Dead Marshes

So this has been bothering me recently. So the dead marshes were an ancient battleground - forgive me, can't remember which battle at the moment.. - and I was just wondering.. Elves, men, and orcs in the marshes yes? Does Tolkien ever mention anywhere or make reference to whether or not Elves body's actually decay when they are killed? Or are the dead in the marshes just somehow preserved?

Just curious. *smiles*