The Geology of Calvin Coolidge State Forest :: BASIC PRINCIPLES AND "TOOLS"

BASIC PRINCIPLES AND "TOOLS"

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.

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.

The third basic principle to command is known as the Law of Faunal Succession. In generalized form this law states that each stratum of rock contains its own distinct group of animal or plant remains, termed fossils, and that these same remains can be recognized throughout the world wherever they occur. Since plants and animals changed through time and because their remains are found throughout the world, it is possible to erect a worldwide time scale based upon animal and plant evolution. In short, the fossils found in particular rock are characteristic representatives of the life at the time that rock originated and the fossils found could have been entombed only at that time. One stratum, therefore, would have a total fossil assemblage quite different from the stratum above or below. Without this time reference chart it would be impossible to reconstruct what did happen during any one time interval in the past.

In addition to these three basic laws it is necessary to mention the rudiments of rock classification. The geologist divides rocks[1] into three major groups which are termed Igneous, Sedimentary and Metamorphic rocks. Igneous rocks are those formed by the solidification of molten material. This molten material was thrust into the outer crust of the earth from below and after cooling became a solid igneous rock such as granite, or in other cases it flowed out over the surface of the earth in the form of volcanic lava. Some small igneous bodies, termed dikes, can readily be seen along State Highway 100A adjacent to Calvin Coolidge State Forest Park (See photograph, Fig. 1).

Sedimentary rocks are formed in quite a different manner and differ in general appearance. These are what might be considered second-hand rocks. They are composed of particles derived from other rocks, igneous, metamorphic or older sedimentary, which have been carried by streams, wind or ice to a place of rest and there cemented into rock. Perhaps you can visualize a river which, throughout its course, runs over rocks of many types. This river would pick up particles of rock from its bed and banks and transport these to a lake or perhaps the sea, where the various transported materials would settle to the bottom in distinct layers. The first layer deposited would become buried under thousands of tons of overlying layers of sediment whose weight and resultant pressure, together with the presence of adequate rock-cementing material such as calcium carbonate or silica, would cause the bottom layer to harden into rock. The layered appearance of sedimentary rocks is one of their most characteristic features and these rocks are said to be bedded or composed of many individual beds of sedimentary rock. Sandstone, composed of sand size particles; shale, originally mud; and limestone, once lime-rich mud, are examples of sedimentary rocks.

Metamorphic rocks result when igneous or sedimentary rocks are subjected to abnormal heat and pressure. Folding or faulting[2] of rocks within the earth’s crust or deep burial beneath overlying rocks or sediments commonly produce metamorphism. The introduction of hot fluids during folding and faulting greatly increase the speed and degree of metamorphic conversion. When igneous or sedimentary rocks are subjected to metamorphism they tend to lose their original appearance as some minerals are completely changed or altered into new minerals and most of the original minerals are oriented in one or more preferred directions. The degree of heat and pressure, type and amount of hot fluids provided and the type of rock undergoing metamorphism will determine the nature of the metamorphic rock to develop. Perhaps the most unfortunate effect caused by the metamorphism of sedimentary rocks, especially when considering the history recorded in the rocks, is that in the majority of cases all fossils originally present are either destroyed or distorted beyond recognition. The absence of fossils makes age determination quite difficult and hinders definition of previous environments.

The rocks seen in and adjacent to Coolidge State Forest Park, with very few exceptions, are metamorphic rocks which were originally sedimentary rocks. Luckily the metamorphism is slight and several pages of geologic history can still be read. Schists, phyllites and quartzites[3], all metamorphic rocks, are well displayed in the Forest Park region.

The parallel arrangement of mica plates and segregation of the darker minerals into distinct layers in the schists and phyllites together with the inherited sedimentary layering in the quartzites, impart a distinctly visible orientation to the rocks seen in the Forest Park. The parallelism of the mica and segregation of the darker minerals in the schists and phyllites is directly related to metamorphic processes and the measurable orientation is called foliation. The original sedimentary layering of the quartzites, little changed through the metamorphism, is referred to as bedding. From all indications the foliation and bedding are practically parallel in this region and since most of the rocks represented are of the metamorphic type, all orientation features will be referred to as foliation.

Figure 2. Block diagram illustrating the dip and strike of foliation. The top of the block is considered an “imaginary horizontal plane.”

The geologist uses the terms dip and strike to describe foliation and uses conventional symbols for plotting purposes (See geologic map). The dip of the foliation is the angle between an imaginary horizontal line and the tilt or downward slope of the foliation. The strike is the compass direction of a line formed by the dipping foliation plane and its intersection with an imaginary horizontal plane (See block diagram, Fig. 2). A glance at the geological map will show that the rocks of the Park area consistently strike north to northwest and dip to the east. If you look at the foliation symbols which are plotted on the geologic map, the straight line of the symbol indicates the strike and the black triangle points in the direction of the dip. The angles of dip have not been included on the map; however, they average forty-five degrees down from the horizontal and toward the east or right margin of the geological map.

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