[Illustration: GEOLOGIC MAP OF CALVIN COOLIDGE STATE FOREST PARK INDEX MAP OF VERMONT]
EXPLANATION TRAIL SECONDARY ROAD (GOOD) SECONDARY ROAD (POOR) STATE ROUTE 100A STATE PROPERTY LINE CONTOUR LINE WITH ELEV. STREAM FOLIATION { FORMATIONS} MISSISQUOI FORMATION BETHEL FORMATION OTTAUQUECHEE FORMATION PINNEY HOLLOW FORMATION TOPOGRAPHY FROM U.S. GEOLOGICAL SURVEY MAP GEOLOGY BY H. W. DODGE, JR.
The Pinney Hollow formation, which is an estimated 2,300 or more feet thick, is primarily composed of pale-green _schist_[5] 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.
[Illustration: Figure 3. The Pinney Hollow Historical Monument embedded in the Pinney Hollow formation. The darker rock consists of pale-green schist and the white areas are quartz layers and lenses.]
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_[6] 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.