Geologic History of Pennsylvania

Features appear in each issue of Pennsylvania Heritage showcasing a variety of subjects from various periods and geographic locations in Pennsylvania.

Pennsylvania and its citizens have made significant contributions to the development of America. Its rich heritage in culture, industry, transportation, agriculture, education, communication, economic development, and many other areas of human endeavor have truly been a keystone in the forging of a growing nation. Its traditions have been recorded in the archives of printed materials, in the structures built by its people, and through the legends of individuals and events of the past. However, thousands of years before this virgin land saw its first European explorer, the earliest history of man in Pennsyl­vania was being recorded through artifacts created by its earliest residents. Equally fascinating is the geologic history of our State that attests to not only the past several thou­sand years, but the age of the earth itself, about four and one-half billion years. This article is a brief summary of the processes and events, as they are currently understood, that have molded our scenic Appalachian Mountains, carved the fertile valleys, and produced its geologic natural resources­ – a vital contribution to State and national economy.

Geology has had a close association with the history of Pennsylvania. The fertility of its agricultural lands; the de­sign of its cities and transportation systems; its water re­sources; and the locations of wells, waste disposal sites, mines and quarries, all depend on geology. The impact of geology on Pennsylvania’s economy is currently about twice as great as that of agriculture. In 1977, the value of minerals produced in the State amounted to about $3.2 billion. Pennsylvania ranks as the fifth largest mineral producer in the nation. Pennsylvania minerals produced today include mostly coal, oil, natural gas, limestone (for cement, agriculture and construction), clay (for brick and tile), sand, and gravel. But iron, zinc, copper, gold and silver have also been produced. Uranium deposits have also been found in the State and may one day provide a source of energy and economic growth.

However. the exploitive and economic interests of man are only one aspect of geology. Geology is a historical science that mirrors the events of the earth’s past, recorded in rock layers in various ways. By compiling pieces of evidence, a geologic history of Pennsylvania can be constructed in much the same manner that written materials, legends, and artifacts can be combined to construct the history of man in Pennsylvania.

In developing a chronology of events in Pennsylvania’s geology, it may be helpful to explore several ideas that have revolutionized the field of geology. The first idea is not new, but it is one of the most fundamental tools in re­constructing events of the past. Each rock unit possesses a characteristic set of features, such as size, shape, and chemical composition of components. By finding modern environments where materials with the same characteristics are accumulating, it may be reasoned that ancient environ­ments were similar to those of today. A second tool of the geologist, recently developed, is the use of radioactive decay in determining the age of rocks. Utilizing these two methods, geologic events, ancient geography, environments, and their ages, can be determined from rock layers.

Historic time is sometimes divided into broad general time units characterized by prevalent attitudes or directions of growth, such as feudalism, medieval period, renaissance, etc. Each time unit can be further subdivided into smaller units; the divisions are approximations and do not have sharp boundaries. Geologic time is similarly organized, but it consists of four large units, called eras, that are based on characteristic life forms. Eras are divided into periods with boundaries defined by physical events like mountain building episodes, etc. (See Table 1). The first era, the Pre­cambrian, represents nearly 9/10 of the age of the earth; this was a time when few organisms fossilized. Many of the materials deposited in that era have eroded away. The re­construction of events in the Precambrian Era is difficult, in much the same manner that it is difficult to reconstruct the history of Pennsylvania prior to the written accounts of early Western settlers. With the Paleozoic Era, an abun­dance of life forms fossilized. Nearly all the rock layers in Pennsylvania are of Paleozoic age, a time when almost all life lived in or near the sea. Because there is a substantial volume of rock and fossil evidence preserved, we know much more about this era than about any other of our States’ past. This geologic evidence has the same effect as a period in Pennsylvania’s history when many prolific writers recorded daily events they had observed. During the Mesozoic Era, giant reptiles – “dinosaurs” – dominated the landscape. A narrow rock band stretching from Bucks to Adams County preserves this part of the Mesozoic Era in Pennsylvania. The Cenozoic Era was dominated by mammals. Only the southeastern tip of the State has a marine record of the Cenozoic Era. However, within the past several million years, ice sheets from the polar north advanced into Pennsylvania leaving a veneer of glacial debris to record this massive event. Ice, perhaps more than one half mile thick covered much of the northern third of the State. Geologic exploration and findings have provided an interesting and valid historical documentation of Pennsylvania over the past several hundred million years, continuing to the very present.

