Structure of a Waterfall EarthCache
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Structure of a Waterfall: Kent
Falls State Park
Quick Description:Kent Falls Brook cascades about 250 feet
over a distance of almost a quarter mile. The highest single
drop, however, is only about 40 feet. This EarthCache
explores the erosional and tectonic forces that created one of the
highest waterfalls in Connecticut.
Location: N.
41o46.58’, -073o25.08’
(Parking Lot Kiosk)
Listed by: CTGEOSURVEY
Purpose: This EarthCache is created by the Connecticut
Geological and Natural History Survey of the Department of
Environmental Protection. It is one in a series of EarthCache
sites designed to promote an understanding of the geological wealth
of the State of Connecticut.
Kent Falls is one of the highest waterfalls in
Connecticut. Kent Falls Brook cascades about 250 feet over a
distance of almost a quarter mile. The highest single drop,
however, is only about 40 feet. Nonetheless it is a
spectacular natural site in Connecticut. Leary (2004,
p.211-212) provides a nice description of the park and its
resources.
Waterfalls usually develop where harder rock is underlain by or
juxtaposed adjacent to softer rock. Hard and soft are used here
merely to denote the material’s relative resistance to
erosion. At Kent Falls, the rocks at the very top are schist
and gneiss which are relatively hard. The rocks over which
most of the fall cascades are marble, calc-silicate gneiss and
schist which are relatively soft.
Kent Falls is located north of the Village of Kent on U.S. rte.
7. Attendants at a kiosk at the entrance to the parking lot
collect a parking fee on weekends and holidays from May
31st through Oct 31st. The kiosk is
located at: N. 41o46.58’,
-073o25.08’.
Figure 1. Geologic and topographic map
(contour interval=10’) of Kent Falls State Park and the
immediate surrounding area. The major river of the area is
the Housatonic River (west of Rte. 7). Kent Falls Brook
flows into the Housatonic River. Kent Falls Brook originates
in highlands underlain by Cambrian aged Manhattan Schist
(Єma). This is underlain by younger rocks, the
Walloomsac Schist (and gneiss) of Ordovician age (Owm). A
major thrust fault separates the two rock formations. The
Walloomsac is underlain by the Stockbridge Marble of
Cambro-Ordovician age (O Є).
Kent Falls Brook flows out of highlands (see Figure 1) underlain by
Cambrian rocks of the Manhattan Schist, which are relatively
resistant to erosion. Kent Falls Brook then cascades from an
elevation of about 650’ over rocks of the Ordovician
Walloomsac Schist and Cambro-Ordovician Stockbridge Marble to an
elevation of about 430’.
The Walloomsac Schist is younger than the Manhattan Schist, so the
observation that it underlies the Manhattan is of some
interest. Normally one expects overlying rocks to be younger
than underlying rocks (Law of Superposition). In this case,
however, an early phase of mountain building (the Taconic event)
that lead to the formation of the Appalachian Mountains caused a
slice of older rock from east of here to be thrust over the
Walloomsac Schist (see Skehan, 2008, p.200-206 for a concise
account of the geologic history of western Connecticut and p.
232-235 for a description of the rocks at Kent Falls State
Park; Leary, 2004, also provides brief geologic
background; Coleman, 2005, provides more detailed, yet highly
readable, background).
An improved trail leads to the following location:
Location: N. 41o46.49’,
-073o24.82’. It is steep in places,
however, has numerous flights of stairs and, therefore, is not
wheelchair accessible. Be careful if you venture out into the
river: some of the surfaces are coated with algae, making
them extremely slippery. Note also the many cliffs and steep
slopes that could pose a hazard to small children. You can
download a description by the Connecticut Geological Survey of the
geologic features along the trail
Geology of Kent Falls State Park and a trail map for Kent
Falls State Park before you visit the park.
Map of Kent Falls State Park
Observations. Keep track of the rocks in the falls as
you go. Notice the tilt (dip) and composition of the layers
(see Figure 2). Some of the layers dip steeply,
70o or more, to the west. Others dip about
45o toward the east. Some of the layers are
thin-bedded and appear dark gray or black (Figure 2A); some of the
thin bedded layers are schistose and contain biotite mica.
