_Introduction_
With a few exceptions, the rock found in Mount Mansfield State Forest is
a mica-albite-quartz schist. This name indicates that it is a
metamorphic rock[1] of a particular composition and texture as described
in the following paragraphs. The schist forms the cliffs at Smugglers
Notch and the bare-rock faces exposed along the crests of the mountains
and elsewhere. It varies slightly in appearance because of variation in
the proportions of the different mineral constituents.
[Illustration: Figure 1. Index mapping showing location of Mount
Mansfield State Forest.]
_Origin of the schist_
An understanding of the origin of the schist is fundamental to
understanding the geology of the Mount Mansfield area. Many million
years before the formation of the Green Mountains, northwestern Vermont
was covered by a shallow sea into which fine-grained sediments were
transported by the ancient rivers. As these sandy and shaly deposits
accumulated on the bottom of the sea, they were buried by progressively
younger sediments of different types. Many of these sedimentary layers
contained shells of the animals that lived and died in these seas, with
the shell remains of the older generations occurring in the bottom
layers. By the time the sea had retreated, the older sediments were
deeply buried beneath the younger sediments. During a period of
mountain-making, these materials were subjected to high pressures and
high temperatures. Physical-chemical changes took place within the
sediments causing recrystallization to form the mica-albite-quartz
schist. In other words, under conditions of heat and pressure the rocks
became plastic and the elements which were dispersed through the
sediments as sand and clay minerals reorganized into different and
larger mineral grains. It is probable that some material was added to
the rocks and some was removed by hot solutions migrating through the
rocks. The overlying younger sediments were also converted to
metamorphic rocks. Because the crystallization of the minerals occurred
under the influence of pressure, platy minerals developed with their
long dimensions at right angles to the pressure. Thus, the resulting
rock developed a layered appearance by the parallel arrangement of the
minerals. Where this layering or banding, which is called _foliation_,
is coarse, the metamorphic rock is a _gneiss_; where it is fine but
pronounced, the rock is called a _schist_. If the original rock was a
limestone or sandstone, the metamorphic product is marble or quartzite,
respectively. In the process of, or following the formation of the
schists, the rocks were crumpled and folded by continued pressure.
During the 380 million years following the metamorphism and folding,
this area has been above sea level and has been subjected to erosion. At
various times the area was uplifted vertically which resulted in
continued erosion of progressively older rocks until the present day
when the overlying rocks have been removed to expose the
mica-albite-quartz schist.
_Age of the mica-albite-quartz schist_
Some readers may wonder how the age of metamorphism can be stated so
specifically—380 million years seems like a long period to be determined
beyond a guess. Such a determination is based on a number of different
factors. Sedimentary rocks can be placed in their general age sequence
by their physical relationships—the rocks deposited on top must be the
youngest. A study of the fossils of successive layers shows that they
occur in a definite sequence with the simpler forms in the oldest layers
and generally the more complex ones in the youngest layers. On the basis
of the fossil evidence and the physical relations, the geologic sequence
of the layers can be established for any given area and their relative
age can thus be indicated on the geologic time scale. Usually such
sequences are established for rather large areas as, for example,
northern Vermont or eastern New York State. In addition, actual age
determinations can be made for some rocks. Many igneous rocks contain
traces of uranium which has been decomposing at a known rate since its
formation. By comparing the remnants of uranium with the decomposition
products, one can assign an approximate age in terms of years to the
igneous rock. By observing the relationship between the dated igneous
rock and any sedimentary rocks in contact with it to determine their
relative ages, it may be possible to assign an approximate age to the
sedimentary rock and the fossils contained within it.
The general age of the original constituents of the mica-albite-quartz
schist of the Green Mountains can be determined only by comparison to
other rocks that can be dated. Any fossils present originally were
destroyed during metamorphism. Igneous rocks containing uranium do not
occur with the schist. However, elsewhere in Vermont, one can determine
that the schist lies beneath rocks containing fossils of Ordovician age
and lies above pre-Cambrian rocks known to be more than 500 million
years old. The Cambrian and Ordovician periods on the geologic time
scale are the oldest periods containing abundant fossils. The period of
the metamorphism is based on evidence at other localities where unfolded
rocks of known age lie over folded rocks. On the geologic time scale the
mica-albite-quartz schist on Mount Mansfield is said to be
Cambro-Ordovician in age, which may be from 380 to 500 million years
ago.
