*** START OF THE PROJECT GUTENBERG EBOOK 62053 ***

                             The Geology of
                       MT. MANSFIELD STATE FOREST


                                  _By_
                          ROBERT A. CHRISTMAN


                     DEPARTMENT OF FOREST AND PARKS
                      Perry H. Merrill, _Director_

                     VERMONT DEVELOPMENT COMMISSION

                       VERMONT GEOLOGICAL SURVEY
                   Charles G. Doll, _State Geologist_


                                  1956

    [Illustration: Cover photo: Smugglers Notch looking northeast from
    the top of Mount Mansfield.]




                       GEOLOGY OF MOUNT MANSFIELD
                              STATE FOREST


                                  _By_
                          ROBERT A. CHRISTMAN




                              INTRODUCTION


Included within Mount Mansfield State Forest are Mount Mansfield,
Smugglers Notch and a number of the mountains of the Green Mountain
range to the northeast and southwest of these famous landmarks. Because
much of the area is easily accessible by trails, ski lifts, and roads
(see the index map of Figure 1), the visitor has ample opportunity to
observe the minerals, rocks, and mountains. Undoubtedly, these
observations have provoked questions which, for lack of sufficient
information, have gone unanswered in the mind of the observer. In the
hopes of remedying this situation—a very grievous situation in the eyes
of a geologist—this brief geologic pamphlet has been written to help the
visitor to the State Forest obtain a greater appreciation of the
handiwork of nature.

To begin with, _geology_ can be defined as the study of the history of
the earth as recorded in the rocks. It includes the study of minerals,
rocks, fossils, the structure of the rocks, and the forms of the land.
Although only a few have been fortunate enough to choose this subject as
a profession, the field is wide open for amateurs.

This report is divided into four sections. The first describes the
rocks; the second deals with the detailed structure of the rocks and the
mountains; the third treats the glacial history of the area; and the
fourth part describes the geology that may be seen at various localities
within the park.




                        DESCRIPTION OF THE ROCKS


                             _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.




                      GLACIAL HISTORY OF THE AREA


                             _Introduction_

The geologic time division previous to the present one is called the
Pleistocene or the “ice age.” During this time, large continental
glaciers advanced over the northern part of North America several times.
The cause of the ice age is not known with certainty and whether
geologic history will repeat itself is a matter of conjecture. However,
it is established that these vast ice sheets covered New England and
that the last ice sheet melted back from the Mansfield area about 12,000
years ago.

If one stands on the crest of Mount Mansfield and looks westward over
the Champlain Valley, it is difficult to visualize this entire valley
completely filled with ice of the continental glacier. Yet, the evidence
shows that the ice sheet was so thick that it completely covered Mount
Mansfield at one time.


                       _Evidences of glaciation_

Two types of evidence, glacial striae and erratics, show that Mount
Mansfield was over-ridden by the continental glaciation. _Striae_ are
scratches in the bedrock which were produced by the sharp edges of rocks
protruding from the sole of the moving glacier. These scratches show the
direction in which the glacier was moving at a particular spot and the
average of many readings gives an accurate value as to the overall
direction of movement of the ice sheet. On the Long Trail between the
Mount Mansfield Hotel and the Chin on Mount Mansfield, striations may be
observed at a number of places. Some of the positions where readings
were made are indicated on the map by the triangular-pointed arrows.
Faint striae may be seen near the entrance of the Mount Mansfield Hotel
and more conspicuous ones are visible on the west side of the roadbed of
the secondary road that intersects the Toll Road just below the Hotel.
Figure 7 shows a photograph taken at Drift Rock in which the striae are
clearly visible.

The average trend of the striae on Mount Mansfield is about N. 50° W.
The movement of the ice is presumed to have been nearly north-south down
the Champlain Valley which was deepened by the erosive action of the
ice. These facts seem to suggest that the movement of ice over Mount
Mansfield was marginal and nearly 45° to the axis of the main ice
tongue.

