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].