Table 1 – Geologic Time Scale

Eras Millions of Years Before the Present Periods
Cenozoic 0
25 Neogene
65 Paleogene
Mesozoic 130 Cretaceous
185 Jurassic
230 Triassic
Paleozoic 265 Permian
310 Pennsylvanian
355 Mississippian
413 Devonian
425 Silurian
475 Ordovician
570 Cambrian
Precambrian 5,000

Many geologic forces have been working at the earth’s surface since its origin, constantly changing and reordering the landscape. Some forces are destructive, some con­structive, as they tear away or rebuild the elevations of continents above sea level. Everyone has witnessed the des­tructive forces of wind, ice, and running water grinding away at the landscape. The products of erosion eventually find their rest beneath the earth’s oceans. The forces of destruction are so effective that the continents of the earth should erode away to sea level in several million years. However, there are young, rugged mountains in the world that have not yet shown severe effects of erosion. There­fore, there must be forces at work within the earth capable of renewing the landscape. Volcanic activity is one of the rebuilding forces, but another important theory of renew­ing the earth’s landscape is a seemingly preposterous new idea referred to as “plate tectonics.” The theory of “plate tectonics” continues to gain more credence annually and suggests that the earth’s surface is divided into about a dozen rigid plates that are capable of moving in various directions over the face of the earth similar to blocks of ice floating freely on an open ocean. Where plates collide mountains form; where they separate, vol­canic activity occurs. In both the boundaries of collision and of separation, earthquake activity is common. How­ever, in the center of the plates, a minimum of volcanic and earthquake activity occurs.

Beneath sea level a whole new geography is intimately associated with rebuilding and destruction of continents. As a continent erodes, the eroded materials accumulate on the continental shelf. The continental shelf has a maximum depth of about six hundred feet beneath sea level; most of the life in the sea exists here. Beyond the continental shelf is the continental slope, a transition zone beginning at a depth of about six hundred feet and continuing to a depth of about eighteen thousand feet (nearly three and one-half miles). At the base of the continental slope is a flat region of the sea floor, the abyssal plain. Rising above the sea floor are submarine mountain ranges continuing around the globe in a pattern similar to the seams of a baseball. It is along the axes of these ridges that plates of ocean crust separate and new ocean floor forms, just as new ice may form between two icebergs moving away from one another on an arctic lake. Another prom­inent feature of the ocean floor is a series of deep oceanic trenches that parallel the edges of some continents. Tren­ches may reach a depth of 35,000 feet (nearly seven miles) below sea level. Where dense oceanic plates collide with more buoyant continental plates, the descending oceanic slab pulls the crust downward producing a trench. Friction between the plates causes melting of some rock material and the rising liquid may create a series of island arcs be­tween the trench and adjacent continent. A modern example of this continent-arc-trench system is the western Pacific, stretching from Japan to New Zealand. The sea floor, being relatively more dense, is ultimately buried as two converging continental plates collide. The buoyant continental masses first come into contact at the sub­merged continental slope and then at the continental shelf, forcing sedimentary layers to be compressed and wrinkled, like two cars colliding head on at 60 m.p.h. The accordion-like deformation produces continental mountains. This process is active today in the Himalayas where India, which was once a part of Africa, is colliding with Asia.

The geologic history of Pennsylvania is complex and has involved several episodes of collisions and separations between adjacent plates. During each of these events, rocks became partially or entirely molten and intensely de­formed, making each preceding event difficult to interpret. Information for the first 3 1/2 billion years of earth history, for Pennsylvania, has been obscured by multiple deformations. The earliest stable material in Pennsylvania is a group of crystalline rocks found in the Reading Prong and in the Philadelphia area. These materials are about one billion years old and are fragments of a large crystalline mass which extended northward to New England and an indefinite distance east of Pennsylvania. About 750 million years ago this crystalline plate separated along a northeast-southeast fracture producing volcanic activity, an eastward moving plate (Europe), a westward moving plate (North America). and an ocean basin between them. Evidence of this separation is seen in the South Mountain where volcanic rocks occur. These volcanics contained copper and iron, in the vicinity of Fairfield and Pine Grove Furnace, and supported a mining industry in that area during the 19th century.

