Shelter Caves at Chatfield Hollow State Park
The rock in Chatfield Hollow State Park is granite gneiss with
minor pegmatite and a few quartz veins. The rock has an interesting
set of fractures that has led to the formation of non-solutional
caves. These caves are formed by frost wedging and rock fall. The
caves are referred to as shelter caves. According to Leary, “Many
native (American) artifacts have been found here, attesting to the
caves’ probable use as shelters.” This EarthCache explores some of
the caves.
Chatfield Hollow State Park provides opportunities for many
activities including swimming, hiking and picnicking. It has a pond
for swimming (changing houses also), picnic shelter, numerous
tables and barbeque sites, and lots of hiking trails, many of which
climb up and over rock outcrops. A trail mapmay be downloaded from
the Chatfield Hollow State Park web site which also contains a
brief description of the geology:
http://www.ct.gov/dep/cwp/view.asp?a=2716&q=325182&depNav_GID=1650
Purpose. This EarthCache is published 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 and
biological wealth of the State of Connecticut and provide
educational outings for families.
Location: N.41 o21.893’, -072 o35.088’,
PARKING LOT
THIS EARTHCACHE INVOLVES CLIMBING ON AND OVER ROCKS: YOU WIL
NEED TO USE YOUR HANDS IN SOME SPOTS. IT ALSO INVOLVES WALKING
ALONG NARROW LEDGES AND CLIFF EDGES. HENCE, THIS EARTHCACHE IS NOT
APPROPRIATE FOR YOUNG CHILDREN OR FOR THOSE WITH AMBULATORY
PROBLEMS.
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Figure 1. Examples of trail roughness. Green blaze may be seen
on trees in left two pictures and on rocks on right picture. Notice
fractures in ledge. |
Directions: Chatfield Hollow State Park is located in the
town of Killingworth, about 15 miles east of New Haven. It is on
State Route 80 about 1.2 miles west of the intersection of Rte. 80
with Rte. 81. The park entrance is on the north side of Rte. 80
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Figure 2. Entrance to Chatfield Hollow State Park in the winter.
Figure 3. Topographic map of the southern part of Chatfield Hollow
State Park and the neighboring Cockaponset State Forest (to east).
Notice the topographic grain of the area (north-south alignment of
hillsides). This is caused by a local set of fractures in the
bedrock. Note also the north-northwest/ south-southeast alignment
of Chatfield Hollow River and Schreeder Pond, which is a regionally
prominent fracture trend. |
Closed during the winter, you can hike in from the parking lot.
An entrance (parking) fee is required during the summer. A free
parking lot (see GPS data above) is found immediately before the
entrance gate.
The rock at Chatfield Hollow is composed of granite gneiss
(pronounced “nice”). Granite gneiss is made-up of small sized
grains of plagioclase feldspar, quartz, and biotite. It has a
“foliation” which is defined by concentrations of biotite (black
mineral) in layers, giving the rock a “grain”. The rock formed when
molten granitic rock (magma) forced its way up and into surrounding
rocks and cooled. This happened when the area was deep underground,
covered by rock that has since eroded away. Geologists at Indiana
University think the magma cooled and congealed about 340 million
years ago. A metamorphic event occurred about 300 million years ago
that produced the foliation. There are also small amounts of
pegmatite, a coarsely crystalline rock of granitic composition.
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Figure. 4. Boulders in parking lot illustrate the two
predominant rock types found at the park. Image on the left is
fine-grained granite gneiss that contains plagioclase feldspar,
quartz and biotite. The biotite is concentrated in layers that may
be seen beneath the pen. Image on the right shows pegmatite. Notice
the mineral grains that make up the rock are larger that those of
the gneiss. Largest crystals are plagioclase feldspar, microcline
feldspar, and quartz. Muscovite mica may or may not be present in
the pegmatite (here it is). Pen used for scale is 5.5”
long. |
What we are interested in is the way the rock has fractured and
how glacial erosion has taken advantage of those fractures to pull
or pluck large blocks of rock off the outcrop. These processes,
along with rock-falls, have produced several caves. Take time
looking at the fractures during your hike.
