The dead faces

[The Saucepan Man]

The description of the Dead Marshes seems to me to be a brilliant combination of the real and the unreal. It is perfectly feasible that the bodies of those who fell in battle were preserved in the boggy conditions and that the "tricksy lights" were the result of the ignition of swamp gases (which is the origin of the will o' wisp myth).

At the same time, they are described in terms that suggest that there is more to them than this. As has been pointed out, Gollum says that the faces cannot be touched, and he also warns Frodo and Sam not to follow the candles of the corpses (just as will o' wisps were not to be followed since they were said to lure people to their death).