Geology of Pennsylvania
The Geology of
Pennsylvaniaconsists of six distinct physiographic provinces, three of which are subdivided into different sections. Each province has its own economicadvantages and geologic hazardsand play an important role in shaping every day life in the state. They are: (listed from the southeast corner to the northwest corner) the Atlantic Coastal PlainProvince, the Piedmont Province, the New England Province, the Ridge and Valley Province, the Appalachian PlateauProvince, and the Central Lowlands Province. [Sevon, W.D. (2000). Physiographic Provinces of Pennsylvania, Map 13. Pennsylvania Geologic Survey. Harrisburg, PA.]
A majority of the rocks in Pennsylvania exposed at the surface are
sedimentaryand were deposited during the Paleozoic Era. A total of four orogenies have affected the rocks of the Commonwealth including the Grenville orogeny, the Taconic orogeny, the Acadian orogeny, and the Appalachian orogeny. The Appalachian event has left the most evidence and has continued to shape the landscape of the state.Schultz, C.H. ed. (2002) The Geology of Pennsylvania. Special Publication No.1. Pennsylvania Geologic Survey and Pittsburgh Geological Society.]
glaciershave also repeatedly visited the state over the last 100,000 years. These glaciers have left some evidence and carved out much of the landscape of the northern tier of the state..]
A rock with high economic value from Pennsylvania is
Anthracite coal. Before mining began, there was an estimated 22.8 billion tons of anthracite in Pennsylvania. As of 2001, 12 billion tons still remain in the ground, most of which is not economically feasible to mine.Edmunds, W.E., (2002), Coal in Pennsylvania (2nd ed.): Pennsylvania Geological Survey, Educational Series 7, p. 17.] American geologistsrecognized the importance of Pennsylvania's coal region and named the Upper CarboniferousPeriod the PennsylvanianPeriod because of the abundance of coal in the state. Despite this, Celestine was proposed as the state mineral in 2002. The proposal however, was not approved by the state legislature. [ [http://www.dcnr.state.pa.us/topogeo/collecting/rocks.aspx Pennsylvania Geological Survey: Rocks and Minerals ] ] [Edwin, C., (2003). Celestine, the Proposed State Mineral. "Pennsylvania Geology", Pennsylvania Geological Survey, Harrisburg, PA. V 33 n 1. ]
Pennsylvania is also home to the famous Drake Oil Well in
Titusvillewhich help give rise to the modern oil industry and two brand name motor oils, Quaker State (now owned by Royal Dutch Shell) and Pennzoil.
Atlantic Coastal Plain
One of the smallest provinces in the state and is confined to
Philadelphia, Delaware, and Bucks counties along the Delaware River. Local relief is less than convert|200|ft|m|0 and much of the bedrockis buried under recent alluvialdeposits. The traditional boundary of the coastal plain is the fall line. The coastal plain in Pennsylvania was once home to thousands of acres of fresh water tidal marsh. This was important in the early development of Philadelphia and Chester. [ [http://www.fws.gov/northeast/heinz/welcome.htm Welcome to John Heinz National Wildlife Refuge at Tinicum ] ] Many of the small tributaries to the Delaware have cut small but impressive gorges into the bedrock including the Ridley Creek, the Chester Creek, and the Wissahickon Creek. Flash floodsare becoming a local problem in the province..]
The piedmont in Pennsylvania is divided into three distinct sections: the Piedmont Uplands, the Piedmont Lowlands, and the Gettysburg-Newark Lowlands. Much of the Piedmont is becoming
urbanizedand developed. Some of the best farmland in the state is in this region, specifically Lancaster and Chester counties.
