River weed parchment

This stuff is a bit strange. I'd never noticed it before, but then again I'd never torn up the entire bottom end of the field with a JCB before. Allow me to explain... there used to be a pond at the bottom end of the field, but it was pretty useless from an ecological point of view because every year it flooded completely (the water table rises and the river bursts its banks and half the field is under up to a meter of water), meaning that no species gets a full lifecycle and any habitats are decimated annually. So last summer, I decided to fill in the pond and dig one somewhere more useful, i.e. in a place where it doesn't flood. I haven't dug it yet - that's 2012's job. Anyway after the pond was filled in, the whole area was cleared by Tom in his JCB - there are some pics on facebook somewhere - and bare earth was left. After the floods went down last month (finally! The first time since October!) we were left with this odd, slimy pond weedy stuff that promptly bleached white in the sun. It looks like some kind of creepy ancient parchment...