As the continents continued to separate, sediments from North America (on the west) accumulated on the continental shelf (eastern Pennsylvania), covering the earlier crystalline rocks. An island arc system, similar to the present day Japanese-New Zealand island chain, may have existed east of North America in the early Atlantic Ocean. During the next several hundred million years, from the late Precambrian to middle Ordovician time, a shallow sea covered most of Pennsylvania. Climates in Pennsylvania during this period were warm and tropical and many lime secreting organisms lived in the shallow sea causing thou­sands of feet of limy mud to accumulate. This limy deposit is now the limestone that exists in the rich agricultural zones of the Lancaster-York and Cumberland-Lebanon Valleys. As a result, these valleys support a lively commercial limestone industry with applications in construc­tion, cement, and agriculture. Since limestone weathers into a rather flat, uniform surface, early settlers found it relatively simple to build highways and subdue the natural environment. The longest valley in the world begins in New York and continues through our “Great Valley” (the Lehigh, Lebanon, Cumberland, Hagerstown valleys). and into Tennessee. Where this valley intersects with major river and gap systems, forming trading crossroads, cities such as Allentown, Bethlehem, Easton, Harrisburg, Carlisle, Chambersburg, York, and Lancaster were established.

In the middle Ordovician time, the early Atlantic Ocean began to close again as the two continental blocks of Europe and North America moved toward one another. This closing caused gradual up­warping to the east of Pennsylvania. As the Ordovician Period closed (about 425 million years ago). the warping became more intense, elevating eastern Pennsylvania above sea level and producing accordion-like fold layers which thrust over one another and melted rocks which were deeply buried. The first gradual uplift brought mud westward into the shallow sea covering the earlier limy deposits. Some of the mud became the slate belts of eastern Pennsylvania. As the deformation became more intense by late Ordovician and early Silurian time, streams ran more swiftly carrying sand and gravel to the shallow sea. The sand and gravel deposits eventually became the Tuscarora and Kittatinny mountains of central Pennsylvania. Volcanic activity during this episode is considered to be the source of chrome, lead, zinc, and other metals deposited in the Piedmont of southeastern Pennsylvania. By the Mid-Silurian time, the mountains of the east were nearly eroded away and a shallow sea again covered central and western Pennsylvania until the Mid-Devonian time. Many of the fossil layers of central Pennsylvania accumulated during this time period. These sediments are also the rocks which weather into many narrow valleys in central Pennsylvania. Reefs in the shallow sea of western Pennsylvania supported an abundance of micro-organisms. After burial in sediments, favorable chemical environments converted these organisms into oil and gas which eventually became stored or trapped in Mid-Devonian sandstone. These reservoirs now contain some of the world’s finest petroleum.

Beginning in the Mid-Devonian time another episode of deformation occurred in a sequence of uplift and erosion cycles which continued for about 100 million years and ended at the close of the Pennsylvanian time. The uplift again began in the east with rivers flowing westward carry­ing red silt and sand into a delta environment covering the Silurian and early Devonian fossil beds. These red delta deposits are several thousand feet thick and are seen in the road cuts along I-80 and through many of the valleys of central and northeastern Pennsylvania. During the Mississippian period, there were several uplifts and erosions. During one of these uplifts, uranium accumulated in the red sandstone of northeastern Pennsylvania. By Pennsyl­vanian time, much of Pennsylvania was covered by a complex of deltas near sea level. Alternate rise and fall of the sea produced swamps on the deltas which served as hosts for massive primitive. tropical forests. Partial decay and sediment covered the forests and gave rise to the Pennsylvania coal fields. By the late Pennsylvanian and early Permian time, the last pulse of deformation caused the Appalachian Mountains to rise. About 250 million years ago the Appalachians must have looked very much like the present day Alps, with great elevations and deformations in eastern Pennsylvania. This last event caused the accordion-like wrinkles seen in the Valley and Ridge Province and the up-warping of the Appalachian plateau. In central Pennsylvania the intense pressures compressed coal into anthracite, while diminished pressure in the western region produced bituminous or soft coal. Geologists believe that during the Permian period North America, South America, Africa, and Europe were joined together into one large supercontinent.