Activity 1. From the parking lot, follow the entrance road north
to the Chimney Rock
Trail head It has a green blaze to guide your hike. Follow the
trail to the location: N.41o21.947’,
-072o35.118’. . This is a rough unimproved trail. Notice
the fractures in the rocks as you pass. There are three prominent
sets: two that are nearly vertical and one that is roughly
horizontal. Note also that the foliation (layering) here is also
nearly horizontal. Horizontal fractures form near the surface of
the earth in many rocks as a result of “unloading”. This occurs
when the weight of the former overlying rock is removed during
erosion. It takes a long time to occur, but the rock eventually
fractures in sheets roughly parallel to the Earth’s surface. The
process is called exfoliation. This occurs most often in rocks that
are even grained and in rocks with planes of weakness that are
parallel or nearly so to the Earth’s surface. Foliation planes are
naturally weak areas of the rock. (Stone masons use the property of
foliation to split the stones during their work.) Here the
foliation planes are nearly parallel to the earth surface and the
rock has exfoliated (see Figure 5-left). This allows rock to slide
horizontally and/or break off the outcrop in large slabs.
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Figure 5. Activity 1 area. Image on left shows the fractured
outcrop; note exfoliation fractures (nearly horizontal fractures
that follow the rock foliation - layering). A second set of
fractures is nearly vertical and is perpendicular to the cliff
face. A third set of fractures is not as apparent but is parallel
to the cliff face. The center and right images show two caves near
the Activity 1 coordinates. The cave on left was formed by rocks
that were dislodged from the ledge and fell in front of an
overhanging layer. Note additional shelters above the cave. If you
look carefully you can see that the cave on right was formed when
large boulder on right was dislodged from the ledge and wedged
outward. |
Sliding of large slabs off the outcrop so that they lean against
the stationary rock is one of the ways shelter caves form. Another
way the caves form is by large slabs being forced out from the
outcrop leaving an overhang (Figure 5-center). In some places the
rock also forms an outside wall to the cave (Figure 5-right). The
force needed to wedge rocks away from the ledge was probably ice
crystallization and resulting expansion. Crystallization of ice is
accompanied by a 9% volume increase. Summer melt-water freezing in
a fracture during the winter can create a pressure up to 2000
pounds per square inch, a pressure more than that necessary to
cause rocks to fracture and fractures to expand. Many rocks broken
and forced away from the ledge during glaciation are taken up by
and may be moved or removed by the glacier. The process is called
glacial plucking.
Continue on the Chimney Rock Trail until it returns to the
starting point. Although it is only a quarter of a mile in
distance, the terrain is difficult, so take your time. Notice three
prominent fracture orientations on this hike.
Cross to the other side of the road and find the continuation of
the trail at the eastern edge of a grassy area. This trail is about
a mile round-trip. The trail is not as well marked here: black
paint has been painted over the green blazes. The trail goes into
the woods and over a bridge that crosses a wet area. Some time
recently a tree fell onto the bridge both damaging it and blocking
the wetland crossing.
Another ledge with shelter caves can be seen east of the bridge.
Eventually the trail passes a cave and indeed goes right into it
(N.41o21.893’, -072o34.946’). The trail goes
through a hole in the roof of the cave (see Figure 6). Here you
will need your hands to help your climb through the cave. The ledge
at the top is slanted and narrow and care must be taken to keep
your balance. Nonetheless, the cave is fun to explore although it
is not very large. This is another area where you can measure the
orientation of fractures.
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Figure 6. Cave through which the trail climbs. Left picture
shows ledge on right and fallen rocks on left and on top of cave.
Picture on right shows top entrance to cave. Note the various
orientations of the foliation, testifying that the rocks are broken
from the outcrop. |
Following the trail out the top of the cave will bring you again
to green blazes on the trees. The trail becomes easier. Follow the
green blazed trail to Buck Road and follow the road to the main
part of the park. The parking lot will be toward your left.
This whole area of Connecticut contains other shelter caves.
Another EarthCache takes you to some shelter caves just “around the
corner”.
http://www.geocaching.com/seek/cache_details.aspx?wp=gc18fdc
(The web-site for that cache is not linked: you will have to
copy and paste it into your server.)
How do people log this EarthCache?
1. Measure the orientation of either set of vertical fractures
with your GPS unit. To do this, you could take two readings along a
fracture you can follow for at least 50 feet and use geometry to
determine the orientation of the line connecting the two points. If
your GPS unit has a compass function, stand facing the direction
the fracture goes (or walk along the fracture) and use your unit to
determine the direction you are facing (walking). Note that the
many of the cliff faces are defined by a fracture.
2. Submit a picture of you or a companion at the entrance of one
of the caves in Activity 1.
Difficulty. 1
Terrain: 4
Type of land: State Park and State Forrest
EarthCache category: Geomorphology
References.
Leary, Joseph, 2004, A Shared Landscape: A Guide and History of
Connecticut’s State
Parks and Forests. Friends of Connecticut State Parks, CT D.E.P,
and CT Forest and Park Association, Hartford, CT, 240p.