Philadelphia Geological Society








Notable Geologic Features of the Philadelphia Region


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




French Creek State Park


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.



• 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.

• French Creek State Park



Nockamixon State Park


Location: Bucks County

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.



• Inners, J. D., Nockamixon State Park: Rocks and Joints, Pennsylvania Trail of Geology, Park Guide 14, Bureau of Topographic and Geologic Survey.

• Nockamixon State Park



Ringing Rocks County Park


Location: Bridgeton Township, Bucks County

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.



• Ringing Rocks County Park

• Geyer, A. R., Hickory Run State Park: Boulder Field, Pennsylvania Trail of Geology, Park Guide 2, Bureau of Topographic and Geologic Survey.



Ringing Rocks


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.



State-Line Serpentine Barrens


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.



• Nottingham County Park

• Nature Conservancy preserves

• Soldiers Delight Serpentine Barrens



Valley Forge National Historical Park


Location: Montgomery and Chester Counties

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.



• 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



White Clay Creek Preserve


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.



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.

• White Clay Creek Preserve



Wissahickon Valley


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 temperatures—400 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.



• Friends of the Wissahickon

• Fairmount Park Commission




Author: C. David Brown


Last revised: 12 September 2016