Photograph of the Blue Ridge Mountains taken in North Carolina.

Rocks of the Blue Ridge and Piedmont

Contents

Image above by "formulanone" (Flickr; Creative Commons Attribution-ShareAlike 2.0 Generic license).

Map showing the area of the southeastern United States occupied by the Blue Ridge and Piedmont physiographic regions.

The area of the southeastern United States occupied by the Blue Ridge and Piedmont physiographic regions.


Overview

The Blue Ridge and Piedmont are distinct physiographic areas, but they share similar types of crystalline igneous and metamorphic rocks. This region was at the center of several orogenic events that occurred throughout the Precambrian and Paleozoic, and many of the rocks found here were metamorphosed by the compressive forces of mountain building. The core of the Blue Ridge mountain range and the Inner Piedmont are the most highly metamorphosed, having been located nearly at the center of the continental collisions; the outer Piedmont is more variably metamorphosed. During the Paleozoic, continental collision compressed the Blue Ridge and Piedmont region further, causing folds, faults, intrusion by magma, shearing, and uplift. The region was pushed over 160 kilometers (100 miles) west, telescoping into a series of folded, thrusted crustal sheets that carried older rocks atop younger rocks, overturning the stratigraphic sequence. The Piedmont was thrust over the Blue Ridge, and the Blue Ridge was thrust over the rocks that lie farther west.


Image depicting how the rust of the Blue Ridge and Piedmont regions were telescoped by compressional forces.

The crust of the Blue Ridge and Piedmont was "telescoped" by the compressional forces of Paleozoic mountain building. Slices of crust were thrust over top of each other, stacking like a deck of cards.


The Brevard Fault Zone, one of the thrust faults that formed during this time period, is today considered to mark the border between the Piedmont and Blue Ridge areas.


Map depicting the position of the Brevard Fault Zone, which separates the Blue Ridge and Piedmont regions.

The Blue Ridge and Piedmont physiographic regions, as divided by the Brevard Fault Zone.


Along this 600-kilometer-long (370-mile long) zone, which stretches from Alabama to Virginia, the rocks were crushed and ground by the tremendous pressure of thrusting along the fault zone, creating cataclastic gneisses, schists, and phyllonite.

Precambrian Rocks & Grenville Orogeny

The Blue Ridge is dominated by rocks of Precambrian origin, including highly metamorphosed igneous and sedimentary rocks formed more than a billion years ago during the Grenville Orogeny—a mountain building event associated with the assembly of the supercontinent Rodinia. These Precambrian rocks are the oldest materials found at the surface in the Southeast, ranging from 1.8- to 1.1-billion-year-old gneisses along Virginia's Blue Ridge Mountains and North Carolina's Roan Mountain Highlands to 1-billion-year-old gneisses in the Georgia and Alabama Piedmont. Grenville-aged rocks were originally sandstone, shale, and limestone deposited in a zone called the Grenville Series (also called the Grenville Belt), a warm, shallow ocean along the eastern margin of proto-North America. During the formation of Rodinia, the Grenville Series sediments were squeezed and pushed up onto the continental margin, forming the Grenville Mountains. The intensity of compression metamorphosed the sedimentary rocks; sandstone became quartzite, gneiss, or schist, limestone became marble, and shale became gneiss and schist.

During the Grenville Orogeny, friction between the converging plates pushed magma into the overlying crust. Some magma rose high enough to intrude through the overlying sedimentary rocks, but it remained well below the surface. These amorphous intrusions eventually cooled and crystallized, forming igneous plutons of granite, anorthosite, and, less commonly, gabbro.


Simplified illustrations show how igneous intrusions form and persist over time.

Igneous intrusions form when cooling magma is trapped beneath the surface. These rocks, which are more resistant than the surrounding material, can later be exposed at the surface through the process of erosion.


As the Grenville Orogeny continued, the cooled plutons and sedimentary rocks of the Grenville Series were later covered by as much as 30 kilometers (19 miles) of sediment! High pressures and temperatures associated with the weight of the overlying material caused further metamorphism of the buried rocks. Today, these resistant igneous and metamorphic rocks can be seen at Old Rag Mountain in Virginia, Blowing Rock in North Carolina, and Red Top Mountain in Georgia, where they have been exposed by erosion.


Photograph of a man standing on the Blowing Rock.

The Blowing Rock, an immense 1.05-billion-year-old cliff of gneiss that stands 1200 meters (4000 feet) above sea level in the Blue Ridge Mountains of North Carolina. The rock was named for an updraft of air funneled toward the cliff by the Johns River Gorge below. Photograph by turcottes78a (Flickr; Creative Commons Attribution-NonCommercial-NoDerivs 2.0 Generic license).


