I had the same reaction as Neil. The brown surface with cracks shows some superficial similarity to the upper surface of some Peltigera species but I think this is probably a non-lichenized fungus. I always carry a razor blade in the field and one of the uses for this is to check for any photobiont partner. In most lichens the algae or cyanobacteria can be detected by simple inspection of a cut or scratched surface under a hand lens. If a fresh-looking part of your mystery had been cut and examined with a lens, this would probably have helped settle the matter of whether lichenized or not. If it was a Peltigera there would be a blue-green coloured layer of cyanobacterial partner; if a non-lichenized fungus there would be no green or blue-green layer (except for degraded fungi which might become home to free-living algae).
Scratching the thallus of a lichen can help indicate which sort of photobiont is present – yellowish green indicates Trentepohlia, fresh green = ‘chlorococcoid’ green alga, blue-green = cyanobacteria.
These queries are always interesting, even if we can’t give a definite identification.
Mark
