Part 2

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. 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.

Almost all of the boulders deposited by the glacier are composed of white granite similar to the rock which underlies the entire area. This indicates that most of the boulders have not been transported very far. However, occasionally boulders are found which are not characteristic of the area and represent rocks brought in from the north. Such boulders which are foreign to the area in which they are found are called erratics. Most erratics in this area are dark-colored metamorphosed rocks in which the minerals are oriented to give the rock a layered pattern. These are called either gneisses or schists depending on whether the layering is coarse or fine. Deposits of the glacier are exposed in two gravel or sand pits near the Stillwater Camp site. These deposits are composed principally of sand but contain scattered boulders of different sizes. A few erratics are found in these deposits—particularly a variety of rock which weathers to a soft, brown porous mass resembling decayed wood. These sandy deposits probably were plastered onto the ground from the sole of the creeping glacier or were simply let down as the glacier wasted away.

Because of the irregular manner in which the glacier may deposit its load of sand and gravel, the topography in such areas is uneven and characterized by poor drainage. At a number of places in Groton swampy areas occur at higher elevation which might normally be expected to be well-drained. Some of these areas have become the sites of beaver dams because they are ideal for damming up the water.

GEOLOGY OF NEARBY AREAS

Ricker Mills

Just south of the park at Ricker Mills some of the oldest rocks in the area are exposed in the railroad cut just north of the highway crossing. These rocks belong to a thick sequence of similar rocks which are collectively called the Waits River formation. Studies in other areas indicate that these rocks belong to the portion of geologic time called the Ordovician period which was more than 350 million years ago.

The Waits River formation represents a series of sediments which accumulated at the bottom of a shallow sea during Ordovician time. These sediments included both limy and sandy beds, and fossils may originally have been preserved in some of the beds. Sediments of other types later accumulated over the Waits River formation during a long period of geologic time, so that eventually the formation became deeply buried. (See Figure 3.) The sea retreated and the rocks were subjected to high pressure and temperatures during a period of mountain-building. The rocks which had been sedimentary were folded and converted to metamorphic rocks by partial melting and recrystallization of the components. As a result the rocks became schists or marbles. Any fossils which may have been present were destroyed or badly altered in the process. This is unfortunate because valuable geologic information as the age of the rocks can be determined from the type of fossils present.