This section is characterized by the
metamorphic rocksthat provide much of the bedrock for this area. The oldest exposed rocks in Pennsylvania are found here and consist of the Baltimore Gneiss.Blackmer, G.C., (2005). Preliminary Bedrock Geologic Map of a Portion of the Wilmington 30- by 60-Minute Quadrangle, Southeastern Pennsylvania. Pennsylvania Geologic Survey, Open-File Report OFBM-05-01.0.] These rocks have a complex history and a vast array of different minerals. Much of the rock was altered during the formation of Rodiniaduring the Grenville orogeny. These rocks eventually provided the platform for the deposition of sediment that would become the Wissahickon Formationduring a rifting of Rodinia. Sea floor spreading continued until a passive margindeveloped along the new Iapetus Oceanand a beach strandline developed. These sediments eventually became the Chickies Formation..]
Siliclasitc and carbonate deposition continued through the Cambrian and into the Ordovician period. During the Taconic orogeny, more igneous intrusions and metamorphism occurred as the ancestral Taconic Mountains were pushed up. The sediments that were deposited in a sea between an island-arc and the Iapetus eventually were squeezed and deformed along a
subductionzone. The sediments deposited in that sea are now located in the Great Valley section. (See below) The sediments placed from the rifting of Rodinia became the roots of the ancestral Taconics and went through their first wave of metamorphism during the Taconic orogeny. Additional waves of metamorphism continued up until the Alleghanian orogeny..]
The lowlands are underlain primarily by more easily eroded rocks such as
limestone, dolostone, and phyllite. These rocks are relatively younger in age then the surrounding uplands and are likely the result of a quiet stretch of shallow sea deposition. Some of the rocks deposited during this time are also found in the Great Valley section but have been separated by the Gettysburg-Newark Lowland section. Relief is low and generally never rises above convert|700|ft|m. Karstterrain is problematic in this section..]
This section is a bit misleading since there are hills as high as convert|1200|ft|m|0 in this section. It is separated from the rest of the Piedmont sections due to the distinctive rock types found here. Also called the Triassic Basin, most of the bedrock are red
sandstone, siltstone, and shale. A few formations are brown and black. The sediment accumulated during the rifting of Pangeain the Triassicage. Also, a basaltic igneous rockcalled diabaseformed dykes and sills later in the Jurassicas the Atlantic Oceanbegan to form. [Faill, R.T. (2004). The Birdsboro Basin. "Pennsylvania Geology", Pennsylvania Geologic Survey, Harrisburg, PA. V 34 n 4.] Much of the rocks from this area have been eroded away, but the more erosion resistant diabase has left hills and small elevated regions throughout the section. The erosion patterns of these rocks played a pivotal role in the Battle of Gettysburg. [Smith, R.C. and Keen, R.C., (2004). Regional Rifts and the Battle of Gettysburg. "Pennsylvania Geology", Pennsylvania Geological Survey, Harrisburg, PA. V 34 n 3.]
A small slice of
Paleozoicrocks, mostly carbonates, exists in Buckingham Township. These rock lie north of the Furlong fault which is an offshoot of the larger Chalfont fault. [Glaeser, J. D. (1966). Provenance, Dispersdal, and Depositional Environments of Triassic Sediments in the Newark-Gettysbrug Basin. Pennsylvania Geologic Survey, G43, Harrisburg, Pennsylvania.] Buckingham Mountainrises south of the valley and comprises quartzite. The other ridge is less prominent and is underlain by the conglomerates of the Stockton Formation. Karst is a localized problem in this area.
Only a small and fragmented province in Pennsylvania called the
Reading Prong. This is the southern end of the Hudson Highlandsof New Yorkand New Jersey(known as the Ramapo Mountainsin New Jersey) and the Taconic Mountainsof New York. The graniticrocks and quartzite of this area are highly metamorphosed and are Pre-Cambrian to Cambrianin age. Hills and ridges are locally steep and rounded at the top and form the hills around Reading, and to the south of the Allentown-Bethlehem-Easton metropolitan area. (See also South Mountain).]
Ridge and Valley
A region in Pennsylvania made famous by
NASA's LANDSATimages. This province is the second largest in the state and is home to the famous anthracite fields. The rocks here are severely folded and contain numerous anticlines and synclines that plunge and fold back over each other. There are numerous thrust faultsthat help create a chaotic mess. Most of the deformation is result of continent to continent collision during the Alleghenian orogeny. There are seven distinct regions of the province and they are listed below. Much of the drainage patterns in the province is trellis.
