The Geology of Groton State Forest

Part 1

THE GEOLOGY OF GROTON STATE FOREST

By ROBERT A. CHRISTMAN

DEPARTMENT OF FORESTS AND PARKS Perry H. Merrill, Director

VERMONT DEVELOPMENT COMMISSION

VERMONT GEOLOGICAL SURVEY Charles G. Doll, State Geologist

1956

GEOLOGY OF GROTON STATE FOREST

By ROBERT A. CHRISTMAN

INTRODUCTION

Geology is the study of the history of the earth as recorded in its rocks. This study explains why certain types of rocks and minerals occur at one place and not another, why the forms of the land differ from one region to another, and why particular animal and plant remains are sometimes preserved as fossils in certain kinds of rocks. The professional geologist makes these studies his business; the amateur finds these studies a fascinating hobby; but the uninitiated person misses much of the pleasure of travel. Anyone who notices the difference between rocks or terrains and wonders “why?”, has a potential for geology. Many fall into this class and it is for them that this booklet has been written. It is hoped that with its aid, the traveler or vacationer may come to know something about the geology of Groton State Forest. The author is confident that those who come into the habit of observing nature and the world around them will find more meaning in life itself. In any case, those traveling with children may find answers to some of their questions about minerals, rocks and mountains.

Groton State Forest is not a geologist’s paradise—as compared to Yellowstone Park or the Grand Canyon—but it does contain interesting rocks and land forms which can be explained geologically. In keeping with the calm, subdued and mature atmosphere of the Vermont countryside, the geology is unobtrusive. There are few jutting cliffs or bare rock exposures; all is mantled with vegetation. If this vegetation could be stripped away—admittedly, a postulation that would destroy the wilderness and charm that belongs to Groton—boulders and gravelly glacial deposits would be seen to fill the valleys. If in turn these boulders and the soil could be stripped away, a continuous floor of rock would be exposed. This would be a geologist’s paradise—square miles of bare rock would be available for study. However, lacking the magic wand to perform this feat, we must be satisfied to glean what information we can from the existing rock exposures.

To use a pun, it can be said that almost all the rocks found at Groton State Forest can be taken for granite. As well as has been determined, all the underlying rock is granite and most of the boulders deposited by glaciers of the last ice age are the same type of granite. To avoid confusion in describing these rocks, the discussion has been divided into two parts: the first deals with the granite of the bedrock, and the second deals with the glaciation of the area and the deposits resulting from it. A third section describes the geology in some of the nearby areas.

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.

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 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.

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