Rocks of the Appalachian/Piedmont

Figure 2.14: Shaded-relief map of the Appalachian-Piedmont valley and ridge in Pennsylvania. Image provided by Ray Sterner, Johns Hopkins University.

The folded, deformed rocks of the Appalachian/Piedmont region record the successive mountain-building events that folded the land into narrow ridges in this area. The rocks of this region were originally sandstone, siltstone, shale and limestone, formed as sediments eroded from the Taconic and Acadian Mountains into the inland ocean basin. Much of the Appalachian/Piedmont rocks are similar to those of the Inland Basin region because they were deposited in the same inland basin, though much closer to the mountains. Many of the sedimentary rocks, however, from the Appalachian/Piedmont Region are no longer sedimentary rocks. They have been squeezed, pushed, faulted and severely deformed in many places because this region was, at various times through geologic history, either the suture area for converging plates or directly adjacent to the uplifting mountains. The Appalachian/Piedmont Rocks were closer to the mountain building than rocks further west, and so felt the effects of the immense pressures of colliding plates much more severely. The northeast-southwest trending narrow ridges and valleys, and the rolling hills of the Piedmont are a result of the stress caused by the intense compression of plates of crust (Figure 2.14). 

Precambrian Rocks 

Figure 2.15: The Precambrian rocks of the Appalachian/Piedmont occur in a nearly north-south line, forming the many ridges of the Appalachian Mountains and revealing the location of the ancient Grenville Mountains (though in some places the Precambrian rock has been thrust westward from its original position).

The oldest rocks of the Appalachian/Piedmont region record the deposition of sediments on the ancient North American coastline more than one billion years ago as sediments eroded from the Grenville Mountains. There are several areas in which Precambrian rock is exposed within the Appalachian/Piedmont Region: the Green Mountains of Vermont; the Berkshire Mountains of Massachusetts stretching south into northern Connecticut; the Hudson Highlands and Manhattan Prong of New York; the Reading Prong; the Trenton Prong; the Baltimore Gneiss; South Mountain and the Catoctin Mountains (Figure 2.15). At each exposure, metamorphosed sedimentary rocks are visible, including gneiss, quartzite, schist and marble. Remember, though, that these metamorphic rocks were once sands, silts, muds and limestone deposited in the warm, tropical Iapetus Ocean from the Grenville Mountains. They were repeatedly subjected to enormous pressures and high temperatures from the colliding continents, recrystallizing to become metamorphic rocks. The Precambrian rock is visible at the surface only because of intense folding of the Appalachian/Piedmont region, which has uplifted layers of rock that were once buried beneath kilometers of crust, and erosion.

Indeed, the erosion-resistant Precambrian rocks have become the "backbone" of the range, helping the mountains resist being worn completely flat. The Precambrian rock and overlying younger sedimentary rocks have been compressed by the collisions of the continents into a giant upward fold. The softer sedimentary rocks were eroded away at the peak of the fold, exposing the resistant Precambrian rocks at the center. The Green Mountains of Vermont clearly expose this backbone of Precambrian gneiss and quartzite. The Hudson Highlands, extending into Pennsylvania as part of the Reading Prong, the Berkshire Mountains, Manhattan Prong and the Trenton Prong, follow the same line of resistant ridges of Precambrian rock.

The Precambrian Baltimore Gneiss, near Baltimore, Maryland, is actually a series of domes. The domes have Precambrian gneiss in the middle, surrounded by rings of quartzite and marble. These domes are not simple upwarps of the crust. The rocks of this region have been squeezed so tightly and have been so complexly deformed, that the folds have overturned, folded again, and later eroded to expose the Precambrian gneiss. The gneiss is a hard, resistant metamorphic rock that has remained a highland, while surrounding softer rocks have worn away.

Basalts, rhyolites and other Precambrian volcanic rocks, as well as Precambrian gneiss and quartzite, are found stretching across the Pennsylvania-Maryland border along the north-south line of Precambrian rock. South Mountain of Pennsylvania and Maryland’s Catoctin Mountains record the rifting of North America and Baltica in the Cambrian. As North America moved away from Baltica and the Iapetus Ocean opened up, cracks in the crust occurred that were similar to the younger Triassic rifts from Pangea. The rifts and fractures in the crust made pathways for emerging lava to pour out across the surface, forming the volcanic rocks seen today.