South Mountain is the northern tip of the
Blue Ridge Mountains. This region is characterized by broad flat ridges with deep narrow valleys. The rocks here are highly metamorphosed igneous and sedimentary rocks with some occasional dolomite. These rocks are Pre-Cambrian in age..]
Great Appalachian Valleyis a long broad valley that extends from Canadato Alabama. In Pennsylvania, the valley is known by three names: (listed from north to south)the Lehigh Valley, the Lebanon Valley, and the Cumberland Valley. Rocks that characterize this region include: limestone, dolostone, slate, shale, sandstone, siltstone, and some scattered basalt. Almost all of the rocks in the Great Valley in Pennsylvania are Ordovicianin age and were deposited during a quiet period before the Taconic orogeny. The limestones and dolostones of this area are extensively quarried in Pennsylvania. These carbonate rocksare used for variety of purposes including, crushed stone, cement manufacturing, fertilizers, and coal-mine dust (reduces acid mine drainage) [Barnes, J.H. and Smith, R.C., II, (2001). The non-fuel mineral resources of Pennsylvania: Pennsylvania Geological Survey, Educational Series 12.] Karst features are problematic in the Great Valley.
This region is not to be confused with the Blue Ridge Mountains but instead, represents the sharp
escarpmentseparating the Appalachian Mountains from the Great Valley. Many of Pennsylvania's water gaps cut through Blue Mountain including Delaware Water Gap, Lehigh Gap, Schuylkill Gap, and Susquehanna Gap. Also along the ridge, many " wind gaps" also exist. (see separate article) The rocks of the Blue Mountain section include mostly Silurianaged sandstone, conglomerate, siltstone, shale, and some limestone. Blue mountain is also known by the names: Kittatinny Mountain(especially in New Jersey) and Hawk Mountain. One of the most prominent rock types of this section is the Shawangunk Formationwhich is named after the Shawangunk Ridgeof New York.
Arguably the most complex and most studied section in the state. This area is home to one of Pennsylvania's most profitable coal fields ever, containing high-grade
Anthracitecoal. The sediments deposited during the MississippianPeriod came from highlands located to the southeast. Waves of mountain building occasionally brought coarser-grained sediments onto the plain. As the mountains eroded, the sediemnts became more fine-grained. As the highlands became more distant (or more eroded) the sands would grade into silt or clay.
North American Platewas near the equator, a tropicalclimate exisisted an allowed dense forests to flourish. Beginning in the Late Mississippian, forestsof Lycopodiophyta, Arthrophyta, Pteridophyta, and Pteridospermatophytabegan to grow in these plains.Oleksyshyn, J. (1982). Fossil Plants From the Anthracite Coal Fields of Eastern Pennsylvania. Pennsylvania Geologic Survey, Harrisburg PA, G72.] As the conditions became more favorable for the dense forests to survive for hundred of thousands of years, much of the dead plant material became preserved in oxygen-depleated environments. The accumulation of this material became preserved in the vast coal deposits. The rise and fall of the mountains, along with changes in sea level occurred numerous times. (often in conjunction) These cyclical stratagraphic events sequences are preserved in the rock record and are often called cyclothems. By the Late Permian, much of the coninental plate collision had subsided; the mountain building however, still continued. All of the sediments deposited during the previous 30 million years became folded and faulted as the Supercontinent Pangeawas finally formed.
It would take 150 million years for the mountains of this area to finally achieve the shapes we see today. These mountains are steep-sided and valleys are canoe-shaped, largely due to its complex folded structure. Most of the coal being mined from this section is from the Pennsylvanian-aged formations. Along with the
Mazon Creek fossilfield in Illinois, a tremendous amount of plant fossils have been studied from this area..] Landslides and acid mine drainage are two principle hazards of the area. In the past, underground mine fires have also been a threat. The Centralia Mine Fire is located within this section.
