The Geology of Groton State Forest

by Robert A. Christman

GRANITE AND RELATED ROCKS

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.

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.

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.

Figure 3. Sequence of events at Groton State Forest shown diagrammatically.

A. The Waits River Formation and other younger formations are deposited from a shallow sea during Ordovician time.

YOUNGER FORMATIONS WAITS RIVER FORMATION

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

C. Erosion removes much of the rock from the area.

GRANITE

D. During the ice age, continental glaciers move over the land and erosion by the ice forms Owlshead Mountain and the basin for Groton Pond.

ICE GRANITE

E. Present topography, exaggerated.

OWLSHEAD MOUNTAIN RICKER MILLS

Evidence that the granite was emplaced into the older rocks of the earth’s crust can be seen at certain locations outside of the State Forest. At Ricker Mills, for example, narrow bodies of granite can be seen cross cutting the older rocks. A fuller description of the geology at Ricker Mills is given in a later section of this report. Another type of evidence showing that the granite came into older rocks is found in the occurrence of fragments of older rock incorporated into the granite. These are called inclusions and represent broken pieces of older rock which were enveloped by the granite. Inclusions are like peach slices in jello in that the surrounding material solidified after they were dropped in. Inclusions were observed in rocks on top of Kettle and Jerry Lund Mountains.

Near the covered picnic shelter at Ricker Pond, one of the large granite boulders deposited by the glacier contains inclusions. Although this boulder has been moved from its original occurrence, it probably has not moved far as it is composed of the white granite which is typical of the area. It is cut by several pegmatitic dikes. The most interesting feature is the occurrence of inclusions of elongate, layered bands of older rocks of gray to dark gray schist.[3] These relations are shown in the sketch of this boulder in [Figure 4]. A careful examination of the schist inclusions reveals that they contain small plates of biotite in a fine matrix of quartz and more mica. The contact between the schist and granite is gradational at places because when the rock was formed the hot molten granite was in the process of melting the solid schist. The schist resembles the rock which occurred in this area before the granite was intruded and which occurs in nearby areas where no granite is exposed. Older rocks of somewhat similar appearance can be seen at Ricker Mills and on top of Jerry Lund Mountain.

The composition of the granite at Groton State Forest is nearly the same as that which occurs throughout this region of Vermont. Incomplete mapping suggests that the granite at Groton is part of a large mass which extends to the southwest to the vicinity of East Barre. Undoubtedly all the granitic rocks of this region are related although they are not continuous at the surface. They were all emplaced at about the same time following a mountain-building episode in which the older rocks were folded and metamorphosed. On the geologic time scale, the granites were emplaced near the end of the Devonian period which is estimated to be more than 300 million years ago.

Figure 4. Sketch of boulder of granite containing pegmatite band and schist inclusions at picnic area at Ricker Pond.

GRANITE PEGMATITE SCHIST

Aplite and pegmatite

Two other types of igneous rocks called aplite and pegmatite occur sparingly in Groton State Forest. Both of these are productions of crystallization of residual fluids or late stage magma related to the granite. These were emplaced along cracks or planes of weakness in the granite after the granite had solidified. When viewed from the surface the aplite or pegmatite generally appear as bands cutting through the granite. However, when the third-dimension is considered it is easily realized that they are tabular or sheet-like in shape. Igneous rock masses having these dimensions are called dikes. At Groton most of the dikes are nearly vertical with a thickness ranging from less than an inch to more than several feet and extending for considerable distances. On Owlshead, one of these dikes is nearly three feet thick. The extent of these dikes is not known because they are only partly exposed, in that they extend beyond the limited areas of rock exposure.

GROTON STATE FOREST

[High-resolution Version]

DREW MTN NIGGERHEAD MTN BLAKE HILL NIGGERHEAD BROOK KETTLE MTN SPICER MTN OWLSHEAD MTN KETTLE POND STILLWATER BK. HARDWOOD RIDGE BEAVER BROOK SILVER LEDGE LITTLE SPRUCE MTN PEACHAM POND DEER MTN DEVIL’S HILL PEACHAM BOG LITTLE DEER MTN OSMORE BK. COLDWATER BK. GROTON POND JERRY LUND MTN RICKER POND RICKER MILLS EXPLANATION GRANITE EXPOSURES SCHIST EXPOSURES TRAIL RAILROAD SWAMPY AREAS CONTOUR LINE WITH ELEVATION CONTOUR INTERVAL IS 100 FEET TOPOGRAPHY FROM U. S. GEOLOGICAL SURVEY MAPS BY ROBERT CHRISTMAN

The pegmatite dikes are coarse-grained, in some cases consisting of individual mineral grains as much as two to four inches in diameter. The mineral composition of the pegmatites is nearly the same as the granite, except that biotite is usually absent. Because of their larger grain size, the minerals can be recognized more easily in pegmatites than in either granite or aplite. Quartz is glassy and breaks with smooth curved fractures. Feldspar is chalky white, or pink, and may occur as tabular crystals with straight-line contacts. It tends to break along definite intersecting planes which can be seen in their reflecting position. Muscovite occurs as “books” of semi-transparent leaves. The large “books” of muscovite are particularly interesting because of the fascinating fact that a mineral sheet can be split along a given planar direction into thinner and thinner sheets until they are too thin to handle. Theoretically the mineral might be split into sheets only as thick as one layer of atoms. The ability of a mineral to break along definite planes is related to its atomic structure and is called cleavage. The cleavage in mica is perfect, whereas the cleavage in feldspar is only poorly developed, and quartz does not possess cleavage at all.

The aplite dikes are composed of nearly the same minerals as granite except that the average grain size is smaller. They are characterized by the absence of dark minerals and muscovite and by a high quartz content which gives the rock a “sugary” appearance. Most of the aplite dikes are less than six inches thick.

Inasmuch as the pegmatite and aplite dikes both cut through the granite, they both must be younger in age than the granite. As is shown by the relations between these two types on Owlshead (reproduced in [Figure 5]), the pegmatite dike is younger because it cuts across the aplite dike. This is the general age relationship for these dikes in this age.

Figure 5. Sketch showing aplite and pegmatite dikes in the granite on Owlshead Mountain. The cross cutting relations show that the pegmatite is youngest and that aplite is younger than the granite but older than the pegmatite. In the distance is Kettle Pond and Kettle Mountain.

GRANITE APLITE PEGMATITE