In order that the geologist can talk about the sequences of rocks,
layers having a similar age and appearance are assigned a formation
name. The schists on Mount Mansfield closely resemble schists in
southern Vermont which belong to the Pinney Hollow formation. However,
because they can not be traced directly, it is possible that the two
sequences are not exactly equivalent. For this reason some geologists
assign the rocks in this area to the Camels Hump formation which has
been named after their abundant occurrence on Camels Hump Mountain,
south of the Mount Mansfield State Forest. Although it would be
geologically correct to use these formational names, they will be
omitted in favor of continued use of the name “mica-albite-quartz
schist.”
The formation which lies over the mica-albite-quartz schist may be seen
in the vicinity of the village of Stowe where the rocks are either a
black, shiny schist or a fine-grained green schist. The formation which
lies under the mica-albite-quartz schist is not exposed in the Mount
Mansfield area.
_Description of the schist_
As the name implies, the mica-albite-quartz schist contains the minerals
mica, albite, and quartz. These mineral constituents are found in all
the schists in the area. Other minerals may be locally abundant or
present in small amounts.
When the schist is examined without a hand lens or microscope, _mica_
appears to be the most abundant mineral. It occurs as small colorless to
white flakes which sparkle and shine in the sunlight. You may recognize
this mineral as the one that is sometimes sold as artificial snow at
Christmas time. Its species name is muscovite, and it has a chemical
composition of KAl₂(AlSi₃)O₁₀(OH)₂. _Muscovite_ is found in various
proportions in most of the rocks in the area. It is a deceptive mineral
upon which to make a percentage estimate because it appears to be more
abundant than it actually is. In most of the rocks it comprises less
than 50 per cent of the minerals. _Biotite_ is the other important
member of the mica group and is distinguished from muscovite by its
black or dark brown color. Biotite occurs in minor proportions in the
rocks of the area, being most abundant on the western slope of Mount
Mansfield. Like muscovite, biotite occurs as small flakes with smooth
flat surfaces.
The orientation of the mica flakes accounts for the pronounced layering
of most schists. All of the flakes are parallel and when folded and
crumpled they give the rock its structure. Where the muscovite is most
abundant, breakage along planes rich in mica produces smooth, shiny
surfaces which may give the rock a slippery appearance. The layers rich
in mica may show in a rather perfect manner the small-scale folding of
the schist.
Albite and quartz are also important constituents of the schists.
Together they are more abundant than mica in the average schist on Mount
Mansfield but because they are not so “showy” they are more easily
overlooked. Both minerals are white and granular. Quartz, which consists
of silica or SiO₂, is the principal constituent of most beach sands. In
the typical mica schist, quartz is glassy in appearance and occurs as
small rounded or irregular grains without plane surfaces. _Albite_ is a
variety of plagioclase feldspar having the composition of NaAlSi₃O₈. It
has a white chalky appearance and occurs in small equidimensional grains
bounded by flat surfaces which break along plane surfaces that reflect
light in certain positions. On the west side of Mount Mansfield some of
the rocks contain so much albite and so little mica that the rock is
granular in appearance.
_Chlorite_, variety pennine, is an important mineral constituent of some
of the schist. Chlorite is characterized by its green color and small
amounts are responsible for giving a greenish cast to much of the
schist. Like mica it occurs as thin sheets which reflect the folding of
the schist.
Garnet and magnetite are locally abundant minerals in the schist.
_Garnet_ occurs as pink to red grains ranging from pin point to pea
size. Most of the grains are rounded although a few occur as
equidimensional crystals which have twelve equally developed faces.
Garnet has a semitransparent, glassy appearance and is harder than a
knife blade. _Magnetite_ occurs as bluish-black metallic masses with
about the same size range as the garnet. Although most are rounded
masses, crystal faces are developed on some. Perfectly developed
crystals occur as _octahedrons_ which is the form consisting of eight
faces, as two four-sided pyramids with their bases together. The larger
grains of magnetite have sufficient magnetic power to attract or deflect
a compass needle. The garnet and magnetite usually do not occur
together, but each may form localized concentrations as lenses or layers
in the schist.
In smaller amounts, usually visible only with a hand lens or microscope,
the schist also contains a green mineral called epidote, a white mineral
apatite, and an elongated black mineral called tourmaline. Locally, as
on the Nose Dive ski run above the Toll Road on Mount Mansfield, slender
needles of tourmaline are visible in the schist.
When a piece of rock is sawed and ground to a thickness of 0.03
millimeters, many of the minerals that appear opaque are found to be
transparent. By their color and their optical properties the minerals
can be accurately identified. On the basis of the amount of each present
the mineral and chemical composition of the rock can be determined.
Figure 2 shows the appearance of a thin section of the
mica-albite-quartz schist from the Forehead of Mount Mansfield. The
parallel orientation of the mineral grains is apparent even though the
photomicrograph represents a very small area of the schist.
