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.]
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.
[Illustration: 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.