Note 4 — THE GEOLOGY OF D.A.R. STATE PARK

The relative rather than the absolute age of the rocks can be determined from a study of their fossil content. These fossils are compared with collections from various places in the world where the standard geologic time scale assigns them a place (see Fig. 4). The Park rocks were deposited during the Ordovician Period. How is a standard geologic time scale put together? Several geologists first worked out the sequences of rocks according to the Law of Superposition in Great Britain and neighboring parts of Europe. When systematic collections of fossils were made from these layers and arranged according to age it was found that certain fossils occurring in rocks in distant areas were identical and occupied the same relative age position. These fossils were considered to be of the same relative age. Fossils found in the Park can be compared with these reference fossils and a relative geologic age can be assigned to them. Absolute ages can be determined in some cases by the use of rates of decay of radioactive elements and in general these ages agree with the relative ages derived through the use of fossils.