Grenville-aged rocks are present in many other parts of the Southeast besides the Blue Ridge, but they are often deeply buried by younger overlying sedimentary rocks. Precambrian rocks are visible at the surface in the Blue Ridge and Piedmont region only because of the intense thrusting and deformation that occurred during Paleozoic mountain building events (especially the Alleghanian Orogeny), uplifting layers of rock that were once buried beneath many kilometers (miles) of crust. The rocks of the Blue Ridge were compressed into a giant anticline, or upward fold, that has become the "backbone" of the Appalachian range, preventing the mountains from being worn completely flat. Softer sedimentary rocks were eroded away at the peak of the fold, exposing the resistant Precambrian rocks at its center. Precambrian rock can also be seen throughout the region where "windows" in overthrust layers have eroded, exposing the ancient bedrock.

four-part illustration depicting the developmental stages of a geologic window.

The development stages of a window. Older rock is thrust over younger rock. Erosion begins through the older rock, eventually exposing the underlying younger rock.


One such example is Grandfather Mountain near Linville, North Carolina. This 750-million-year-old mass of rift basin conglomerate was covered by a 1-billion-year-old block of crust during the Alleghanian Orogeny, then metamorphosed by the pressure of thrust faulting. A window later eroded in the overlying thrust sheet, revealing the rock that we see at Grandfather Mountain today.


Photograph of Split Rock.
 The surface of Split Rock, a large weathered boulder at Grandfather Mountain, reveals large pebbles typical of meta-conglomerates. Photograph by turcottes78a (Flickr; Creative Commons Attribution-NonCommercial-NoDerivs 2.0 Generic license).

There are several such geologic windows in the Southeastern states, although not all of them expose Precambrian Grenville rock.


Map showing locations of geologic windows in the Blue Ridge and Piedmont regions.

Outstanding geologic windows in the Blue Ridge and Piedmont regions (shaded) of the southeastern United States, including Cades Cove, Tuckaleechee Cove, Wear Cove, Grandfather Mountain, Toxaway Dome, Brasstown Bald, Shooting Creek, and Pine Mountain.

Late Precambrian and Early Paleozoic Rocks

Beginning around 570 million years ago during the late Precambrian and early Cambrian, North America began to rift apart. As the rifts enlarged, many became basins that eventually filled with sediment eroded from the Grenville Mountains. Remnants of these ancient rift basins can found in the rocks at Mt. Rogers in Virginia, Reelfoot Lake in Tennessee, Grandfather Mountain in North Carolina, and outcroppings near Lynchburg, Virginia. The last sediments to fill the rift basins, known as the Chilhowee Group, were deposited in the early Cambrian.


Map showing outcrop locations of late Precambrian and early Cambrian rocks.

Outcrop locations of late Precambrian and early Cambrian (Chilhowee Group) rocks of the Blue Ridge and Piedmont regions of the southeastern United States.


Over time, the rift basin sediment was compacted and cemented together to become conglomerate, sandstone, siltstone, and shale. These rocks were metamorphosed to slate, phyllite, and quartzite during later orogenic events, and they are often referred to as "metasedimentary" due to the fact that their sedimentary structures are often well preserved.

Rifting and Widening of the Iapetus Ocean

As a result of continental rifting and the widening of the Iapetus Ocean, volcanic activity was common along the margin of North America during the late Precambrian and early Cambrian. Rifts and fractures in the crust made pathways for emerging lava that poured out across the surface for a period of several million years, covering over 10,300 square kilometers (4000 square miles) of land and cooling to form basalt. The Catoctin Basalt underlies Maryland’s Catoctin Mountains and caps many of the peaks in easternmost West Virginia as well as Virginia's Shenandoah Mountains. This basalt, originally a dark-colored volcanic rock, was highly metamorphosed during the formation of the Appalachian Mountains, and became a fine-grained dark green to light grey greenstone. Although most Shenandoah greenstones are found as boulders or jagged cliffs, the cooling basalt occasionally contracted to form polygonal structures called columnar joints.


Photograph of a man standing next to greenstone rocks that exhibit columnar jointing.
Columnar jointing in the greenstones of Shenandoah National Park, Virginia. Photograph by Anna Kim (Flickr; Creative Commons Attribution-NonCommercial-NoDerivs 2.0 Generic license).

Areas where new lava flows advanced over older ones are often marked by breccia, a chaotic layer of cemented sediments and rock fragments (see an example of a meta-volcanic breccia from the Catoctin Formation here).

At Mt. Rogers in southwestern Virginia, there is evidence of an explosive rift-related Precambrian volcano that formed around 750 million years ago. Mt. Rogers is named after William Barton Rogers, Virginia's first state geologist, who was famous for his studies of Appalachian Mountain geology. The lava from Mt. Rogers eventually cooled to form rhyolite sections as much as 750 meters (2500 feet) thick in some areas. Beneath these volcanic flows, exposures of diamictite in the Konnarock Formation record evidence of the Neoproterozoic "Snowball Earth" glaciations that occurred between 759 and 543 million years ago.


