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.
[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.
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.
[Illustration: Figure 4. The Ottauquechee formation as exposed
approximately nine-tenths of a mile north of the Park entrance road
and on the west side of State Highway 100A. Note the massive
quartzite layers above, the black phyllite below.]
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.
[Illustration: Figure 5. Diagrammatic cross sections illustrating
the geologic history of the Calvin Coolidge State Forest Park.
Vertical scale greatly exaggerated. Patterns to designate formations
in cross section A are repeated in cross sections B and C.
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.