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French Creek State Park,
Berks and Chester Counties
Nockamixon
State Park, Bucks County
Ringing Rocks County
Park, Bucks County
Ringing Rocks, Montgomery
County
State-Line
Serpentine Barrens, Chester County
Valley Forge National
Historical Park, Montgomery County
White Clay Creek Preserve,
Chester and New Castle Counties
Wissahickon
Valley, Philadelphia County
Location: Berks and Chester
Counties
Physiography: Gettysburg-Newark Lowland Section of the Piedmont
Province
A wide range of rock ages can be seen in the park, from
some of the oldest rocks to the youngest volcanic rocks in Pennsylvania. Small
boulders of Precambrian gneisses, about 1 billion years old, are found east of
Mount Pleasant. Nearby are weathered blocks of late Precambrian
to early Cambrian (about 550 million years old) quartzite. A large area
of the park is covered by Triassic sandstones and conglomerates. These rocks
were deposited in a rift basin, probably underwater, during the breakup of the
supercontinent Pangea about 210-250 million years
ago. Lastly, a 200-million-year-old Jurassic diabase
sheet is exposed in the northeast area of the park. This magmatic intrusion,
also associated with continental rifting, was emplaced below ground and has
subsequently been exposed by erosion. Magnetite ore bodies formed by this
volcanic activity were the source of iron produced at the adjacent Hopewell
Furnace National Historic Site.
Resources:
Inners, J. D., and W. B. Fergusson, French Creek State Park: Piedmont Rocks and Hopewell Furnace, Pennsylvania Trail of Geology, Park Guide 6, Bureau of Topographic and Geologic Survey, 1996.
Physiography: Gettysburg-Newark Lowland Section of the Piedmont
Province
This park, located in the early Mesozoic Newark
basin, exhibits examples of sedimentary, igneous, and metamorphic rocks in a
continental rift environment. In the Late Triassic Period (about 220 million
years ago), the supercontinent Pangea began to rift
apart, eventually forming the Atlantic Ocean. The tectonic stresses associated
with the rifting also disrupted what is now the North American continental
margin, and the Gettysburg-Newark basin is one manifestation of these forces.
The elongate, northeast-trending basin was sometimes filled with shallow lakes
and at other times contained rivers and floodplains. The sediments deposited in
the basin during the Late Triassic and Early Jurassic can be seen in the park today as the shales and
siltstones of the Lockatong and Brunswick formations.
A portion of the diabase
sill, or sheet, is also exposed in the park. The rifting of Pangea
caused upwelling of magma from the mantle. The magma nearly reached the surface
and intruded the sedimentary rocks in the basin about 200 million years ago.
Heat from the diabase sill metamorphosed the adjacent
rocks; hornfels, the resulting contact metamorphic
rock, can also be found in the park. Hornfels was
removed from the Tohickon quarry for construction. At
the quarry one can find hornfels samples containing
large epidote crystals, up to two inches across.
Resources:
Inners, J. D., Nockamixon State Park: Rocks and Joints, Pennsylvania Trail of Geology, Park Guide 14, Bureau of Topographic and Geologic Survey.
Physiography: Gettysburg-Newark Lowland Section of the Piedmont
Province
This park encompasses a diabase
boulder field, one of the largest in the eastern United States. The boulder
field formed during the Pleistocene Epoch ice ages (10,000 to 1.8 million years
ago). Although this part of the state was not glaciated, the cold climate
caused the ground to be frozen as permafrost. This tundra environment was
accompanied by periglacial geologic activity, and the
boulder field formed by two distinct periglacial
processes. First, the well jointed (that is, fractured) diabase
bedrock was progressively broken apart by cyclic freezing and thawing of water
in the cracks (frost wedging). Second, these large fragments slowly moved down
a gentle slope to accumulate into a field (solifluction).
In the summer, the surficial ice would melt, and the
water-saturated soil would lubricate the gradual creep of the boulders downhill
on top of the permafrost.
Today the boulders field is called
"Ringing Rocks" because the boulders emit distinct tones when struck
by a hammer. This unusual property is a consequence of the iron content of the diabase. The size of the boulder and its contacts with the
adjacent boulders also affect the sound.
Resources:
Geyer, A. R., Hickory Run State Park: Boulder Field, Pennsylvania
Trail of Geology, Park Guide 2, Bureau of
Topographic and Geologic Survey.
Location: Lower Pottsgrove Township, Montgomery County
Physiography: Gettysburg-Newark Lowland Section of the Piedmont
Province
This diabase
boulder field, similar to Ringing Rocks County
Park, is located in Ringing Hill Fire Company Park. Directions: 1.2 miles
north of Pottsdown on Pennsylvania Route 663 (North
Charlotte Street); enter at intersection of highway and White Pine Lane.
Location:
Chester County
Physiography: Piedmont Upland Section of the
Piedmont Province
Serpentinite is a metamorphic rock, composed partly of
the mineral serpentine, formed by hydrothermal alteration of rocks usually
found in the upper mantle. Serpentinites are often
associated with ophiolites, sections of the oceanic
crust and mantle that have been tectonically emplaced on continental margins
during subduction. Owing to a high level of toxic
metals and a deficiency in nutrients, serpentinite
outcrops sustain only certain types of plants. These desert-like barrens are
ecologically unique and host rare plant and animal species.
