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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: 22 March 2002