_Erratics_ are boulders moved and deposited by glaciers. Often these
boulders are found in environments foreign to them. On the high parts of
Mount Mansfield, a few boulders other than the mica-albite-quartz schist
are found which have reached their present position by glacial
transportation. A particularly interesting erratic is Drift Rock. One
may note by referring again to Figure 7 that the glacial striae pass
beneath the large boulders indicating that the boulders were not at
their present position when the striae were formed. Thus, these boulders
are aptly named for they must be considered erratics even though their
composition is the same as the surrounding rocks. As a logical
speculation it is probable that they were plucked out of the bedrock
just a short distance to the northwest of their present position and it
is possible that these were the very boulders that formed the striae.

Other evidence of glaciation is the presence of _cirques_ which are the
high mountain basins in which mountain glaciers originate. Small
mountain glaciers probably existed on Mount Mansfield, particularly in
the east side of the mountain where the topography has the suggestive
form of cirque-like walls and basins.

Melting of a continental glacier causes deposition of the sand and
gravel scooped up and transported by the glacier. Some of this material
is plastered beneath the moving glacier, some is simply let down as a
blanket and terminal accumulations, and some is transported by streams
of melt water to form deposits in which the sands and gravels are
somewhat sorted in size by the action of the water.

    [Illustration: Figure 7. Drift Rock, glacial erratics along the Long
    Trail on Mount Mansfield. Note the glacial striae trending away from
    the viewer and passing beneath the boulders.]

The blanket type of glacial deposits probably were present over much of
the Mount Mansfield area at one time, but have been removed from the
upper mountain slopes by erosion. Such deposits are present at lower
elevations but are only rarely discernible because of the heavy
vegetation and soil cover. Sand and gravel deposits derived from
post-glacial streams are located in some of the lower valleys as, for
instance, along the highway north of Barnes Camp.

North of the old road and northeast of the camping area along one of the
main streams on the west side of Mount Mansfield, fine-grained, sandy
lake deposits occur at an elevation of about 1900 feet. The nature of
the deposits indicates that they were formed in ponds marginal to the
ice sheet when the continental glacier occupied the Champlain Valley but
did not extend over the crest of Mount Mansfield.




                 DESCRIPTIONS OF INDIVIDUAL LOCALITIES


                    _Spruce Peak and Sterling Pond_

Spruce Peak is the mountain on the east side of Smugglers Notch. It may
be reached by several trails or the ski lift. An excellent view of
Smugglers Notch and the surrounding country is obtained from the summit.
Near the summit along the access road to the ski lift, unweathered
mica-albite-quartz schist is exposed and the folding can be clearly
seen.

Sterling Pond lies to the northeast of Spruce Peak in a shallow
depression along the divide of the mountain. This location is anomalous
for a pond because the amount of higher land around it is so small that
only a limited amount of drainage area is available for the accumulation
of rain and snow. Yet, the outflow of water is almost continuous during
the summer. The basin occupied by the pond was probably scoured out by
the glacier.

The Sterling Pond area contains talc deposits which probably would be
commercial if they were not so inaccessible for mining. These deposits
have been studied by A. H. Chidester of the United States Geological
Survey and a report is obtainable from the U. S. Government. The
occurrence of the talc, as mapped by Chidester, is shown in the sketch
of Figure 8.

Talc has the composition of Mg₃Si₄O₁₀(OH)₂ and is a soft flaky white
mineral. The talc-bearing rocks in this area are white, light gray, or
light green and usually are irregularly stained yellow-brown by the
weathering of an iron-bearing mineral that occurs with the talc. Because
of its extremely soft nature, the talc does not form prominent rock
exposures. As shown by the figure, the talc can be most easily observed
along the shore of Sterling Pond, south of the Green Mountain Club
cabin, or along the trail to Smugglers Notch near the top of the first
rise from the pond. Here the talc is exposed in the trail as low,
rounded, slippery knobs of “messy looking” yellow-brown rock.

As part of the pond is underlain by talc, it is probable that the
softness of the talc was a factor in the differential erosion of the
basin by the continental glacier.

The talc is believed to have originated by the alteration of a body of
_ultramafic_ igneous rock, which is characterized by having a low silica
content and a high magnesium content. Sometime during the
mountain-building period, the ultramafic igneous rock invaded the
pre-existing rocks from an unknown source within the earth’s crust. It
is believed that at a later date the original minerals in the igneous
rock were altered to talc and other minerals by the action of hot
ascending solutions composed principally of water.

