Part 2

The Pinney Hollow formation, which is an estimated 2,300 or more feet thick, is primarily composed of pale-green schist with abundant white quartz layers and lenses sandwiched between the dominantly greenish colored rock. Originally these metamorphic rocks were sand, mud and sandy mud deposited on a relatively shallow sea floor. After burial these sediments hardened into thin layers and lenses of sandstone, shale, and sandy shale. Metamorphism has since converted many of the shale minerals into mica flakes and some of the sandy shales produced crystals of the mineral garnet. The pale-green color of the Pinney Hollow schist is caused by the presence of the green mica-like mineral, chlorite, which developed during the process of metamorphism.

NORTH 20 WEST 25 RODS FROM THIS SPOT STOOD THE LOG CABIN OF Jonathan Pinney & Priscilla Grover Pinney WHO SETTLED HERE A.D. 1800. HERE WERE REARED THEIR TEN CHILDREN— AND WHEN THE SECOND GENERATION CAME THEY CALLED IT PINNEY HOLLOW

No fossils have been found in the Pinney Hollow formation of this region: however, evidence from other parts of Vermont indicate that these rocks are of Cambrian or perhaps Ordovician age. This dates these rocks as approximately 500 million years old.

The Ottauquechee formation, next above the Pinney Hollow formation and therefore younger in age, was named for the Ottauquechee River along which this formation is well exposed and was first studied in detail. The Ottauquechee River is located just north of Calvin Coolidge State Forest Park where it flows through the towns of Bridgewater and Bridgewater Corners. You can see this formation along State Highway 100A approximately nine-tenths of a mile north of the Park entrance road. Here, the formation crops out on the west side of the road, or on your left if traveling toward Bridgewater Corners (See photograph, Fig. 4). A hike along the recently diamond-blazed Coolidge Forest trail, which leads from behind the Ranger’s cabin to the Picnic area, affords an almost continuous view of this formation. The highly resistant quartzite layers within the Ottauquechee formation are very conspicuous features along this trail as they form several ridges trending across the footpath.

The Ottauquechee formation, estimated to be 3,000 feet thick, contains rocks which contrast sharply with the underlying pale-green schists. The formation consists of alternating black phyllite and layers, up to several feet thick, of dark- to light-colored quartzite. The black carbonaceous color of the phyllites and dark quartzites indicate the presence of life, either animal or plant, during the original deposition of the sediments forming these rocks. The phyllite was, before metamorphism, a black shale which formed from black muds rich in organic matter. From studies done in areas where black muds are accumulating today it seems quite probable that the ocean waters of Ottauquechee time were of a restricted nature, that is to say, the waters were stagnant due to the lack of oxygen. Pinney Hollow shallow marine water conditions remained, but for some reason, such as a barrier of islands or shallow water sand bars, the circulation of ocean waters was hindered, leading to the accumulation of stagnant organic matter on the ocean bottom. This organic matter gives the Ottauquechee phyllite and some quartzites a dark to black color.

No fossils have been found in the Ottauquechee formation of the Coolidge Forest Park, but, as with the Pinney Hollow formation, evidence from other areas indicates a Cambrian or perhaps Ordovician age for this formation.

Above the Ottauquechee rocks lie those of the Bethel formation. Approximately 2,000 feet thick, this formation is composed mainly of pale-green schist with interlaminated thin layers of white to greenish quartzite. The rock presents a distinct banded appearance and contains conspicuous crystals of garnet and the black mica, biotite. The best view of this schist is found along State Highway 100A and slightly over two miles north of the Park entrance road. The relatively steep rock cliffs which line the west side of this road are composed of the resistant Bethel formation. In the northeast portion of Coolidge Forest Park several excellent exposures of this formation in contact with the underlying Ottauquechee formation can be seen, but, to reach these areas you would have to leave the beaten trail. If you are interested in visiting these contact locations it would be best to consult the geologic map for approximate directions.

A. Horizontal layers of early paleozoic sedimentary rocks were formed. This cross section extends beyond the limits of the Park area. Thicknesses of rock formations are approximately proportional.

B. During late Ordovician or Devonian time these sedimentary layers were folded and the rocks metamorphosed.

C. Calvin Coolidge State Forest Park as it appears today.]

A MISSISQUOI FORMATION BETHEL FORMATION OTTAUQUECHEE FORMATION PINNEY HOLLOW FORMATION B C SHELTER VILLAGE SLACK HILL

The conditions under which the Bethel formation was originally deposited are similar to those which existed during Pinney Hollow time. Shale, sandy shale and thin-bedded sandstones formed under shallow water marine conditions; however, restriction with accompanied organic accumulation did not occur. This formation is thought to be of either Cambrian or Ordovician age.

The easternmost, and therefore youngest formation to be found in the Park area is known as the Missisquoi formation. This formation, which is only partially represented here, is made up of dark phyllite, light blue-gray schist with conspicuous crystals of garnet and biotite, and gray quartzite. The rocks of this formation can be seen along Broad Brook which flows north along the eastern border of Coolidge State Forest Park.

This formation is considered Ordovician in age with the conditions of deposition being quite similar to those which existed during Ottauquechee time. Once again the ocean waters were restricted and organic-rich black muds accumulated in quantity on the ocean floor.

