Part 1

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

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.

_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.

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.

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.

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.

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.

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.