To many of you rocks are just “rocks” and very little thought has been given to any history which might be gained from their study. Probably even fewer of you realize that literally billions of years of Earth history can be derived directly from the rock record. Most geologists consider the Earth to be nearly four billion years old with man entering the picture a mere million or so years ago. No human was present to record the events of billions of years of changing land and seas, violent earth movements or the gradual evolution of life through the last 500 million years. The study of rocks and their contained evidence of past life offers the only clarification for the extremely long past history of the Earth. In order to unravel this past history the geologist accepts, with some reservations, three basic principles or laws. The Law of Uniformitarianism provides an extremely important link with the past. This law states that the physical and chemical forces which are attacking or building up the surface of the earth today have operated in much the same way during past geological time. This means that our observations of present day environments, such as streams, deltas, lakes and oceans can be applied, after slight modification, to the past as recorded in the rocks. For example, study of present day deltas, such as the Nile or Mississippi River delta, has led to the discovery of many past deltaic deposits now preserved as layers and lenses of rock. This distribution pattern of sands and muds, the nature of the life forms within each environment and many other such criteria aid the geologist in his interpretations. The Law of Superposition provides a physical order to the many layers of rock which form the geological record. Compare, for the moment, the vast numbers of rock layers (strata) with the layers of a layercake. As a layercake is built up each individual layer is placed one over another, with the bottom or base layer the first to be positioned and followed by successively overlying layers. If you think of the development of this cake in terms of time, the base layer is the oldest, the uppermost the youngest. The Law of Superposition follows this example and states that the lowermost stratum in a sequence of rock strata is the oldest or the first to form, while the stratum above is younger and formed at a later time. Some reservations do exist; however, in the Forest Park under immediate consideration this general law does apply. [Illustration: Figure 1. Igneous dike cutting the Pinney Hollow formation. The dike is the darker rock which trends toward the upper left of the photograph. This dike is located approximately 100 yards south of the Pinney Hollow Historical Monument and on the east side of State Highway 100A.]