Cambrian-Ordovician Rocks

The Cambrian and early Ordovician sediments record the ancient North American shelf and slope sediments. Sandstone and shale were the dominant rocks formed from the eroding sediments of the continental highlands and limestone was formed from the abundant shelled organisms in the inland ocean. With the collision of the Taconic volcanic islands, the original limestone, sandstone and shale were metamorphosed in many areas, forming the marble, quartzite and slate that make up the bulk of the Appalachian/Piedmont region. The Cambrian and Ordovician rocks underlie the Champlain Valley of Vermont, the Taconic Mountains and highlands of western New England and eastern New York, and the prominent ridges and valleys further south in Pennsylvania, New Jersey and Maryland. In some areas, particularly the Marble Valley of western Massachusetts, Connecticut and southern Vermont, the less resistant marble is extensively exposed at the surface, forming a wide valley of fine marble that was once vigorously quarried for buildings and monuments.

The Taconic Mountain rocks are unusual because older Cambrian rocks (which should be beneath younger rocks) are overlying younger Ordovician rocks. This unusual situation occurs because the sediments deposited on the continental shelf and slope of ancient North America were shoved and stacked up onto the coast. Older rocks from the continental shelf and slope were thrust on top of younger rocks from the Inland Basin, causing no small amount of confusion when geologists first tried to unravel the history of the area. The older Cambrian rocks are a resistant cap atop the less resistant Ordovician sedimentary rocks, forming the ridge of the Taconic Mountains (Figures 2.16 and 2.17).

Figure 2.16: Cambrian rocks exposed in the Appalachian/Piedmont.

Figure 2.17: Ordovician rocks exposed in the Appalachian/Piedmont.

Figure 2.18: The Serpentine Belt exposed in the Appalachian/Piedmont.

Along a line from the middle of Vermont through western Massachusetts and Connecticut, southeastern New York, Pennsylvania and Maryland are small exposures of very unusual dark rocks that are part of ophiolite sequences (Figure 2.18). Ophiolites are made of deep-sea sediments, oceanic crust and upper mantle material that are rarely seen at the Earth’s surface. The line of ophiolite exposures is located along the ancient suture line between North America and the volcanic island terranes of the Taconic mountain building in the Ordovician period. These igneous rocks, known as the Ultramafic Belt, are very rich in magnesium and iron, but very low in silica, typically forming basalts, gabbros and peridotite. The peridotite, derived from the upper mantle, is often altered slightly through metamorphism to a greenish rock called serpentinite.

Ophiolites are recognized by their particular sequence of rocks that are not usually found at the surface. The sequence includes sedimentary rock from the ocean floor underlain by flows of pillow basalt. The pillow basalts form as lava pours out of cracks in the oceanic crust and cools very quickly in the seawater, creating a pillow-shaped mass of lava. Beneath the pillow lavas are sills of gabbro, a dark igneous rock formed from cooling magma beneath the surface. The lowest layer in the ophiolite sequence is composed of peridotite, a rock formed from the upper mantle layer of the Earth that is rarely seen at the surface. The subduction of the oceanic plate also caused igneous intrusions beneath the volcanic islands.


When North America was on its collision course with Baltica, the oceanic crust in between the continents was being pushed beneath the continental crust of the approaching North America. As the oceanic crust was subducted, some of the deep-sea sediments overlying the crust, the oceanic crust itself, and perhaps rock from the upper mantle, were scraped off the descending plate and did not get shoved back down into the mantle. Instead, the scraped off ophiolite was left stuck on the continental crust. Subsequent erosion exposed this odd group of rocks that is so unlike the surrounding rock of the continental crust. The ophiolites are significant in the geology of the Northeast because they record the subduction of the oceanic plate beneath the Taconic volcanic islands as they collided with North America. Figure by J. Houghton.

During and after the Taconic mountain-building event, sediments were deposited into the Iapetus Ocean basin and the inland ocean basin east and west of the mountains, mixing with and then covering the limestones that had been building up along the margin of North America prior to mountain building. Volcanic ash within these rock layers indicates volcanic activity occurring as the volcanic islands collided with the continent. The sediments of the Queenston Delta record the deposition of eroding sediment from the Taconic Highlands as well as the changing shoreline as the basin filled.

Light-colored vs. dark-colored igneous rocks

Dark-colored igneous rocks generally come from either mantle magma or melting oceanic crust at a subduction zone. Oceanic crust is already dark, dense and rich in iron and magnesium. The dark color originates from the iron and magnesium. The dark color originates from the iron and magnesium as well as a relatively low percentage of silica, and characterizes rocks such as basalt and gabbro. Light-colored rocks are formed from continental crust that is melted from the pressure of overlying rock or friction from colliding plates. Continental crust-derived sediments may also form light-colored rocks. Light-colored igneous rocks are very rich in silica and lack significant amounts of iron and magnesium, and include rocks such as granite. The abundance of silica also makes light-colored igneous rocks less dense than oceanic crust. Thus, continental crust, with a density of 2.7 g/cm³, is rarely subducted when plates collide because it is too buoyant to be pulled under another plate. Oceanic crust on the other hand, with a density of 3.2 g/cm³, is very dense and more easily pulled under an approaching plate.

