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
Cover photo: Smugglers Notch looking northeast from the top of Mount Mansfield.
By ROBERT A. CHRISTMAN
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.
Figure 1. Index mapping showing location of Mount Mansfield State Forest.
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.