America's Volcanic Past -
Pennsylvania

Volcanic Highlights and Features:
[This list is just a sample of various Pennsylvania volcanic features or events and is by no means inclusive.]

• Pennsylvania
• Pennsylvania Regions
• Pennsylvania - Brief Geologic History
• Pennsylvania's Volcanic Rocks
• French Creek State Park
• Gettysburg
• Gifford Pinchot State Park
• Governor Dick Hill
• Hopewell Furnace National Historic Site
• Millers Point, Mill Creek Valley, Morgantown, and St. Peters
• Nockamixon State Park
• Philadelphia
• Samuel S. Lewis State Park
• South Mountain (Caledonia and Pine Grove Furnace State Parks)
• Valley Forge National Historical Park
• White Clay Creek Preserve

Pennsylvania's Igneous Rocks:²
Igneous rocks have formed in Pennsylvania at several widely separate times in the past. The oldest are of many compositions and have complex associations. They are not truly igneous anymore, having been transformed into metamorphic rocks. Among these are metamorphosed rhyolite on South Mountain, in Adams, Cumberland, and Franklin Counties in south-central Pennsylvania. In the southeastern part of the state, a large area labeled on the geologic map as having rocks of Lower Paleozoic and Precambrian age contains metamorphosed felsic rocks, such as granite, and smaller bodies of metamorphosed mafic rock. In Lancaster, Chester, and Delaware Counties are small bodies of extremely mafic, or ultramafic, rocks that are believed to have formed from magma originating in the mantle. Because some minerals, such as olivine, that are found in these rocks are unstable at the earth's surface, they have been severely altered to the mineral serpentine, forming a rock called serpentinite. The complex associations of rocks in the southeast support the idea that the region is one dominated by microplates. Many small landmasses were carried toward what is now the east coast of North America hundreds of millions of years ago. They eventually piled up against each other, giving southeastern Pennsylvania a varied assortment of rock types.

Pennsylvania's Metamorphic Rocks:²
Two major types of metamorphism are recognized. One is contact metamorphism, during which rock is altered by the intense heat of a nearby body of molten rock that has intruded from a greater depth. Among the more common contact metamorphic rocks is hornfels, a hard, fine-grained rock that is really a baked shale. Hornfels is found in southeastern Pennsylvania where shales were altered by igneous intrusions during the Jurassic Period. They are located immediately adjacent to the igneous rocks. The other major type, referred to as regional metamorphism, involves rocks over a large area that have been dramatically transformed by some major even, such as deep burial. Rocks in Pennsylvania that have been altered by regional metamorphism can be found near the surface in the southeastern part of the state, although at greater depth they probably could be found under the entire state. The metamorphic rocks in southeastern Pennsylvania are quite diverse and are believed to have been derived from both igneous and sedimentary rocks. On the geologic map, they are included among the rocks of Lower Paleozoic and Precambrian age. The metamorphism and diversity of these rocks possibly result from the conditions to which eastern North America was subjected several hundred million years ago.

Pennsylvania's Diabase:⁴
A much younger group of igneous rocks, which formed during the Jurassic Period, less than 200 million years ago, cuts across this older assortment. They consist of a dark, medium- to fine-grained intrusive rock, diabase. Diabase is a mafic rock that is believed to have formed from magma originating in the mantle. It has a different mineral composition than the older ultramafic rocks that have altered to serpentinite. The origin of the diabase found in Pennsylvania is related to the movement of large, continent-sized plates. The diabase is found in a place that was being rifted, or pulled apart, when North America and Africa were separating, forming the Atlantic Ocean. Magma from the mantle entered the space that separated the two sides, forming igneous rock that, with sediments, filled the widening gap.

Pennsylvania's Granite and Gneiss:²
Gneiss and granulite are the coarsest grained metamorphic rocks. They usually contain more quartz and feldspar than the finer grained rocks, but often have bands of platy minerals such as mica running through them. Some are believed to have formed from the metamorphism of igneous rocks such as granite, although these could be formed from a wide variety of parent rocks.

