How do you explain that fossils date the rocks and rocks date the fossils?
Initially, rock strata were only dated *relative* to each other (strata A was older than strata B which was above it, but how much older was impossible to say). Estimates of age from rates of deposition could be made – but these were difficult to make with any degree of certainty. But then radioisotope dating was developed, and it became possible to absolutely date strata (by a number of different means, Uranium/Lead, Potassium/Argon, etc.). From this it became possible to find “Index Fossils” – fossils which are only found in strata of a particular age. For example, Scaphites are only found in rocks from the Cretaceous (145 to 65 million years ago), and Trilobites are only found in rocks from the Cambrian (540 to 488 million years ago). These can be used as a rough guide to a rock formation’s age before radiography is performed to confirm the absolute age.