[Illustration: Figure 2. Photograph taken through a microscope at a
thin section of the schist. The white and gray grains are quartz and
albite and the dark colored elongate grains are either muscovite or
chlorite. The magnification is about one hundred times normal size.
Even at this scale, the layering of the minerals is clearly
visible.]
Other varieties of the schist occur less abundantly in the area. These
contain the same minerals as the mica-albite-quartz schist but in
different proportions. If the mica is most abundant, as it is locally on
Mount Mansfield, the schist may be smooth or highly crinkled and have a
very shiny appearance. If the albite is most abundant the schist is more
uniform and granular in appearance and the rocks are more massive. Small
scale folding is usually absent. Such albite schists occur on the west
side of Mount Mansfield, particularly along the lower part of the Maple
Ridge Trail and in the cliffs south of the Forehead along the Long
Trail.
If the quartz is most abundant, but mica and albite are present in
considerable quantities, the rock may have a granular, layered
appearance. Locally some of the rocks consist almost entirely of quartz
and are classified as quartzite. These rocks have a dense, fine-grained,
sugary appearance and generally are gray to bluish gray in color. They
are hard rocks and often form minor ledges in cliff exposures or are the
resistant rock at the top of small waterfalls in some of the creeks.
Most of the quartzite in the area occurs in narrow layers less than a
foot thick. Although these layers cannot be traced, they are most
abundant on the east side of Mount Mansfield at various localities about
one-third of the way up the mountain.
At places, vein-like masses of glassy, milky white quartz occur in the
schist. In these, the quartz is massive and without evidence of
individual grains and is often fractured unevenly. The quartz occurs as
localized lenses in the schist, particularly at the noses of the folds.
The small, white boulders of quartz of this type are conspicuous along
some of the trails.
A special and somewhat unique type of rock occurring at Sterling Pond is
described in the description of Spruce Peak and Sterling Pond.
_Structure of the mountain and the rocks_
The position of the Green Mountains is a function of the structure of
the rocks and their resistance to erosion. At the same time that the
mica-albite-quartz schist was being developed under conditions of heat
and pressure the region was tightly folded by the same forces. It is
likely that this folding continued after the metamorphism during the
declining stages of mountain-making. This period of mountain-building
probably raised the rocks to a higher level but it was the later
repeated uplifts and erosion of the overlying rocks which finally
produced the present mountain topography.
It is postulated that the folding and crumpling of the schists were
accompanied by a westward movement of large masses of rock. That is,
segments of the earth’s crust are believed to have been pushed westward
by pressure from the east. Thus, it is believed by many geologists that
the rock which now occurs in the Green Mountains may have been derived
in early times from an area ten to forty miles to the east.
The basic structure of the Green Mountains is an anticlinorium, a large
complex fold. An _anticline_ is an upward fold in which individual rock
layers if traced through the _structure_ have a shape similar to that of
an arch; the opposite structure is a syncline in which the individual
layers are shaped like a trough or basin. An _anticlinorium_ is a large
anticline upon which are superimposed many smaller anticlines and
synclines. Figure 3 is a diagrammatic sketch showing the relation of the
topography to the structure of the rocks in the Mount Mansfield area.
The structure of the rocks is reflected in the topography of Mount
Mansfield, but such correspondence is not necessary, for the form of any
hill or mountain is a function of its erosional history and resistance
of the rock to erosion. In some folded areas, the rock in the trough of
a syncline is so resistant to erosion, that it persists in hills or
mountains after neighboring anticlines have been more deeply eroded to
form valleys.
[Illustration: Figure 3. Diagrammatic sketch showing the relation
between the topography and the structure of the Green Mountains in
the Mount Mansfield area. The section has been drawn to
approximately pass through The Chin, Smugglers Notch, Spruce Peak
and Sterling Pond, and looking N 20° W.]
THE CHIN
SMUGGLERS NOTCH
SPRUCE PEAK
STERLING POND
The smaller folds are like “little fleas on bigger fleas on bigger
fleas” in that many little folds may be superimposed on larger ones.
These anticlines and synclines range in amplitude from fractions of an
inch to thousands of feet. Many are miniature anticlinoria themselves
and could be used as scale models of the structure of the entire
mountain range. The small folds, or crenulations, in the schist have
weathered differentially so that the more resistant layers stand out in
relief, emphasizing the shape of the folds. The photograph in Figure 4
shows the small-scale folding. It will be noted that the anticlinal
folds are asymmetrical with the west side dipping more steeply than the
east side.
If a comparison is made between the structure of the mountain and that
of an asymmetrical arch, to carry the simile one step further, it may be
imagined that the axis of the arch may be either horizontal or inclined.