French Creek State Park:³
Both French Creek State Park and Hopewell Furnace National Historic Site straddle the boundary between two geologic divisions of the Piedmont. Southeast of Hopewell Lake, metamorphic rocks of the "Crystalline Uplands" form Mount Pleasure and nearby hills; these rocks range in age from about 580 million the 1 billion years old. The rest of the area is underlain by 185- to 200-million-year-old sedimentary and igneous rocks of the "Mesozoic Lowlands". (See Millers Point below)

Gettysburg:⁴
The region in which the Gettysburg campaign was conducted falls into four roughly parallel bands. From southeast to northwest, these are the Piedmont proper; the Triassic basins, including the Culpeper basin in Virginia and Maryland and the Gettysburg basin in Maryland and Pennsylvania; the Blue Ridge of Virginia and its continuation, South Mountain, in Maryland and Pennsylvania; and the Great Valley, including approximately the northern half of the Shenandoah Valley in Virginia, the Hagerstown Valley in Maryland, and the Cumberland Valley in Pennsylvania. Each of these four regions played a distinctive role in the Gettysburg campaign.

Piedmont:⁴
After the campaigns of 1862, the Confederates went into winter quarters on the south bank of the Rappahannock River, the Union armies on the northern bank. The Rappahannock and its tributary, the Rapidan, which joins it about 10 miles west of Fredericksburg, flow eastward across the entire 50-mile width of the Piedmont in steep, easily defended valleys, and were in effect the outermost defense line of Richmond. In this area, the Piedmont rises from approximately sea level at Fredericksburg, Washington, and Baltimore, to about 400 or 500 feet near the Blue Ridge. The exposed rocks, mostly of Precambrian age, are granite, gabbro, and hornblende gneiss in the southeastern part and the Wissahickon Schist in the northwestern part of the Piedmont. The rocky roads of this region of ridges and ravines were hard on men, animals, and equipment, so were to be avoided by the armies of the sixties. To use a term coined later by military geologists, the "trafficability" of the roads was poor. Although the Gettysburg campaign started in the Piedmont, both armies left it as soon as possible.

Blue Ridge:⁴
In Virginia, the Blue Ridge is high and rather narrow, formed of tightly folded and metamorphosed quartzite and volcanics, gneiss, and other resistant rocks mostly of early Paleozoic age. The crest of the ridge drops from about 4,000 feet south of Thorntons Gap to 1,500 feet at the Potomac, where it is known locally as Loudoun Heights. An extension, Elk Ridge, continues about 10 miles into Maryland; the southern end of Elk Ridge is known as Maryland Heights. At the Potomac River, the main ridge is offset about 3 miles to the east, and an extension, Short Mountain, continues for about 10 miles into Virginia. Near the Maryland-Pennsylvania State line, a spur of South Mountain, the Catoctin Range, swings eastward and southward into Virginia, passing a few miles west of Frederick, Md. The Bull Run Mountains in Virginia are an interrupted extension of the Catoctin Range. South Mountain in Pennsylvania and northern Maryland is wider and more complex in structure than the Virginia Blue Ridge, though it is formed by the same rock types. Altitudes of the crest in that region range from about 1,500 feet at the Potomac to 2,000 feet near Carlisle.

Great Valley:⁴
In the Gettysburg campaign area, the Great Valley is from 10 to 20 miles wide and averages about 15 miles. It is underlain by limestone and shale, mostly of Ordovician age, and owes its existence to the incompetent nature of these formations as compared to the harder ridge-forming rocks on either side. In Virginia and West Virginia, the valley drains northward through the Shenandoah River into the Potomac. The divide north of the Potomac is a few miles north of Chambersburg, standing at an altitude of about 650 feet. From that area, the Hagerstown Valley is drained by Conococheague Creek, which flows into the Potomac at Williamsport, Md. The Cumberland Valley drains northeastward into the Susquehanna at Harrisburg through Conodoguinet Creek. The Great Valley was and is a fertile region, from which the Confederates, once across the Potomac, impressed a vast quantity of supplies of all kinds. Because of the gentle grades and deep soil, the trafficability of the valley roads was good, and the Confederates reached Chambersburg in excellent condition.