    [Illustration: Figure 8. Sketchmap showing the location of the
    talc-bearing rocks at Sterling Pond.]

  MAP OF THE STERLING POND AREA
    FOREST BOUNDARY
    Green Mountain Club cabin
    STERLING POND
    To Smugglers Notch
    EXPLANATION
    Talc-bearing rock
    Solid areas are positions of exposures
    Postulated boundary of talc body beneath pond
    Trail
    To Spruce Peak
      From report by A. H. Chidester

The Long Trail passes by Sterling Pond, where the Green Mountain Club
has erected a small cabin overlooking the pond. Figure 9 is the
picturesque view of Mount Mansfield taken from the cabin.


                           _Smugglers Notch_

All of the rocks exposed at Smugglers Notch (Figure 10) are the
mica-albite-quartz schist which locally contains garnet. The large rock
boulders in the Notch were derived from the cliffs forming the walls of
the Notch. The gradual processes of weathering and breaking up by
freezing cause large slabs of the rock to become loose and eventually
break off the cliff faces to careen down the mountainside to the valley
below, just as King Rock did within historical time. At the north end of
the Notch, the large accumulations of such boulders form a _talus
slope_, the name for rock accumulations at the base of a cliff. The
irregular stacking of these rocks have formed Smugglers Cave. Smaller
openings extend further back under the “rock pile” to where the ground
is considerably colder and where the heavier cold air has sunk. Drafts
of this air escaping at the foot of the talus is noticeably cool.

The Smuggler’s Face, Elephants Head, the Singing Bird, and the Hunter
and His Dog are freaks of nature and a product of man’s imagination.
Their existence is due to the haphazard nature in which the rocks on the
cliff faces have broken along joint surfaces.

The origin of the Notch is not completely known. The steep walls and the
narrowness of the Notch suggest that it could not have been formed by
the headward erosion of two streams or by glacial action. It seems most
likely that it was formed by the erosive action of an ancient river that
once flowed through the area. Because of the high elevation, the only
time when such a river could have existed is when the Champlain Valley
was filled with ice on the west side of Mount Mansfield, so that the
normal drainage of water to the west was blocked by the ice. It is
possible that the conditions were such that the water supplied by the
melting glacier could only drain to the south through the Notch. After
the ice retreated the drainage system was abandoned in favor of lower
outlets and eventually the drainage was developed to Lake Champlain in
the west. The Notch was modified by the headward erosion of the present
small streams. Except for the shape of the Notch, the only evidence for
this hypothesis is the occurrence of a weakly-bedded, well-sorted
deposit of sand at the north end of the Notch at an elevation of 2050
feet. Such a deposit of sand is characteristic of standing water, which
occurring at this elevation indicates that some vastly different
drainage system must have existed in the past.

    [Illustration: Figure 9. The Chin and Bear Head on Mount Mansfield
    viewed from Sterling Pond.]

    [Illustration: Figure 10. View of Smugglers Notch and Spruce Peak
    from ski slope on Mount Mansfield.]


                              _Big Spring_

On the south side of the Smugglers Notch road at an elevation of about
1800 feet is the Big Spring which furnishes a tremendous output of cold
spring water. The source of the water is not known although it is likely
that, like most springs, it is derived from an underground drainage
system. The spring probably originates by the seepage of water derived
from the winter snows and rainfall in Spruce Mountain through a joint
system within the mountain. No buried stream channels could be located
between the spring and the base of the massive overhead cliffs. The
belief that the spring is related to Sterling Pond is unfounded. It is
unlikely that surplus water could be drained from Sterling Pond because
it already maintains a delicate balance between the supply of water from
rain and snow and the output to the stream flowing to the north.


                           _Mount Mansfield_

The summit of Mount Mansfield when viewed from a distance has the
resemblance of a face with an exaggerated distance between the nose and
upper lips. Accordingly, these peaks are named, from south to north, the
Forehead, Nose, Upper Lip, Lower Lip, Chin, and Adam’s Apple. All these
points are readily accessible by the Long Trail and the area of the Nose
may be reached by the Toll Road or the Ski Lift. From all the points
along the crest an excellent view may be obtained. On clear days Lake
Champlain and the Adirondack Mountains in New York State may be seen to
the west, and Mount Washington in New Hampshire may be seen to the east.

