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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: 12 September 2016