Culpeper and Gettysburg Basins:⁴
The exposed rocks in the Culpeper and Gettysburg basins are sandstone and shale that were deposited in Triassic time in the downtilted western part of the Piedmont, which abuts the Blue Ridge and South Mountain. In Early Jurassic time, these sediments were intruded by sills and dikes of diabase. As the roads generally avoided the diabase outcrops, trafficability across the Triassic sediments was good, much like that in the Great Valley. Like the Confederates, the Union troops took advantage of geologic conditions to expedite their movements. The Gettysburg basin, from its southern end near Frederick, widens northeastward toward the Susquehanna River; in southern Pennsylvania, it is a wide, fairly level plain, except for the diabase outcrops. The town of Gettysburg, on this plain, was the hub from which radiated ten roads. Such was the road pattern that the Confederates, whether they came from Cashtown, Carlisle, or York, had no choice but to pass through Gettysburg.

Gettysburg Sill:⁴
The Battle of Gettysburg was essentially an effort by the Confederates to drive the Union army from the outcrop of the Gettysburg sill (York Haven Diabase) south of the town of Gettysburg. This outcrop is shaped like a fishhook extending northward about 3 miles from Round Top through Little Round Top and Cemetery Ridge to Cemetery Hill, then east and south to the barb of the fishhook, Culps Hill. Round Top stands at 785 feet above sea level, Little Round Top at 650 feet. Between Little Round Top and Cemetery Hill, the ridge drops to about 570 feet. For comparison, the elevation of the town of Gettysburg is about 500 feet. Seminary Ridge, the Confederate position on the second and third days of the battle, stands throughout most of its extent at about 560 feet, but rises northward to 650 feet at Oak Ridge. Seminary Ridge is the trace of a diabase dike (Rossville Diabase) that apparently is an offshoot of the westward-dipping Gettysburg sill.

Devils Den:⁴
The Union position at Gettysburg had a weakness that became more and more evident as the battle progressed; owing to the geology, it could not be entrenched. The resistant diabase sill is so close to the surface that it was impossible for the soldiers to "dig in," and what little protection they could obtain was provided by existing stone walls, outcrops of rock such as Devils Den (a mass of diabase boulders), and isolated boulders. Because of its inability to solidly entrench its position, the Union army suffered disproportionate casualties (about 23,000 men killed, wounded, and missing), considering that it was the defending force throughout the battle.

Gifford Pinchot State Park:³
Gifford Pinchot State Park contains examples of the three major rock classes: igneous, metamorphic, and sedimentary. The most common rock present in the park is the igneous rock diabase, formed below the earth's surface and originally hot and liquid (molten). Diabase underlies nearly all of Gifford Pinchot State Park except along the northern and southern boundaries. Here, the diabase and the metamorphosed sedimentary layers, both of which are resistant to weathering, form two ridges that outline the park. The diabase may be seen exposed (at many park localities), at many places along the lake shore, and as scattered boulders in many areas of the park.

Governor Dick Hill:³
The view from the top of Mt. Pisgah encompasses several hundred square miles of rolling Piedmont landscape to the north, east, and south. Probably the most distant feature visible on a clear day is Governor Dick Hill, an 1,150-foot knob of Triassic-age (190 million years old) diabase, or "traprock". This knob of intrusive igneous rock is near Mt. Gratna, Lebanon County.

Hopewell Furnace National Historic Site:³
Both French Creek State Park and Hopewell Furnace National Historic Site straddle the boundary between two geologic divisions of the Piedmont. Southeast of Hopewell Lake, metamorphic rocks of the "Crystalline Uplands" form Mount Pleasure and nearby hills; these rocks range in age from about 580 million the 1 billion years old. The rest of the area is underlain by 185- to 200-million-year-old sedimentary and igneous rocks of the "Mesozoic Lowlands". (See Millers Point below)

Millers Point, Mill Creek Valley, Morgantown, and St. Peters:³
Millers Point is a spectacular rocky crag of diabase on the edge of a ragged escarpment overlooking the Schuylkill River valley 3,600 feet northeast of the crest of Chestnut Hill. The diabase of Millers Point and Mill Creek valley is part of the Morgantown pluton, a great bowl-shaped body of intrusive rock that crops out around the eastern and northern edges of French Creek State Park, and occurs at a depth of about 1 mile in the vicinity of Hopewell Lake. The boundary of the pluton north of Millers Point is a large fault along which rocks on the south side have been dropped down relative to rocks on the north side. Southeast of the park, the outcropping edge of the pluton makes a sharp turn through St. Peters and continues in a westerly direction to Morgantown. The diabase was at one time extensively quarried as a source of dimension stone and aggregate. At St. Peters, 1.5 miles southeast of the park, the French Creek Granite Company operated a dimension-stone quarry that was first opened in 1885. When polished, the diabase made an attractive dark stone for ornamental and architectural use. The polished rock was marketed as "black granite".