For the next 50 to 60 million years, the Appalachian Mountains of Pennsylvania eroded nearly to sea level. By late Triassic time, warping, rifting, and volcanic activity occurred in eastern Pennsylvania as North America separated from Europe once again. This separation continues today, after nearly 180 million years of drifting, increasing the width of the Atlantic Ocean at the rate of about one inch per year. The Triassic warping and rifting created valleys which were filled in with red continental sediment. Early dinosaur remains and footprints left in soft mud give a clue to the environment of that time. The red sediment is in­truded with a dark volcanic rock (diabase), producing tree­-covered hills on the present landscape of southeastern Penn­sylvania, from Adams County to Bucks County. The best examples of diabase volcanics are Cemetery Ridge, Semin­ary Ridge, and Devil’s Den, on the Gettysburg Battlefield. One reason why so many casualties occurred in the Battle of Gettysburg was that neither army could dig trenches in the hard diabase rock which outcrops there. Associated with the diabase intrusions along with the Triassic Lowlands are massive iron ore deposits like those found at Cornwall and Morgantown, Pennsylvania. Until the Lake Superior deposits were discovered, Cornwall was the largest producer of iron ore in North America. Many colonial forges, like Valley Forge, were established where limestone quarries and wood supplies (for charcoal) were near transportation routes to the Triassic iron ore. Some of Pennsylvania’s lead and zinc were probably deposited at the same time.

Erosion continued during the Jurassic, and by Cretaceous time, part of eastern Pennsylvania was again under a shal­low sea. For the past 65 million years there has been a gradual uplifting of Pennsylvania, with maximum dis­placement along a north-south line through central Penn­sylvania. This uplift increased the ability of streams to erode deep valleys into soft rocks, such as limestone and shale, leaving high ridges where resistant rocks such as quartzite and sandstone come to the earth’s surface. Because the layers are warped, the ridges and valleys are es­sentially parallel to one another, trending in a northeast-southwest direction. During this episode, a series of rapids and small water falls developed on streams and rivers in the extreme southeastern part of the State, where the hard rocks of the Piedmont end and the soft sediments of the Atlantic coastal plain begin. This area is referred to as the “Fall Line,” the head of navigation for major rivers. It was a source of energy for growing industries in the eighteenth century. The success of Philadelphia partly resulted from its proximity to the Fall Line.

During the last million years, glaciers which accumulated in two major centers of Canada invaded northeastern and northwestern Pennsylvania several times. The ice was several thousand feet thick. While advancing, the glaciers widened valleys, changed the course of some streams, created lakes, and pushed debris (moraine) in their paths leaving mounds of deposits where their forward motion ceased. The primary channel to carry meltwater from the receding ice was the Susquehanna River. The torrent of water associated with this event carved the scenic Sus­quehanna water gap and adjacent flood plain. The flood plain made a natural flat surface for the construction of rail­road beds and highways into the valleys of central Penn­sylvania, establishing trade routes. It continues to be a potential problem for water flooding.

The complex evolution of geologic events has indeed had a substantial impact on Pennsylvania and its citizens, and continues to play an important and interesting role in its future development. Each time period made its own unique contribution to Pennsylvania’s geological history and can be geologically documented through research and study. Through proper planning, management, and an understanding of geologic processes, the wealth of natural resources and esthetic beauty of Pennsylvania can continue to be a keystone in the development of its culture and economy.



Benson, Allan P., et al., American Association of Petroleum Geolo­gists Geological Highway Map, North East Region, Tulsa, Okla­homa, 1976.

Bird, J. M., and Dewey, J. F., “Lithosphere Plate Continential Margin Tectonics and the Evolution of the Appalachian Orogen,” Geological Society of America Bulletin, Vol. 81 (1970), 1031-1060.

Dewey, John F., “Plate Tectonics,” Scientific American, May. 1972. Dietz, Robert S., “Geosynclines, Mountains and Continent Build­ing,” Scientific American, March, 1972.

Levin, Harold L., The Earth Through Time, Philadelphia, 1978.

Seyfert, C. K., and Sirkin, L.A., Earth History and Plate Tectonics, New York, 1973.

Willard, Bradford, Pennsylvania Geology Summarized, Pennsylvania Geological Survey, E.S. 4, 1978 reprint.


Joyce J. Way gave typing and editorial assistance and John Purvis assisted in drafting maps and diagrams.


J. Ronald Mowery, Associate Professor of Geology at Harrisburg Area Community College, is President of the Harrisburg Area Geological Society, and member of the National Association of Geology Teachers and the American Geological Institute.