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.
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.