Becky Burcher, age 13, of Newport News, Virginia, for her question:

How do geologists tell the age of fossils?

As a rule, a fossil is found in the rocky layers in which it has been buried for ages. Except where the earth's crust has been deformed, older strata tend to be below layers that were formed later. Experts know roughly when the different strata were formed. Hence, the bedrock in which a fossil is found gives a general idea of its age. Nowadays, radioactive materials are used to give more precise dating of fossils. These techniques are highly complex and very specialized.

Traces of carbon 14 and other radioactive materials are present in the elements that living plants and animals absorb from their environments. These unstable materials break down at a fixed rate into smaller .atoms. When life ceases, no more elements are absorbed,  but the radioactive atoms continue to decay on schedule. Experts examine bone, wood and other fossil remains for these elements. They compare the quantities of an original radio¬

active substance and the elements it has formed during the process of decay. Since radio¬ active decay progresses at a fixed rate, this ratio can reveal when a plant or animal stopped living. This, of course, is the age of the fossil.