The Geology of Pennsylvania consists of six distinct physiographic provinces, three of which are subdivided into different sections. Each province has its own economic advantages and geologic hazards and plays an important role in shaping everyday life in the state. From the southeast corner to the northwest corner of the state, they include: the Atlantic Plain Province, the Piedmont Province, the New England Province, the Ridge and Valley Province, the Appalachian Province, 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 sedimentary and were deposited during the Paleozoic Era. Almost all of the metamorphic and igneous rocks are confined to the southeast portion of the state. 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. The Pennsylvania terrain has also been affected by continental rifting during the Mesozoic era.Schultz, C.H. ed. (2002) The Geology of Pennsylvania. Special Publication No.1. Pennsylvania Geologic Survey and Pittsburgh Geological Society.
Pleistocene glaciers have 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. In 2001, 12 billion tons still remained in the ground, most of which was not economically feasible to mine.Edmunds, W.E., (2002), Coal in Pennsylvania (2nd ed.): Pennsylvania Geological Survey, Educational Series 7, p. 17. American geologists recognized the importance of Pennsylvania's coal region and named the Upper Carboniferous Period the Pennsylvanian Period 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.Pennsylvania Geological Survey: Rocks and MineralsEdwin, 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 Titusville which helped give rise to the modern oil industry and two brand name motor oils, Quaker State (now owned by Royal Dutch Shell) and Pennzoil. Pennsylvania also has reserves of natural gas from both deeply buried source rocks and coal-bed areas.
Atlantic Coastal Plain One of the smallest physiographic provinces in the state is confined to Philadelphia, Delaware, and Bucks counties along the Delaware River. Local relief is less than and much of the bedrock is buried under recent alluvial deposits. On the geologic map, "Trenton Gravel" is used to describe most of these sediments. Much of the alluvial sediments that exist here are sand, silt, and clays.D. W. Greenman, D. R. Rima, W. N. Lockwood, and Harold Meisler, Ground-water resources of the Coastal Plain area of southeastern Pennsylvania (1961). 375 p., 22 pls. 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.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 floods are becoming a local problem in the province.
Piedmont 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 urbanized and developed. Some of the best farmland in the state is in this region, specifically Lancaster and Chester counties.
Piedmont Uplands
This section is characterized by the metamorphic rocks that 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. They are similar in many respects to their cousins in northern and central Maryland, immediately to the south. Much of the rock was altered during the formation of Rodinia during the Grenville orogeny. These rocks eventually provided the platform for the deposition of sediment that would become the Wissahickon Formation during a rifting of Rodinia. Sea floor spreading continued until a passive margin developed along the new Iapetus Ocean and 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 subduction zone. 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.
Piedmont Lowlands The lowlands are underlain primarily by more easily eroded rocks such as limestone, dolomite, and phyllite. These rocks are relatively younger in age than 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 . Karst terrain is problematic in this section.
Gettysburg-Newark Lowlands This section is a bit misleading since there are hills as high as 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 Pangea in the Triassic age. Also, a basaltic igneous rock called diabase formed dykes and sills later in the Jurassic as the Atlantic Ocean began 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.
Buckingham Valley A small slice of Paleozoic rocks, mostly carbonates, exists in Buckingham Township. These rocks 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-Gettysburg Basin". Pennsylvania Geologic Survey, G43, Harrisburg, Pennsylvania. Buckingham Mountain rises 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.
New England A small and fragmented province in northeastern Pennsylvania called the Reading Prong is akin to the crystalline bedrock found in much of New England. This is the southern end of the Hudson Highlands of New York and New Jersey (known as the Ramapo Mountains in New Jersey) and the Taconic Mountains of New York. The granitic rocks and quartzite of this area are highly metamorphosed and are Pre-Cambrian to Cambrian in age. Hills and ridges are locally steep and rounded at the top and form the hills around Reading, and to the south of the Lehigh Valley on South Mountain.
Ridge and valley This region in Pennsylvania, made famous by NASA's LANDSAT images, is the second-largest in the state and 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 faults that 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 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 Valley The Great Appalachian Valley is a long broad valley that extends from Canada to 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, dolomite, slate, shale, sandstone, siltstone, and some scattered basalt. Almost all of the rocks in the Great Valley in Pennsylvania are Ordovician in age and were deposited during a quiet period before the Taconic orogeny. The limestones and dolomites of this area are extensively quarried in Pennsylvania. These carbonate rocks are 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.
Blue Mountain
The Blue Mountain or Blue Ridge region, like the ridgelines to its north and west, is one of a series of near parallel ridges that run for tens of miles, and are equally likely to be called Ridge or Mountain. Hence Blue Mountain is not to be confused with the Blue Ridge Mountains but instead, represents the sharp escarpment, a step in elevation separating the Appalachian Mountains from the pastoral basin famous as the landscape associated with the Pennsylvania Dutch in southern Pennsylvania called the Great Valley A physiographic province, as are these section titles. Many of Pennsylvania's water gaps cut through Blue Mountain including Delaware Water Gap, Lehigh Gap, Schuylkill Gap, Susquehanna Gap, and Swatara Gap. Also along the ridge, many "wind gaps" also exist. The rocks of the Blue Mountain section include mostly Silurian aged 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 Formation, which is named after the Shawangunk Ridge of New York state.