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.