The chain of serpentine barrens found along
the Pennsylvania-Maryland border is one of only three such occurrences in North
America. Called the Baltimore Mafic Complex, the underlying
serpentinite body formed in the Cambrian Period
(about 490 million years ago) and was probably deformed and attached to the
continent by the Taconic orogeny in the Ordovician
Period (approximately 450 million years ago). The igneous precursor rocks to
the serpentinites may have originated as part of an
oceanic plate or perhaps as a magmatic intrusion into the crust of an island
arc. The barrens were a major locale for chromite
mining in the 19th century.
In addition to Nottingham County Park, serpentine
barrens can be explored at several Nature Conservancy preserves, including the
Chrome and Goat Hill Serpentine Barrens in Chester County.
Physiography: Gettysburg-Newark Lowland Section of the Piedmont
Province
The geologic units found in
the park are primarily Cambrian and Triassic sedimentary rocks; the
unconformity that exists between rocks of these two ages represents a time gap
of some 300 million years. The Cambrian units include detrital
sedimentary rocks of the Chickies and Antietam
Formations (Chilhowee Group), which include
sandstones, siltstones, and mudstones, as well as chemical sedimentary rocks
such as the Elbrook and Conestoga Formation limestones and the Ledger Formation dolostone.
These Cambrian strata reflect a time of changing sedimentary environment as the
continental margin subsided and the sea deepened as it moved inland. Stromatolites in the Ledger dolostone
reveal evidence for algae colonies some 500 million years ago.
Folding and other deformation
in these rocks was caused by the Taconic orogeny
which occurred in the Ordovician (about 450 million years ago) as well as
subsequent Paleozoic orogenies. The breakup of Pangea about 250 million years ago formed the Newark rift
basin wherein the Triassic Stockton formation red sandstones and shales were deposited. The sedimentary environment at that
time was above sea level, and sediments were deposited from streams and
sometimes in lakes. In the park one can see the Cambrian dolostone
in contact with the Triassic sandstone although these two rocks formed over 300
million years apart. Any geologic record from that interval has been erased by
erosion, leaving only the unconformity.
Resources:
Wiswall, C. G., Valley Forge National Historical Park: The Geologic History, Pennsylvania Trail of Geology, Park Guide 8, Bureau of Topographic and Geologic Survey, 1993.
Valley Forge National Historical Park
Location:
Chester County, Pennsylvania and New Castle County, Delaware
Physiography: Piedmont Upland Section, Piedmont Province
Metamorphic rocks
of the Wissahickon Formation (Glenarm
Supergroup) are displayed in the preserve. The parent
rocks of the Wissahickon were mudstones, siltstones,
and sandstones that were deposited from the late Precambrian through the
Ordovician. Metamorphism occurred during the Late Ordovician Taconic orogeny (about 450 million years ago); the resulting rocks
seen here are schists. Examples of amphibolites are
also found; they are probably the product of metamorphosed volcanic basalt
flows and/or diabase sills. Pegmatites
(large crystals of quartz and feldspar) were hydrothermally deposited in the Wissahickon schists. These
materials were uplifted in the Devonian Period (approximately 380 million years
ago) by the Acadian orogeny.
Resources:
Faill, R. T., White Clay Creek Preserve: A Scenic
Valley and the Arc Corner, Pennsylvania/Delaware
Trail of Geology, Park Guide 20, Pennsylvania Topographic and Geologic
Survey, 1991.
Location: Philadelphia City and
County
Physiography: Piedmont Upland Section, Piedmont Province
Wissahickon Valley is a rugged area of Fairmount Park
bordering Wissahickon Creek. This is a great place to
see early Paleozoic metamorphic rocks; the best outcrops are on the hiking
trail along the east side of the creek. These rocks were originally sediments
deposited in the shallow Iapetus Ocean covering
Pennsylvania in the late Precambrian, Cambrian, and Ordovician (roughly 600 to
460 million years ago). When compacted, these materials formed clay-rich
sedimentary rocks (for instance, shale).
In the Ordovician Period, the eastern edge of
the North American plate began subducting beneath an
offshore island arc in the Iapetus Ocean. Continuing subduction brought the island arc closer to Pennsylvania,
leading to its eventual collision with the continent in the Late Ordovician
(about 450 million years ago). This event is called the Taconic orogeny. A mountain belt formed northeast of Pennsylvania,
but metamorphism and deformation also occurred in the state. The seafloor sedimentary
rocks were buried and subjected to high pressures and temperatures400 to 600
degrees Celsius (750 to 1100 degrees Fahrenheit). This metamorphism converted
the strata to rocks such as schist and gneiss. Some of the schists
in the park contain small garnet crystals, recognizable by their dark red,
translucent appearance. Other outcrops demonstrate that the rocks experienced
stresses causing them to flow during metamorphism.
Resources:
Last revised: 25 May 2009