Nockamixon State Park:³
Cropping out within the park is a stratigraphic sequence that includes sedimentary, igneous, and metamorphic rocks. The igneous rock, diabase, or "trap" as it is commonly known, is a dark-gray, medium crystalline rock composed predominantly of the two minerals labradorite (a gray feldspar) and augite (a black pyroxene). Originally, the diabase was a hot and molten liquid (magma) that was intruded into the preexisting sedimentary rocks at considerable depth beneath the surface of the earth. As the magma was emplaced, it forced the sedimentary layers apart and baked the adjacent shales and argillites into a tough, maroon to dark-gray metamorphic rock called hornfels. The topography of Nockamixon State Park and the surrounding countryside is largely due to the structural configuration, and to the erosional resistance of the rock units. Diabase is the most resistant of all the rock units and forms Haycock Mountain and the rocky hills in the northwestern portion of the park.

Philadelphia:⁶
The geologic map from Bascom and others (1909) shows that the Philadelphia area is underlain by three distinct geologic rock types: (1) the limestone and sandstone rocks of the Northern Piedmont, (2) the complex metamorphic bedrock of the Southern Piedmont, and (3) layers of sediments that occupy the Coastal Plain. The limestone and sandstone rocks of the Northern Piedmont were deposited in an ancient shallow marine environment and are approximately half a billion years old. The metamorphic bedrock of the Southern Piedmont ranges from half a billion to more than one billion years in age and was shaped by the extreme heat and pressure of geologic forces deep in the Earth. Within the Coastal Plain, sediments were deposited during the last 75 million years, forming a layer of sand and gravel on top of the older Southern Piedmont metamorphic bedrock.

Samuel S. Lewis State Park:³
The view from the top of Mt. Pisgah encompasses several hundred square miles of rolling Piedmont landscape to the north, east, and south. The scenery nearer at hand, however, is entirely developed on complexly folded and faulted, metamorphosed rocks that range in age from Precambrian to Ordovician (600 to 450 million years ago). Except for some volcanic rocks at the bottom of the geologic sequence, these Piedmont rocks were originally sedimentary sandstones, conglomerates, limestones, dolomites, and shales formed from nearshore and shallow marine deposits.

South Mountain: (Caledonia and Pine Grove Furnace State Parks):³
About 600 million years ago, in late Precambrian time, South Mountain was the site of considerable volcanic activity. Over a period of several million years, volcanoes erupted intermittently through fissures and vents in the basement rock. Initially the flows were composed of basalt (a dark-colored volcanic rock), but this composition changed to rhyolite (a lighter-colored volcanic rock) in the later stages of activity. As widespread volcanism waned, the environment of the South Mountain area began to change. The land subsided, and the terrestrial volcanic environment became a shallow sea environment. During Early Cambrian time, clastic sediments were deposited on top of the volcanics. These sediments were transported to the sea floor by streams and rivers from landmasses to the southeast and north of the South Mountain area. About 330 million years ago, deformation and mountain building began to occur in this area. The sediments deposited over a period of 240 million years in the ancient sea (now sedimentary rocks) were folded, faulted, and lifted high above the water by a series of powerful crustal movements. During this regional deformation, the conditions of high temperatures and pressure changed the internal character of the sedimentary rocks. Most of the sandstones and shales were metamorphosed to quartzites and phyllites. The early volcanic rocks became metarhyolites and metabasalts. This mountain-building stage occurred as several pulses of activity. By Triassic time, 100 million years later, tectonic activity had finally ceased. For the past 230 million years, the South Mountain area has remained above sea level and has gradually eroded to its present elevation and topography.