Of the local structures, good exposures of the mica-albite-quartz schist
occur along the crest with magnetite and garnet locally abundant. The
rocks on Sunset Ridge, which extends west from the Chin, can be seen
clearly to be dipping at gentle angles to the west.

The Chin has an elevation of 4393 feet, which is the highest point in
Vermont. Most of the schist in this area is nearly horizontal or dipping
slightly to the west. However, minor folds are present everywhere and an
average reading is difficult to obtain. At the summit much of the schist
contains large black grains of magnetite. The summit is reached by the
Long Trail along the crest of the mountain from the Toll Road, by the
Long Trail from Barnes Camp via Taft Lodge, or from the west by the
trail up Sunset Ridge. An excellent view of the Lake of the Clouds and
the Adam’s Apple is obtained a short distance north of the summit of the
Chin.

    [Illustration: Figure 11. “Subway” formed by separation of the rocks
    along a joint on west slope of Mount Mansfield.]

Lake of the Clouds which lies north of the Adam’s Apple and Bear Pond
which lies north of Bear Head Mountain are both small shallow bodies of
water. The slight depressions in which they occur were probably scooped
out by the erosive action of the glacier.

    [Illustration: Figure 12. Cave of the Winds. Mount Mansfield, as
    seen from the east slope of the Chin. The Mount Mansfield Hotel and
    the Nose are in the distance. The rocks in the foreground show the
    beginning stages of downhill slippage of a large mass of rock away
    from the joint.]

Between the Chin and Mount Mansfield Hotel are a number of interesting
trails. The Subway and Canyon trail on the west side of the mountain
follow, for part of the way, joints in the rock which have been enlarged
by the downhill slippage of the western block. Figure 11 shows the
nature of one of these passageways. On the east side of the mountain,
the Cliff trail passes through a similar joint called “Wall Street.” The
Cave of the Winds, reached by a trail just north of the Lower Lip, has
formed along another north-south joint. The block which has moved
downhill has tilted into the mountain and rubble has filled the gap at
the top to form the cave. Figure 12 shows the appearance of the cave as
seen from the eastern slope of the Chin. This photograph shows smaller
joints on the Chin which have just begun to be enlarged by slippage of
the downhill block. It is possible that some of these open joints date
back and partially owe their origin to glacial erosive action.

Drift Rock which is located along the Long Trail south of the Upper Lip
has already been described as an erratic boulder moved by the glacier.
The glacial striae may be seen in the bedrock northwest of the boulders.
Garnet crystals are very abundant in these boulders and specimens of the
small red garnets may be obtained here.

The Nose is easily reached along the trail from Mount Mansfield Hotel.
From the summit the view is excellent to the south towards Camels Hump,
which is one of the prominent peaks of the Green Mountain range. The
mica-albite-quartz schist on top of the Nose has many small folds and
crenulations.

South of the Forehead along the Long Trail, cliffs of an albite-rich
variety of the mica-albite-quartz schist form obstacles which have been
surmounted cleverly by the Green Mountain trail-markers.

In the southern part of Mount Mansfield State Forest the Long Trail
passes through Nebraska Notch in the vicinity of Taylor Lodge. This
notch also was formed by the erosive action of an ancient river that
flowed across the mountain at this point, but which has long since been
abandoned.

    [Illustration: The cliffs at Smugglers Notch as seen looking south
    from the west wall of the Notch.]




                               FOOTNOTES


[1]Rocks are classified as being either igneous, sedimentary, or
    metamorphic. _Igneous_ rocks form by the solidification of molten
    material; _sedimentary_ rocks form by the accumulation of sediments
    derived from older rocks; and _metamorphic_ rocks form by
    recrystallization of older rocks under conditions of high
    temperatures and pressures.


    [Illustration: MOUNT MANSFIELD STATE FOREST]




                          Transcriber’s Notes


—Silently corrected a few typos.

—Retained publication information from the printed edition: this eBook
  is public-domain in the country of publication.

—In the text versions only, text in italics is delimited by
  _underscores_.







End of the Project Gutenberg EBook of The Geology of Mt. Mansfield State
Forest, by Robert A. Christman

*** END OF THE PROJECT GUTENBERG EBOOK 62053 ***