Detached from the rest of Pennsylvania's anthracite fields, this canoe-shaped valley is also known as the
Wyoming Valleyand is home to the cities of Scrantonand Wilkes-Barre. The whole structure of the section is a double plunging synclinewith sharp mountain ridges on either side of the valley. The ridges meet just north of Carbondale. The North Branch of the Susquehanna Riveralong with the Lackawanna Riverflow through this valley. Large-scale coal mining along with its accompanying industry, railroads, have long been abandoned.
Unlike the southern and middle anthracite fields, the anthracite valley has been recently glaciated repeatedly. This has left many talus slopes at the base of
Moosic Mountain, and the soils often contain large boulders making excavation difficult.
This region has also seen its landscape altered by glaciation and the
fluvialprocesses of the Susquehanna River. Most of the ridges in this region are parallel to the streams that drain the area. The Susquehanna also cuts through many of the mountain ridges leading some to believe that the Susquehanna is an ancient river system that existed even before the recent continental glaciation. (Some speculate as far back as the JurassicPeriod) [Inners, J. D., 1998, Rocks and ruins of the “Upper Grand”—An illustrated trail guide to the geology and historical archeology of Lehigh Gorge State Park: Pennsylvania Geological Survey, 4th ser., Open-File Report 98–03, 4 p., Portable Document Format (PDF).] None of the mountains in this section rise above convert|1700|ft|m|0 and the river valley is as low as convert|250|ft|m|0.
The standard long, narrow, and steep sided ridges with narrow valleys that define the state in LANDSAT photos. Many of the valleys have karst features due to carbonate rocks that reside in them. Road building generally follows the valleys and rarely cuts across the ridges. The
Pennsylvania Turnpikecut tunnels into the ridges rather than scale the mountain tops. Mount Nittany, Tuscarora Mountain, Jacks Mountain, Wills Mountainand Sideling Hillare five prominent mountains in this section.The section contains Cambrian through Pennsylvanian-aged sediments all deposited into the Appalachian Basin. During the Appalachian orogany, these sediments became folded, faulted, and moved around. Only during the past few million years has the landscape we see today take shape. The relatively softer or easily weathered rocks became valleys while the harder and erosion-resistant rocks became the mountain ridges. The development of this landscape continues to this day.
By far the largest province in the state, most of the rocks in this region are not folded and faulted and sit relatively flat. In western Pennsylvania, large
bituminous coalfields exist in rocks with a similar age as the rocks in the anthracite region. Many of the folds in the province are high amplitude and stretch for miles. In glaciated sections, steep canyons developed and much of the terrain have many glacial features. The drainage pattern in this area is dendritic.
Glaciated Pocono Plateau
The Pocono Mountain section of Pennsylvania is the same (geologically speaking) as the
Catskill Mountainsof New York. The red-green-gray sedimentary rocks of the Catskill Formationare the predominant bedrock type in the Poconos. The elevation of the plateau is between convert|1200|ft|m|0 and convert|2300|ft|m|0 with only a few steep hills such as Camelback Mountain (Big Pocono). Much of the rock sits in gently dipping horizontal beds, unlike the neighboring Appalachian Mountain section..]
Glaciated Low Plateau
Considered a part of the Pocono Plateau, this area lies to the north of the poconos and contains many of the same types of rock. The local relief is less than that of the Pocono region and bounded to the southeast by the Delaware River. The
Big Bushkill Creekcuts a gorge through this section and has many waterfallsespecially around the area of Resica Falls Scout Reservation. Dingmans Fallsand Bushkill Fallsare waterfalls within the Delaware Water Gap National Recreation Area. Also a part of the Endless Mountainsregion of Pennsylvania.
Glaciated High Plateau
Also an extension of the Catskill Mountains of New York, this section generally has higher elevations than the low plateau section as well as deeper valleys. Younger strata also outcrops in this area with a few minor coal beds. The uplands are rounded or flat along mostly broad hills. An excellent example of the escarpment that divides this section are Ricketts and Ganoga Glen located within
Ricketts Glen State Park.