Some layers are thick and are a lighter color: these are
marble. The upper part of the falls is underlain by
Walloomsac Schist and Manhattan Schist (Figure 3). The
Manhattan Schist is garnet bearing and can be distinguished from
the Walloomsac because of that. Folds in the rocks are not
easy to see. Figure 2 shows cross sections of two of the few
folds that can be easily seen. Fold are anticlines (up-fold)
and synclines (down folds).
Skehan (2008) thought that the alternating dip direction and
repetition of stratigraphy indicate that Kent Falls Brook cascades
over a series of folds in the rock (see Figure 4). The folds
were created by compressive earth stresses that caused the rock
layers to buckle several hundred million years ago. At the
time the rock layers were buried beneath younger rocks and were
under great pressure and elevated temperature. When rocks get
hot they tend to fold rather than break when sufficient stress is
applied to them. The steeper westerly dips on the folds
suggest that the stress necessary to fold the rocks was directed
from east to west, which is the same orientation as the stress
necessary to thrust the Manhattan Schist over the younger
Ordovician rocks (the fault is exposed in the upper part of the
falls, but it is difficult to recognize …it likely is at the
base of the upper falls illustrated in Figure 3A and schematically
shown on Figure 4).
If you did not notice potholes on the way up, look for them on the
way down in the various ledges and pools of the falls.
Figure 2.
Complex folds in the lowermost fall. Picture to above right;
sketch of the outcrop on above left. Beneath the outcrop
sketch is our interpretation of what the wall of a trench blasted
out of the ledge would look like (cross section). The rocks
have been squeezed into a complex series of folds. This
happened millions of years ago when the rocks were buried several
kilometers deep and were hot. The up-folds (look like an
inverted letter U) are called anticlines; the down folds are called
synclines.
Figure 3. Anticlinal fold in bed of Falls
Brook below the upper most fall (see Figure 3). Sketch of the
layers is shown on right.
4A.
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4B.
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4C.
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Figure 4.
A. Manhattan Schist at top of Kent Falls. Upper-most fall
plunges along a fracture, over a garnet-mica schist. Fault
may be under the overhang at base of fall.
B. At top of upper fall, a fold in schist dips about
45o to east at right, but about 45o toward
the west on left side.
C. Brownish-pink garnets of moderate size in Manhattan Schist
adjacent to top of upper falls.
Figure 4.
Schematic sketch-diagram (no scale followed) showing profile of
Kent Falls and a vastly simplified of the interpreted rock
structure. Upper right is slice of the Cambrian age Manhattan
Schist that is thrust over (on top of) the younger Ordovician age
Walloomsac Schist. The Walloomsac overlies the
Cambro-Ordovician Strockbridge Marble. All the layers have
been folded (only three folds are shown schematically here;
there are many more). The folding causes repetition of
erosion- resistant layers of the Strockbridge Marble, forming the
waterfall cascade.
References:
Coleman, M.E., 2005, The Geologic History of Connecticut
Bedrock. State Geological and Natural History Survey of
CT, Spec. Pub. 2, 30p.
Leary, Joseph, 2004, A Shared Landscape: A Guide and History of
Connecticut’s State Parks and Forests: Friends of
State Parks and Connecticut Department of Environmental Protection,
240p.
Skehan, James W., 2008, Roadside Geology of Connecticut and
Rhode Island: Mountain Press, Missoula, MT., 288p.
How to get credit for this EarthCache:
1. In your log, include an image of you, your companions, or your
GPS unit at the base of the upper fall.
2.Locate a fold in the layers and send an explanation of why you
think it is a fold. Also include a photo of the fold in your
log.
Difficulty: 1.
Terrain Rating: 2.5 Toddlers and very
young children will not have an easy time on the trail, but if
it’s a nice warm summer day the whole family can have fun in
pools at the base of the falls. The lower fall (and picnic
area) is handicapped accessible.
This EarthCache was written by Randolph Steinen and
Teresa Gagnon for the Connecticut State Geological Survey, a part
of the Connecticut Department of Environmental Protection.
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