Photograph of a diamictite rock from the Konnarock Formation of Virginia.
Diamictite in the ~750 million-year-old (Neoproterozoic) Konnarock Formation of Virginia. Photograph by James St. John (Flickr; Creative Commons Attribution 2.0 Generic license).

The rocks of the Blue Ridge form the spine of the Appalachian Mountain Range and the western part of its core, whereas the rocks of the Piedmont form the foothills of these mountains and include the eastern part of the Appalachians. Most ancient rocks in the Blue Ridge are related to major Precambrian and Cambrian tectonic events, from the Grenville Orogeny to Cambrian rifting. However, most Piedmont rocks actually formed somewhere other than North America and were attached to the continent in a patchwork of volcanic islands, fragments of land (exotic terranes), and former ocean-bottom sediments.

Many Piedmont rocks are metamorphosed to varying degrees, and it is difficult to determine their exact origin or age of formation. Nevertheless, they are separated into two basic divisions, the Iapetus rocks and the Avalon rocks, based on their inferred origins.

Ordovician Rocks, the Iapetus Ocean, and the Taconic Orogeny

The Iapetus rocks (also known as the Inner Piedmont) include sediments deposited in the ancient Iapetus Ocean, which continued to widen throughout the Cambrian. These sedimentary rocks were once part of a wide carbonate bank that formed along the continental margin after eroded sediment dwindled from the nearly worn-down Grenville Mountains. During this time, the Southeast (and most of proto-North America) was entirely underwater. Sandstone and shale were the dominant rocks generated from eroding sediments in the continental highlands, and limestone formed from carbonate sediments and shelled organisms living in the ocean.

Between 500 and 460 million years ago, the direction of plate movement shifted. The Iapetus Ocean began to close as the continental plates once again moved toward each other, and the Taconic volcanic island arc developed at the subduction zone where the plates came together. As these islands approached North America, compression metamorphosed the limestone, sandstone, and shale, forming marble, quartzite, slate, phyllite, and schist. The Murphy Marble, which stretches across northern Georgia into North Carolina, dates from the Taconic Orogeny, where it was metamorphosed from limestone formed at the bottom of the Iapetus Ocean.


Photograph of a piece of Murphy Marble.
Murphy Marble, collected from a quarry near Tate, Georgia. Specimen is 7.6 centimeters (3 inches) wide. Photograph by James St. John (Flickr; Creative Commons Attribution 2.0 Generic license).

Marble is also quarried extensively from the Piedmont Uplands in Alabama, where it is the official state rock.

Evidence of the Taconic island arc's collision with North America can be seen throughout the Piedmont, where Ordovician-aged metamorphosed sedimentary rock from the volcanic islands is interlayered with metamorphosed volcanic rocks such as slate (originally ash) and greenstone (originally basalt).


Map showing locations of metamorphic and volcanic rocks in the Blue Ridge and Piedmont regions.

Locations of metamorphic and volcanic rocks in the Blue Ridge and Piedmont regions of the southeastern united states related to compression and accretion during the Taconic Orogeny.


The Hillabee Greenstone in Alabama is one such remnant of the Taconic island arc. Igneous intrusions resulting from the collision (e.g., granite, gabbro, and diabase) are located along the suture zone where the Taconic volcanic islands and ocean bottom sediments collided with the margin of North America, forming the Taconic Mountains.

Map showing locations of granite intrusions in the Blue Ridge and Piedmont regions.

Locations of granite intrusions related to the Taconic Orogeny in the Blue Ridge and Piedmont regions of the southeastern United States.


Small exposures of dark rocks called ophiolites are found along the Taconic suture zone, stretching from northern Georgia to southwestern Virginia. These rocks are composed of former deep-sea sediment, oceanic crust, and upper mantle material. Ophiolites appear when a subducting oceanic plate fractures, leaving behind a slice of oceanic crust on land, and they are among the only places where mantle rock can be seen on the Earth's surface. The resulting rock sequences are some of the most helpful tools we have for studying oceanic crust.

Simplified illustration showing the structure of an ophiolite sequence.

Structure of an ophiolite sequence.


An ophiolite sequence includes sedimentary rock from the deep sea, such as chert, underlain by pillow basalts that were extruded into the water at a mid-ocean ridge. Below the pillow basalts are intrusions of basalt known as sheeted dikes, formed as the mid-ocean ridge pulled apart. Below the basalt is gabbro, the plutonic version of basalt, and finally peridotite, the rock that composes the Earth's upper mantle. Peridotite is commonly altered slightly through metamorphism into a greenish rock called serpentinite.

Sample of serpentinite from the New England Orogen, New South Wales, Australia. Model by "Earth Sciences, University of Newcastle" (Sketchfab).