This section is home to the
Grand Canyon of Pennsylvaniaand some of the most remote areas of the state. As the name implies, the streams of this area have cut deep valleys with steep sided slopes of the surrounding ridges. Some of the gorges are at least convert|1000|ft|m|0 deep. Much of the area was forested at the end of the 19th century, and much of the area is owned by the Pennsylvania Bureau of Forestry.
Allegheny Frontsection includes the abrupt escarpment that divides the Ridge and Valley Province from the Allegheny Plateau. The region is a large broad ridge with a steep ascent from east to west and rolling hills away from the ridge. The Allegheny Front reaches its highest elevation in Pennsylvania at Blue Knob, convert|3136|ft|m|0, an unusual bulge along this symmetrical ridgeline. Streams that cut into the ridge are often shallow and steep.
This section includes Pennsylvania's highest point, Mount Davis which stands at convert|3213|ft|m|0 above sea level. Many of the mountains are long and broad with relatively shallow and broad valleys. Unlike the Appalachian Mountain section, the streams of this area have not cut deep and well defined valleys into the earth. Much of the drainage pattern is dendritic with a little trellis where erosion resistant rocks have created higher and more well defined ridges. Elevations increase to the south, and Mt. Davis resides only convert|4.5|mi|km|0 from the
Marylandborder. A few of the ridge tops contain some low-volatile bituminous coal fields including the Broad Topfield..] This region is also home to two national stories: the Quecreek Mine Rescueand the crash of United Airlines Flight 93.
Located in the southwest corner of the state, the Waynesburg hills are another major coal producing area for the state. Much of the 64.4 billion tons of bituminous coal that is remaining in the state resides under these hills in near horizontal beds. The hills are narrow and steep sided with some deeper valleys..]
Pittsburgh Low Plateau
Another section that is a significant coal producer. It is similar to the Waynesbrug hills section except for higher local relief and deeper valleys.
Landslidesand mine subsidenceare common hazards..]
This section consists of high, broad, and flat uplands cut by sharp and shallow river valleys. Much of this area was not covered by the Late Wisconsinan glacier but there is evidence of pre-Wisconsinan glaciers in the area.
Northwestern Glaciated Plateau
This section has been influenced by glaciers and many of the valleys cut into the bedrock trend northwestward- in the direction of the retreating glaciers. There are many signs of glaciers including
kames, eskers, kettles, and moraines. This section is home to Pennsylvania's largest natural lake, Conneaut Lakeas well as one of the longest eskers in the state, West Liberty Esker. [Van Diver, B. B. (1990). Roadside Geology of Pennsylvania. Mountain Press Publishing Company, Missoula Montana. ISBN 0878422277] Some of the drainage patterns have shifted and only a few of the streams flow into Lake Erie.
Along with the Coastal Plain Province, the smallest province in the state, the central lowlands are a part of the
Great Lakesarea and exists along a glacial escapement adjacent to Lake Erie.
The following is a list of notable Pennsylvania geologic features noted for their beauty and/or uniqueness.
Grand Canyon of Pennsylvania
Hickory Run State Park
McConnells Mill State Park
Moraine State Park
* Presque Isle
Promised Land State Park
Ricketts Glen State Park
Bear Valley Strip Mine(including the "Whaleback")
Delaware Water Gap
* Ringing Rocks
* The Pinnacle
Notable Rock Formations
List of mapped rock formations in Pennsylvania"
Pennsylvania has been updating its base geologic map last printed in 1980. New research has shifted the names of several formations and promoted or demoted many different sequences on the stratigraphic chart.
Geology of Bedford County
* [http://www.dcnr.state.pa.us/topogeo The Pennsylvania Geologic Survey]
* [http://www.dcnr.state.pa.us/topogeo/maps/map13.pdf Physiographic Provinces of Pennsylvania]
* [http://www.dcnr.state.pa.us/topogeo/education/es4.pdf The Geologic Story of Pennsylvania]
United States topic
Geology of the United Statesby political division
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