THE GEOLOGIC HISTORY

The early geologic history of Calvin Coolidge State Forest Park is known from the rock formations discussed in the previous section of this pamphlet. The Pinney Hollow, Ottauquechee, Bethel and Missisquoi formations of Cambrian and Ordovician age were deposited, essentially horizontally, from the shallow seas which covered this area 550 to 450 million years ago (See cross section A, Fig. 5). During some of this time these seas were at least partially restricted as evidenced by the Ottauquechee and Missisquoi black phyllites. Other Ordovician formations were deposited on top of these four formations, but have been completely stripped away by erosion during the millions of years since their deposition. Some of these later Ordovician sediments contain great thicknesses of volcanic rocks which probably covered the Coolidge Park area.

Sometime after these later Ordovician rocks formed, great stresses within the earth’s crust folded and in some places faulted these older rocks. The pressure and resultant heat created during these earth movements converted the rocks into metamorphic schists, phyllites and quartzites which are seen today (See cross section B, Fig. 5). Exactly when these diastropic events took place is still open to question, but they probably occurred near the end of Ordovician time or perhaps millions of years later during the last phases of the Devonian Period.

Probably since late Paleozoic time the Coolidge Park area has been subjected to breaking-down rather than building-up processes. Eventually this whole region was reduced to a flat plane with only a few hills, called monadnocks, rising above the general flatness of the landscape. Still later in the history of Coolidge State Forest Park this flat plane was lifted high above its former position and streams began to shape the land into its present form. The more resistant rocks were lowered by the forces of erosion at a much slower rate than the less resistant types. The hills and valleys which you see today are primarily the result of this general uplift followed by the wearing away of the softer rocks at a more rapid rate than the harder rock types. Since the formations trend in a north-north-west direction, the hills and valleys are also oriented in this general direction, as the different resistive qualities of the rocks are presented in this pattern.

The story is not complete without mention of the last modifying influence to affect this region. Approximately one million years ago the first of perhaps three glacial advances began. These large glacial masses, termed continental glaciers, slowly advanced southward over the northern sections of the United States and covered a vast region with a thick sheet of ice. As these ice masses rode over the Coolidge Park region they tended to round-off irregular features of the landscape and to fill in certain low regions with their transported rock debris. The rather smooth outlines of the hills in Coolidge Forest Park and the sand and gravel deposits so apparent adjacent to nearby streams tell the story of these glacial advances and subsequent withdrawals. Other evidence of the overriding ice is seen in the presence of scratch marks, termed glacial striae, which were produced by fragments of rock as they were dragged by the moving ice across the underlying rocks. Along the east bank of Broad Brook and approximately two hundred yards south of the bridge at Five Corners, abundant glacial striae are clearly displayed. The orientation of these striae indicates that the glacier producing them moved from north north-west to the south south-east.

Since the recession of the last ice sheet, which took place several thousand years ago, the general appearance of the Coolidge Park region has changed very little. The streams have cut their valleys somewhat deeper and some of the glacial sands and gravels have been redistributed. A few thousand years ago Coolidge State Forest Park looked quite the same as it does today (See cross section C, Fig. 5).

SIDE TRIPS OF GEOLOGICAL INTEREST

Gold Panning

There are several locations immediately adjacent to the Calvin Coolidge State Forest Park where some gold can still be obtained from stream sands and gravels. The abandoned town of Five Corners offers the park visitor ample chance to “try his luck.” In fact, several local people still obtain small gold nuggets and a good deal of gold dust from the streams in the vicinity of Five Corners (See geologic map for geographic location). Other possible “panning” locations would include Reading Pond Brook, Buffalo Brook and Kingdom Brook. The gold which is found is probably derived from nearby gold-bearing quartz veins.

The equipment needed for an afternoon of panning is relatively simple to obtain. A wide, rather shallow pan with gently sloping sides is really all that is needed. Gravel and sand, which you find concentrated on the upstream side of natural rock riffles in the stream bed should be placed together with water in your pan. Then the pan should be rotated and the lighter rock materials decanted. The heavy gold will be concentrated in the bottom of the pan.

Asbestos and Talc

For you who might be interested in mineral collecting it is suggested that a trip to an old pit near Five Corners might prove rewarding. Walk down Broad Brook from the bridge at Five Corners until you locate the first small tributary entering on your left. Turn left up this tributary and continue to walk for approximately one hundred yards. On your left you will see the remains of a pit dug into the side of the stream bank which, at this location, is lined with rocks of the Ottauquechee formation. You will quickly note the fibrous asbestos in the walls of this pit; however, if you wish to collect specimens of this mineral be extremely careful and avoid standing beneath any overhanging rock material. Talc, which feels “soapy” to the touch, is also abundant in this immediate area. This asbestos and talc occur in a lenticular-shaped body which is perhaps eight hundred feet wide and of an undetermined length. It is not certain when these minerals formed; however, it was probably in the early history of the region.

Garnet

Good specimens of garnet can be found in the rocks of the Bethel formation. These garnets are not of gem quality but still offer good collecting for the amateur mineralogist. Good collecting localities exist along State Highway 100A and at several different spots within the Park itself. A hike along the northern boundary of the Park would intersect several excellent collecting spots.