The Geology of Groton State Forest

by Robert A. Christman

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

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.