Valley Forge National Historical Park³
The rocks now exposed in Valley Forge National Historical Park began forming about 580 million years ago. At this time, the coastline of the ancient North American continent was flooded by rising seas. Sand accumulated along beaches, and muds and limy sediments were deposited offshore. Over time, the continued rise in sealevel produced a layered stack of sedimentary rocks having sandstone at the base and carbonates on the top. The early period characterized by sediment accumulation began to change about 480 million years ago. For the next 200 million years, the Valley Forge area experienced pulses of mountain-building activity that subjected the rocks to deforming forces accompanied by increases in temperature and pressure. The most intense event to affect the region occurred near the beginning of this period when a volcanic island chain collided with the North American continent. The collision caused originally horizontal layers of sedimentary rocks to be compressed into folds and to thrust inland along large faults. At the end of this period, all of the earth's continents were assembled into a single landmass called Pangea. Africa had collided with North America, producing the Appalachian Mountains in central Pennsylvania. Once the mountain-building activity ceased, natural weathering processes began wearing down the mountains. About 230 million years ago Pangea began to split apart. This rifting event marked the birth of the Atlantic Ocean. The separation of Africa from North America was accompanied by a stretching of the crust over a broad area. In response to the stretching, blocks of the earth's crust dropped down along faults, forming long, narrow valleys, or basins. The park is located near the southern edge of one of these -- the Newark basin. The basin is filled with sedimentary rocks that are stained red as a result of oxidized iron, and with rocks that were once molten. In places, the basin-filling rocks are over 3 miles thick. For the last 150 million years or so, the eastern half of North America has been geologically quiet. Under such conditions, weathering and erosion act as the dominant agents shaping the land surface. The present topography of the park is the result of these processes.

Rocks in the Park:³
All of the rocks in the park are sedimentary in origin; that is, they formed at the earth's surface either as accumulations of particles derived from preexisting rock or by some chemical process. The Chickies and Antietam Formations originated as deposits of sand and mud eroded from the continent. Calcite if the major constituent of limestone, the predominant rock type of the Elbrook and Conestoga Formations, which underlie the valley to the south of the park. Dolostone is a rock largely composed of dolomite. The Ledger Formation, a dolostone, underlies the area east and south of Mount Joy. The red sandstones and shales of the Stockton Formation, which underlie the northern and eastern parts of the park, were deposited around 200 million years ago in the Triassic Period. During this time, Pangea was breaking apart and the Newark basin was being formed. The area around the park was very different then. Periodically, earthquakes and volcanic eruptions jarred the region.

White Clay Creek Preserve, Pennsylvania/Delaware:³
White Clay Creek forms a scenic valley incised in the rolling Piedmont terrain of southeastern Pennsylvania and northwestern Delaware. Some 600 million years ago, the Preserve was part of a large continental area that subsided and was covered by a shallow sea. Through time, sediment composed of sand, silt, and mud spread over the sea floor. At various intervals, volcanoes poured lava onto these deposits. Gradually the sediments hardened into sedimentary rocks. About 460 million years ago, an immense mountain-building episode folded and heated the rocks and completely changed their character. The rocks in the Preserve "cooked" at elevated temperatures and pressures for some 70 million years, long enough for the new minerals to develop. Approximately 390 million years ago, the Preserve was uplifted and cooled, which halted the metamorphism. Since then, the minerals have remained largely unchanged. The lava flows became very dark gray amphibolites. Nearly black hornblende dominates these rocks; interspersed feldspar grains tend to be medium gray to white.

Excerpts from:
1) USGS/NPS Geology in the Parks Website, 2001
2)Pennsylvania Geological Survey Website, 2001, John H. Barnes, 1991, Rocks and Minerals of Pennsylvania
3) Pennsylvania Geological Survey Website, 2001, Pennsylvania Trail of Geology, Park Guides
4) Pennsylvania Geologic Survey Website, 2001, Andrew Brown, 1962, Geology and the Gettysburg Campaign
5) Pennsylvania Geologic Survey Website, 2001, Geologic Map of Pennsylvania, Revised 2000
6) USGS Open-File Report 00-224, Mapping Buried Stream Valleys in Philadelphia, Pennsylvania

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