Silurian-Devonian Rocks

Figure 2.19: Silurian and Devonian rocks exposed in the Appalachian/Piedmont.

Silurian and Devonian rocks are found primarily in the southwestern-most part of the Appalachian-Piedmont region (Figure 2.19). These rocks are very similar to the Silurian and Devonian rocks of the adjacent Inland Basin. They record deposition in the inland ocean and the collision of Baltica with North America, which formed the Acadian Mountains and renewed deposition in the inland ocean. The main distinction between the Silurian and Devonian rocks of the Appalachian/Piedmont region and the Inland Basin region is the compression and metamorphism of the Appalachian Piedmont rocks. 

Mississippian-Pennsylvanian-Permian Rocks

Figure 2.20: Mississippian, Pennsylvanian and Permian rocks exposed in the Appalachian/Piedmont.

The rocks of the Mississippian, Pennsylvanian and Permian periods of the Appalachian/Piedmont are also only found in the southwestern area of the region (Figure 2.20). Again, these rocks are very similar to exposures of the same age in the adjoining Inland Basin, recording the lush vegetation and swampy deposits of the receding inland ocean shoreline and deeper-water sediments. However, the rocks of the Appalachian/Piedmont were metamorphosed in many places. The soft coal seen in the Inland Basin is present as very hard anthracite coal in the Appalachian/Piedmont region. 

Triassic-Jurassic Rocks

Figure 2.21: Triassic-Jurassic rift basins exposed in the Appalachian/Piedmont.

Dissecting the southeastern tip of New York, northern New Jersey, eastern Pennsylvania and Maryland, are two connecting basins filled with rocks dating back to the Triassic and Jurassic (Figure 2.21). The northernmost of the two is called the Newark Basin, and the southern is called the Gettysburg Basin. In the adjoining Exotic Terrane region, a similar basin occurs in Massachusetts and Connecticut known as the Connecticut Valley rift basin. In Connecticut, there are a few other mini-versions of the large rift basins, where smaller faults formed tiny basins that preserved Triassic- and Jurassic-aged sediments.

The basins formed as blocks of crust slid down the fault planes (rifts) during the late Triassic and early Jurassic when Pangea was breaking apart. The basins that formed expose characteristic reddish-brown sedimentary rocks and ridge-forming basalt, an igneous volcanic rock also known locally as "traprock". Periodically the basins were filled with water, forming shallow lakes and depositing thin, dark layers of sediment typical of lake deposits. 

The rift valley igneous rocks were formed when magma pushed up through fractures in the crust and either poured out on the surface of the basin as flows of lava, or cooled and crystallized as igneous intrusions before reaching the surface (Figure 2.22). The igneous intrusions, typical within the rift basins, formed rather shallow within the crust. The relatively cold temperatures of the upper crust forced the magma to cool quickly.

Figure 2.22: The Palisades Sill is an igneous intrusion; the Wachtung Mountains are a volcanic extrusion of lava. Figure by J. Houghton.

The same magma that formed the Palisades Sill continued to rise towards the surface. The rising magma cut through the overlying layers and burst out onto the surface, spreading basaltic lava over the basin. These lava flows are recorded by the Wachtung Mountains of New Jersey, left standing because of the resistance of the tilted lava beds in comparison to the weaker sedimentary rocks above and below. 

Colors of sedimentary rocks: what do they tell us about the environment?

Color in rocks may be an important indicator of the type of environment in which the rocks were formed. The red-brown color so common in the rift basins of the northeast is present because of iron within the rock that has been oxidized (rusted!), which tells us that the rock formed in a seasonally hot and dry climate on land, where the iron could be exposed to the air and oxidized. Red sedimentary rock is also found in the Silurian rocks of the Inland Basin, reflecting a time of shallow seas in which the ocean floor sediments were often exposed above water and allowed to oxidize. In some marine environments, where iron is reduced rather than oxidized, rocks may take on a greenish hue. However, in well-oxygenated, deep marine conditions, red clays may form.

In contrast, most shales are gray or black in color, reflecting the abundance of organic material that can accumulate in quiet-water settings and preserve in fine-grained rocks that are relatively impermeable to oxygen-rich pore water. Shales are most commonly formed in quiet waters where tiny particles have time to settle out to the sea or lake floor, where there is very little oxygen to aid in the decomposition of the organisms, so the sediments retain a black color from the carbon of organic material. The darker the shale, the more organic material that is preserved within! The presence of certain minerals may also affect the color and aid in the interpretation of the environment of deposition. Green sedimentary rocks may indicate the presence of the mineral glauconite, found only in marine environments.