Geologists sample rocks and test for pollens, trapped air samples, water content..... loads of things in a similar manner to ice core sampling in the Antarctic. Geologists can, and do, report on findings wrt AGW. Yet some have findings indicating one thing while others can interpret another way. It seems (and happy to be proven wrong) that the analyses are less exacting that ice core sampling. Theres more conclusive evidence to be found in ice than in stones.
