[Illustration: D. During the ice age, continental glaciers move over
the land and erosion by the ice forms Owlshead Mountain and the
basin for Groton Pond.]
ICE
GRANITE
[Illustration: E. Present topography, exaggerated.]
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.
[Illustration: Figure 4. Sketch of boulder of granite containing
pegmatite band and schist inclusions at picnic area at Ricker Pond.]
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.
[Illustration: GROTON STATE FOREST]
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.
[Illustration: 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