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with the petrographic microscope. Some rocks would be chemically analyzed. This takes about 40 pounds of fresh rock per sample which has to be broken off from the outcrop into small pieces, packed in heavy plastic bags, and carried back to the vehicle. Since some outcrops were a mile or more from where we parked, I was happy to leave this task to the younger geologists. Chemical analysis, among other things, can show whether rocks from different locations that look alike are in fact the same rock.
Determining the age of the rock is also important. It’s sometimes possible to tell from the field relations whether one rock is older or younger than another rock. Absolute ages are determined by isotopic dating, which is based on the radioactive decay of certain elements in the rock. This can tell within a few million years when a billionyear-old rock formed.
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