Middle Paleozoic Rocks and the Acadian Orogeny

The Avalon rocks (also known as the Outer Piedmont) were accreted to the margin of North America during the late Devonian Acadian Orogeny. These rocks include the Avalon microcontinent (made up of volcanic sediment, sandstone, mudstone, and intrusions) and the surrounding ocean basin sediment (made up of mud, ash, and sand) on either side of the microcontinent. During the microcontinent’s collision with North America, the Avalon rocks underwent varying degrees of metamorphism based on their distance from the center of the collision. Marine sediments became argillite, slate, gneiss, schist, phyllite, and quartzite; preexisting intrusions were metamorphosed to amphibolite, greenstone, serpentinite, metagabbro, and metabasalt. The Carolina Slate Belt, a weak to moderately metamorphosed section of Avalon rocks, stretches over 970 kilometers (600 miles) from Georgia to Virginia. Located in the Outer Piedmont, the belt includes argillite, slate, schist, and phyllite and contains significant gold deposits.


Map showing the extent of the Carolina Slate Belt within the Piedmont region.
Extent of the Carolina Slate Belt within the Piedmont region of the southeastern United States.

The collision of Avalon with North America also resulted in igneous intrusions throughout the Piedmont, similar to earlier intrusions formed during the Ordovician. Some of these intrusions formed pegmatites.

Late Paleozoic Rocks and the Alleghanian Orogeny

Africa collided with North America during the Alleghanian Orogeny of the late Pennsylvanian and Permian, uplifting the Appalachian Mountains and resulting in the formation of the supercontinent Pangaea. The collision resulted in intense metamorphism of the Blue Ridge and Inner Piedmont, moderate metamorphism in the Outer Piedmont, westward thrusting of the crust, and igneous intrusions throughout the Blue Ridge and Piedmont region. Stone Mountain in Georgia is a granitic and feldspar-rich pluton that formed deep below the Earth's surface during the Alleghanian Orogeny, and was later exposed by erosion.


Aerial photograph of Stone Mountain, Georgia.
Aerial view of Stone Mountain, DeKalb County, Georgia. Image by Nate Steiner (Flickr; public domain).

Arabia Mountain and Panola Mountain, two smaller granite outcroppings east of Stone Mountain in DeKalb County, were formed during the same intrusive event. Stone Mountain is 8 kilometers (5 miles) in circumference, and continues underground for up to 14 kilometers (9 miles) at its deepest point. Granite from the mountain was quarried extensively from the 1830s through the early 1900s, and the stone was shipped worldwide for use in buildings and structures as far ranging as the locks in the Panama Canal, the federal gold depository at Fort Knox, and the Imperial Hotel in Tokyo. Today, the mountain is famous not only for its geology but for the enormous basrelief carving on its north face that depicts the confederates Jefferson Davis, Robert E. Lee, and Thomas "Stonewall" Jackson. Given the carving's connections to racism and white supremacy, calls have been made in recent years to remove it from the side of Stone Mountain.

Mesozoic Rocks and Breakup of Pangaea

During the late Triassic and early Jurassic, Pangaea broke apart. Rifts formed in the crust along the margin of North America (as well as along the margins of Africa and western Europe), and blocks of crust slid down fault planes to form rift basins of varying size. The basins were periodically filled with water, forming shallow lakes in which were deposited thin, dark layers of poorly sorted sediment that solidified into red-colored sandstone and shale. Magma pushed up through fractures in the rifted crust, pouring out on the surface of the basin as lava or cooling and crystallizing below ground as igneous intrusions. The Southeast's Triassic- and Jurassic-aged rift basin deposits are part of a sequence of rocks known as the Newark Supergroup, which can reach thicknesses of up to six kilometers (four miles). They are found at the surface in Virginia and North Carolina, where they expose characteristic reddish-brown sedimentary rock and igneous basalt or diabase, also known locally as "traprock." There is also a very small, poorly exposed basin at the surface in South Carolina called the Crowburg Basin. While there are many other rift basins in eastern North America, most are now buried by younger sediment.

Diabase dikes that formed during the Triassic and Jurassic rifting period are found not only in the region’s rift basins, but also throughout the Piedmont. North and South Carolina claim the largest diabase dike in the eastern United States, "the Great Diabase Dike," which extends across the border between the two states for 35 miles. The dike is more than 300 meters (1000 feet) wide in sections. Diabase from this dike is exposed near Forty Acre Rock in Lancaster County, South Carolina, a large exposure of granite that was emplaced during the Alleghanian Orogeny.


Aerial photograph of Forty Acre Rock, South Carolina.

Drone-capture aerial photograph of Forty Acre Rock near Kershaw, South Carolina. Photograph by Ed McDonald (Flickr; Creative Commons Attribution-NonCommercial 2.0 Generic license).