_Occurrence of the granite_
Ledges of light-colored granite occur at the summits of most of the
mountains and hills in the State Forest area and are found occasionally
at lower elevations. They are conspicuous on Owlshead, Silver Ledge,
Little Deer, Big Deer, Niggerhead and Spicer Mountains; smaller ledges
also occur on Kettle, and Little Spruce mountains, Hardwood Ridge and
the low hills east of Groton Pond. At lower elevations, granite is found
at the outlet of Groton Pond, along the railroad tracks west of Groton
Pond and at Stillwater Brook, along Osmore brook and at several other
minor locations. These locations are shown diagrammatically on the map
by a black dot. These dots indicate where the granite occurs but nothing
about the extent of the exposure. If every location of exposed rock were
marked with a dot, certain parts of the map, for instance, the west side
of Niggerhead, would be solid black and the contour lines which show the
elevation would be obscured completely.
All these rocks are presumably part of one large mass of granite which
extends deep below the surface of the earth. Most of this body of
granite is hidden by the soil and bouldery glacial deposits, so that its
exact areal extent is not known. It appears likely that it extends to
the southwest to the vicinity of East Barre.
_Description of the granite_
The granite found at Groton State Forest is a gray to white,
medium-grained rock with the mineral grains all about the same size.
Surfaces exposed to weathering are generally darker in color and
frequently are covered with scales of dark colored lichen. If the rock
is broken to reveal an unaltered surface close examination will disclose
individual mineral grains of mica, feldspar and quartz. _Mica_ occurs as
very small plates which appear either white or colorless, called
_muscovite_, or as black shiny plates called _biotite_. The _feldspar_,
which is the most abundant mineral in the granite, has a chalky white
appearance and may occur as tabular grains which reflect light from
their flat surfaces when held in the proper position. _Quartz_, which
contains only silicon and oxygen, the two most common elements in the
earth’s crust, is a transparent, glassy mineral which has no flat
surfaces. It may appear gray because one can look down into the glassy
mineral where there is no light source.
A specimen of granite from Owlshead was studied with a microscope after
it had been cut and ground to a thickness of only 0.03 millimeters. Many
minerals, which ordinarily appear to be opaque, are transparent when
ground this thin. By their various optical properties, the different
minerals can be identified and the composition of the rock can be
determined. Figure 1 shows a photograph, taken through a microscope, of
one of these thin sections of granite. By careful examination of the
thin section and by measuring the areal extent of the different minerals
present, the rock was determined to contain, by volume, 35 percent
quartz, 60 percent feldspar (in proportions of 25 percent _microcline_
feldspar, KAlSi₃O₈ and 35 percent _plagioclase_ feldspar, NaAlSi₃O₈) and
5 percent mica (in proportions of 4 percent biotite and 1 percent
muscovite). Although it is a member of the granite family, this rock
should, in strict terminology, be called a quartz monzonite rather than
a granite to indicate more precisely the mineral composition. Because of
slight differences in composition, granite from the same body may
elsewhere be correctly called granodiorite, quartz diorite or granite
proper, depending on the relative amounts of the two feldspars and
quartz. In this report these close distinctions have not been made and
the rock is simply called granite.
_Cracks in the granite_
Two kinds of natural breaks, or cracks, occur in the granite in the
State Forest area. _Joints_ are breaks which occur along plane surfaces
and _exfoliation_ is the name given to the breakage along curved
surfaces related to the exposure of the rock. Granite, as contrasted
with other rocks, is characterized by its uniformity of texture and
massiveness, so that any cracks present are conspicuous.
[Illustration: Figure 1. Photomicrograph of a thin section of
granite from Owlshead Mountain. The mineral with the grid pattern
(upper left) is a feldspar named microcline which has the
composition of KAlSi₃O₈. The one with the indistinct striped pattern
(lower center) is a feldspar named plagioclase, variety oligoclase,
which has the composition of approximately NaAlSi₃O₈. The patterns
for these minerals result from different portions of the same
mineral grain having different orientations, called twinning, so as
to give a different optical appearance. The clear white mineral
(right center) is quartz. The dark gray mineral with the fine lines
(upper center) is biotite and the smaller, lighter-colored, elongate
mineral to the right of the biotite is muscovite. The other minerals
are feldspar and quartz in different orientations. The actual
diameter of the clear white quartz grain (right center) is about
four-tenths of a millimeter so that the photograph is a
magnification of about one hundred.]