The chances that it is inclined are much greater than the chance that it
is exactly horizontal. Thus, most anticlines or anticlinoria are
inclined along their axes and the amount of the dip of the line
connecting the points along the crest of the fold is called the
_plunge_.
Most of the folds in the Mount Mansfield area plunge about ten degrees
to the south. This plunge is expressed in the dip of the crests of the
minor folds, particularly in the crenulation of the mica layers. Viewed
at a distance the trace of the fold-crests form a series of parallel
lines on the smooth mica-rich surfaces. This type of structure is called
the _lineation_ and is expressed on the geologic map by small arrows.
The dominant lineation is north-south. Although Figure 5 is a sketch of
a small fold showing the different structural elements, it might be
taken as a diagrammatic sketch of the regional structure.
Evidence that the structure of the rocks is even more complex is shown
locally by the presence of east-west lineations. The intersection of
this secondary lineation with the dominant south lineation produces a
checker board appearance on some rock surfaces. A system of east-west
trending folds is traced by some of the quartz lenses. The significance
of the east-west structures is hypothetical, but they are believed to
have been mostly obscured by the younger structural features.
[Illustration: Figure 4. Folding and crenulations in the
mica-albite-quartz schist near the Chin on Mount Mansfield. As the
photograph is looking to the north, it may be noted that the folds
are asymmetrical with axial plane of the folding dipping east.]
With the description of the rocks completed, the question which arises
next is how to represent these three-dimensional contortions on the map.
Figure 6 illustrates how the attitude of a particular layer may be
expressed in terms of dip and strike. It is apparent that the _dip_ of
the rock layer may vary from 0° to 90° and is measured as the angle
between its plane and a horizontal plane. Also, it is apparent that the
trend of the bed, or the _strike_, may correspond to any direction of
the compass and can be measured as the intersection of that plane and a
horizontal plane. The maximum dip is always at right angles to the
direction of the strike.
[Illustration: Figure 5. Diagrammatic three dimensional sketch
illustrating the relations between outcrop patterns of folds on
vertical planes perpendicular and parallel to the trend of the
folding (front and sides of the block) and on a horizontal surface
(top of block). Cut-away section of the block shows the folds and
lineation lines on a given foliation surface. These folds can be
more clearly visualized if the upper portion of the diagram is
covered.]
The dip and strike are used to measure the position and attitude of the
layers of the rocks. In the case of the mica-albite-quartz schists these
planes are called _foliation_ planes. If the structure of the rock is an
anticline, most of the strikes of the foliation are parallel, but the
dips are in different directions on either side of the crest. At the
crest of the fold the foliation is horizontal if the fold is not
plunging. On Mount Mansfield where the plunge is about ten degrees to
the south, foliation along the crest strikes about east-west and dips
about 10° south. Away from the crest, the dip of the sides of the
anticline begin to be expressed in the readings so that the strike
directions “swing back” toward the north-south direction. The majority
of the layers on the east side of the mountain strike northeast and dip
to the east with the angle of dip increasing away from the crest of the
anticlinorium. On the west side of the mountain they trend to the
northwest and dip to the west with the dips becoming steeper away from
the crest. In addition to these variations in the dip and strike over
the anticlinal crest, the smaller folds give local abnormal readings.
For these reasons many of the dips and strikes shown on the geologic map
represent the averages of a number of readings, and those of the minor
folds and crinkles have been omitted in order to simplify the picture.
[Illustration: Figure 6. Three dimensional diagrams showing
variations in dip and strike. Plane in A strikes N 45° W and dips
45° SW; B strikes north-south and dips 60° east; and the plane in C
strikes N 45° E and dips 30° SE.]
Another structural feature of the schists is the breakage of the rocks
along definite plane surfaces called _joints_. These usually occur in
systems formed by a number of parallel joints. The joints formed as a
result of stress and strain operating on the rocks during periods of
mountain-making and vertical uplifts. Information as to the nature of
these forces might be obtained if all the joints were carefully recorded
and plotted on a map.
On Mount Mansfield some of the prominent topographic features appear to
be controlled by joints. Much of the north-facing cliff on the Nose is
controlled by a joint trending N. 65° W., and a similar face on the
Lower Lip is controlled by a joint trending N. 60° W. Along the crest of
Mount Mansfield a number of joints trend about north-south. Joints of
this system in the steep cliffs on either side of the crest of Mount
Mansfield have been separated further by the tendency of the rocks to
creep down slope under the force of gravity. These joints form the
canyons or narrow passageways which are traversed by some of the trails.
On Maple Ridge at about 3300 foot elevation the trail crosses a joint
trending N. 50° E. which is conspicuous for its four-foot width and the
extent and the straightness of the break. A number of joints belonging
to this system are found along Maple Ridge.