>Is there not some method involving the last time a soil surface was exposed to sunlight?<
Yup, it's called thermoluminescence. All a bit beyond me I'm afraid but -
"Although the best known form of luminescence dating is thermoluminescence (or TL), there are several scientific methods which can specify the date of certain artefacts or soil sediments by measuring the amount of light energy they have trapped in their crystals. To put it simply, certain minerals, such as quartz, feldspar, and calcite, store energy from the sun at a known rate. This energy is lodged in the imperfect lattices of the mineral's crystals. Heating these crystals (such as when a pottery vessel is fired or when rocks in a fire-place are heated) empties the stored energy, after which time the mineral begins absorbing energy again. TL dating is a matter of comparing the energy stored in a crystal to what "ought" to be there, thereby coming up with a date-of-last-heated. In the same way, more or less, OSL (optically stimulated luminescence) dating measures the last time an object was exposed to sunlight. Luminescence dating is good for between a few hundred to several hundred thousand years, making it much more useful than carbon dating."
Puts a whole new shine on crystals and sunlight (and I've just copyrighted that ;-)
