The Geology of Groton State Forest

by Robert A. Christman

INTRODUCTION

Geology is the study of the history of the earth as recorded in its rocks. This study explains why certain types of rocks and minerals occur at one place and not another, why the forms of the land differ from one region to another, and why particular animal and plant remains are sometimes preserved as fossils in certain kinds of rocks. The professional geologist makes these studies his business; the amateur finds these studies a fascinating hobby; but the uninitiated person misses much of the pleasure of travel. Anyone who notices the difference between rocks or terrains and wonders “why?”, has a potential for geology. Many fall into this class and it is for them that this booklet has been written. It is hoped that with its aid, the traveler or vacationer may come to know something about the geology of Groton State Forest. The author is confident that those who come into the habit of observing nature and the world around them will find more meaning in life itself. In any case, those traveling with children may find answers to some of their questions about minerals, rocks and mountains.

Groton State Forest is not a geologist’s paradise—as compared to Yellowstone Park or the Grand Canyon—but it does contain interesting rocks and land forms which can be explained geologically. In keeping with the calm, subdued and mature atmosphere of the Vermont countryside, the geology is unobtrusive. There are few jutting cliffs or bare rock exposures; all is mantled with vegetation. If this vegetation could be stripped away—admittedly, a postulation that would destroy the wilderness and charm that belongs to Groton—boulders and gravelly glacial deposits would be seen to fill the valleys. If in turn these boulders and the soil could be stripped away, a continuous floor of rock would be exposed. This would be a geologist’s paradise—square miles of bare rock would be available for study. However, lacking the magic wand to perform this feat, we must be satisfied to glean what information we can from the existing rock exposures.

To use a pun, it can be said that almost all the rocks found at Groton State Forest can be taken for granite. As well as has been determined, all the underlying rock is granite[1] and most of the boulders deposited by glaciers of the last ice age are the same type of granite. To avoid confusion in describing these rocks, the discussion has been divided into two parts: the first deals with the granite of the bedrock, and the second deals with the glaciation of the area and the deposits resulting from it. A third section describes the geology in some of the nearby areas.

GRANITE AND RELATED ROCKS

Occurrence of the granite

Ledges of light-colored granite occur at the summits of most of the mountains and hills in the State Forest area and are found occasionally at lower elevations. They are conspicuous on Owlshead, Silver Ledge, Little Deer, Big Deer, Niggerhead and Spicer Mountains; smaller ledges also occur on Kettle, and Little Spruce mountains, Hardwood Ridge and the low hills east of Groton Pond. At lower elevations, granite is found at the outlet of Groton Pond, along the railroad tracks west of Groton Pond and at Stillwater Brook, along Osmore brook and at several other minor locations. These locations are shown diagrammatically on the map by a black dot. These dots indicate where the granite occurs but nothing about the extent of the exposure. If every location of exposed rock were marked with a dot, certain parts of the map, for instance, the west side of Niggerhead, would be solid black and the contour lines which show the elevation would be obscured completely.

All these rocks are presumably part of one large mass of granite which extends deep below the surface of the earth. Most of this body of granite is hidden by the soil and bouldery glacial deposits, so that its exact areal extent is not known. It appears likely that it extends to the southwest to the vicinity of East Barre.

Description of the granite

The granite found at Groton State Forest is a gray to white, medium-grained rock with the mineral grains all about the same size. Surfaces exposed to weathering are generally darker in color and frequently are covered with scales of dark colored lichen. If the rock is broken to reveal an unaltered surface close examination will disclose individual mineral grains of mica, feldspar and quartz. Mica occurs as very small plates which appear either white or colorless, called muscovite, or as black shiny plates called biotite. The feldspar, which is the most abundant mineral in the granite, has a chalky white appearance and may occur as tabular grains which reflect light from their flat surfaces when held in the proper position. Quartz, which contains only silicon and oxygen, the two most common elements in the earth’s crust, is a transparent, glassy mineral which has no flat surfaces. It may appear gray because one can look down into the glassy mineral where there is no light source.