Joints are more conspicuous of the two types, and typically belong to a
general system so that at a given location they tend to be parallel. On
top of Owlshead, for example, the most prominent joints trend N.25°W.
(read: North twenty-five degrees to the west) with dips[2] that are
vertical or dipping steeply to the southwest. Another set of joints
trends N.10°E. with dips that are vertical or dipping steeply to the
southwest. Joints represent the breakage of the rock due to stress and
strain. Some joints result from tensional forces set up within the rock
itself by contraction due to cooling of the originally hot solidified
rock. Other joints result from larger-scale forces within the earth’s
crust which cause earthquakes and general movement of land masses. An
exhaustive study of all the rocks in a large area would be required to
determine conclusively the origin of the joints on Owlshead.
In addition to the nearly vertical joints, a third set of nearly
horizontal joints may be observed on cliffs. These joints are called
_sheeting_ and apparently are related to the depth from a former
topographic surface which existed at the time the sheeting originated.
The vertical joints and sheeting are important qualities of a rock to be
considered in choosing a rock for commercial quarrying. Not only do
these factors effect the ease of quarrying, but they also determine the
amount of waste material which would have to be removed and discarded
because of poor size and shape.
Exfoliation is the term for breakage due to the disintegration caused by
decomposition of the rock on surfaces exposed to the weather. It is
characterized by the scaling off of concentric shells of altered rock to
produce a convex surface. Rocks showing exfoliation surfaces are not
common at Groton. One of the best developed exfoliation surfaces,
illustrated in Figure 2, occurs at the base of the cliffs on the south
side of Owlshead Mountain.
Once joints have formed, they are enlarged by weathering. In particular,
rocks are pushed apart by a “frost wedging.” When water freezes it
expands by about one-tenth of its volume. If it is confined it may exert
a pressure of as much as 138 tons per square foot. In this manner, huge
blocks may be pushed apart. If they are at the edge of a cliff, or part
of the cliff itself, they may eventually break off and fall to the slope
below. The accumulation of broken rock at the base of a cliff is called
_talus_.
_Origin and age of the granite_
The granite originated in the interior of the earth many million years
ago as a molten mass, called _magma_. This magma moved upward through
the earth’s crust by a process of melting the pre-existing rock or by
forcefully pushing it aside. When it reached its present position it
became cooler and minerals began to crystallize out. However, as is
shown in Figure 3, it is important to understand that the surface of the
land was not in its present position and that the magma actually cooled
beneath a considerable thickness of other rocks. These overlying rocks,
now gone, acted as an insulator and prevented the magma from cooling too
quickly. If the magma had risen through these rocks and reached their
upper surface, it would have formed a lava flow similar to those of
present-day volcanoes and would have cooled much more rapidly. Rocks
formed near the surface are characterized either by being fine-grained
without visible crystals or by having a few large crystals in a
fine-grained matrix; they never have a uniformly,
medium-to-coarse-grained texture. Thus, the texture of the granite at
Groton State Forest proves that it cooled slowly and indicates that, at
the time of cooling, the granite was not at the surface. This is a
reasonable postulation because studies of the regional geology indicate
that a large amount of rock has been removed from this area by erosion
through the long periods of geologic time.
[Illustration: Figure 2. Exfoliation surface on the south side of
Owlshead Mountain. Joints, of the sheeting type, are visible in the
granite cliff. The decomposition of the granite by weathering in the
niches has resulted in small patches of soil. The boy in the upper
right gives the scale.]
[Illustration: Figure 3. Sequence of events at Groton State Forest
shown diagrammatically.]
[Illustration: A. The Waits River Formation and other younger
formations are deposited from a shallow sea during Ordovician time.]
YOUNGER FORMATIONS
WAITS RIVER FORMATION
[Illustration: B. The sedimentary rocks are folded and metamorphosed
and the granite is intruded into the older rocks and crystallizes
during Devonian time.]
WAITS RIVER FORMATION
GRANITE
[Illustration: C. Erosion removes much of the rock from the area.]