The Geology of Mt. Mansfield State Forest

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 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.

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