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_[1] 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[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.

YOUNGER FORMATIONS WAITS RIVER FORMATION

WAITS RIVER FORMATION GRANITE

GRANITE

ICE GRANITE

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.

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.

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.

GRANITE APLITE PEGMATITE

GLACIATION

Although the causes of the ice ages remain a matter for conjecture, the fact is established that the northern part of North America was covered by a thick sheet of moving ice several different times beginning about a million years ago. As the effect of the last glaciation erased much of the evidence of previous glaciations, the present topography can be related to that last one. Rather accurate dating by measuring the radioactive decay of Carbon 14, indicates that the ice of the last glaciation retreated from the area about 12,000 years ago. Because the climates between the four glaciations were as warm, if not warmer, than our present-day climate, geologists have speculated that the world may now be in a warm period and that another ice age is scheduled to occur some time in the distant future.

The effect of continental glaciation upon a land mass is twofold. First, the glaciation acts as an erosive agent which tends to scoop out the areas of softer rock and wear down the areas of more resistant rock. Secondly, when the glacier begins to melt, it drops large quantities of gravel and boulders which had become incorporated within the glacier. Most of this material is picked up by the glacier as it moves over the land; some falls onto the glacier where it occupies a valley. Some of the sand, gravel and boulder deposits left by the glacier are distinctive in form and composition and others are characterized by their complete lack of distinctive shapes, and the utterly chaotic nature of the material deposited. The deposits at Groton State Forest seem to be the latter type.

_Erosion and deposition by the glacier_

The shape of Spicer, Owlshead, Little Deer and Big Deer mountains are primarily the result of the erosive action of the glacier as it continually moved southward over the land for a great number of years during the last glaciation. When a continental glacier encounters a hill or mountain of resistant rock, it tends to scour the rock on the up-ice side of the hill and to “pluck out” the rocks on the leeward side. For this reason these mountains have broad gentle slopes on the side from which the glacier came and they drop off sharply on the side from which material was removed by plucking action. The last part of Figure 3 illustrates how these mountains may have been formed. Such prominent rock exposures which have been subjected to glacial erosion originally showed deep scratches, called _glacial striae_, cut by cobbles dragged along the bottom of the glacier. Unfortunately, on most prominences in Groton State Forest exfoliation of the rock has erased these markings; but it is possible that striae may be found on recently uncovered rock exposures.

The depressions in which Groton and Osmore ponds are located probably represent areas in which the glacier scooped out material to a greater depth than elsewhere either because of channeling of bottom flow between topographically prominent features, or because of subtle differences in rock hardness.

When the glacier retreated, that is when it was melting faster than it was advancing, it dropped material in a helter-skelter manner. _End moraines_, which are ridges of gravel formed where the front of the glacier was stationary because of a close balance between rates of movement and melting, are not evident in Groton State Forest. As far as can be determined, the material was deposited irregularly over the entire area, so that boulders dropped by the glacier are found everywhere. These are particularly noticeable around the lakes where the fine material has been removed and the soil and forest cover does not hide the boulders.