because I found the whole mess rather entertaining. This is what happened (no mud, just loads of dirt). The problem was that I was encountering serious difficulties getting back into my pedals:
I was wondering if it's just part of the fine text in being a MTBer, or if there are any tricks to avoid the caking-up of dirt. (It's probably one of those things that just comes with experience, but, uh, I don't have any!) Or, even, if I should maybe revert back to platforms until I learn the trail better, and THEN go back to employing clipless pedals.