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.