THE ROCKS OF CALVIN COOLIDGE STATE FOREST PARK AND THE STORY THEY TELL
The rocks of Calvin Coolidge State Forest Park can be divided into four distinct units or formations[4]. The [geologic map] shows these formations as distinct bands, each band representing a different formation and its intersection with the ground surface. Each formation is, so to speak, stacked upon the next older with the oldest forming the western band and the youngest on the east. Remember that these metamorphic rock formations were originally sedimentary in nature and were deposited in a nearly horizontal position on the floor on an ancient sea. The fact that they are now tilted indicates that earth movements have taken place sometime after the sedimentary rocks hardened but before the present time.
The Pinney Hollow formation, the oldest formation of rocks seen in the Forest Park, was named for the excellent exposures of this formation in Pinney Hollow, a valley located along the western border of Coolidge Forest. You can readily see this formation, in its most typical development, along State Highway 100A about three-tenths of a mile south of the Park entrance road. Here, a historical monument which commemorates the naming of Pinney Hollow, is embedded in this formation (See photograph, [Fig. 3]). The lean-to, tent platform and picnic areas are situated on this formation and its rocks are easily observed. It is interesting to note that some of the Pinney Hollow rocks seen in these camping and picnicking spots differ from the typical rocks seen along State Highway 100A. In fact, some of the rocks which are dark and are classed as phyllites, can be confused with the overlying Ottauquechee rocks. A transition zone, several hundred feet wide, exists between these two formations and in which the two distinct rock types are intermingled. This “mixture” zone indicates that there was no break in the original deposition of these two formations and therefore the rock record during this time is complete.
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.
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