great smoky mountains geology

This is a view of Charlie’s Bunion as my Venture Crew and I trekked our way through the Great Smoky Mountains National Park on the Appalachian Trail several years ago. Thunderhead Mountain, Charlie’s Bunion and Sawteeth are my favorite parts of the trail.

Although the Rocky Mountains are taller and the peaks are more jagged, the Appalachian Mountains are the oldest mountains in the United States. Age . . . that’s actually why the Appalachians are smoother and rounder . . . a billion years of erosion. The Great Smoky Mountains are the highest peaks in the Appalachian mountain range. I grew up in the Smokies, and I get back to them as often as I can. When I was a boy growing up in Western North Carolina, I never wondered how the Smokies were formed, I just enjoyed hiking them, sitting on the balds, peering out over the vast expanse of blue and purple mountain tops. Now that I am older I find it fascinating to study the geologic history of the areas in which I hike.

Journey Back
So sit back and enjoy at journey back through time when an ancient sea flooded what is now the eastern United States, submerging the remnants of an old mountain range. The sea slowly deposited layers and layers of sediment onto the ocean floor. The intense pressure of thousands of feet of sediment compressed these layers into metamorphic rock. Almost 300 million years ago, the sea added yet another layer of limestone sediment that was composed of fossilized marine animals and shells. The stage was set for the formation of the Appalachian Mountains. [01]

As a result of the eons-old shifting of the earth's tectonic plates (large sections of the earth's crust), Africa and North America collided about 250 million years ago. This caused the older, underlying layer of metamorphic rock to tilt upward and slide over the younger limestone rock, slowly creating a towering mountain range, the Appalachians. The older rocks, known as the Ocoee Series, now compose most of the Great Smoky Mountains. Charlie’s Bunion (pictured above), Sawteeth and Chimney Tops are dramatic examples of how the rock layers tilted and buckled to form steep cliffs and pinnacles. In Cades Cove, erosion of the overlying metamorphic rock reveals the limestone layer beneath. [01]

During the ice ages, massive boulders were created by alternating freezing and thawing of the rock. You can see boulder fields on the Cove Hardwood, Noah "Bud" Ogle and Roaring Fork Motor Nature Trails. The Smokies originally looked more like the Himalayas than the rounded mountains we see today. The relentless erosive force of water has sculpted their present-day appearance. Water run-off has also helped to carve the alternating pattern of V-shaped valleys and steep ridges. Landslides caused by a torrential downpour in 1951 created the large V-slash on Mount LeConte, and rock slides in 1984 briefly closed Newfound Gap Road. As you explore the park, look for how water continues to sculpt the land. [01]

A look at the rocks and minerals in the park
Although the nature of the rocks of the Great Smoky Mountains is puzzling, they are geolocially interesting because they contrast the Paleozoic sedimentary rocks of the Appalachian Valley on the northwest with the metamorphic rocks and granite of the Blue Ridge on the southeast. Like the rocks of the Appalachian Valley, most of those in the Great Smoky Mountains are sedimentary. The rocks of the Appalachian Valley are made up of a variety of fossil-bearing limestone, sandstone, and shale. Those of the mountains are a great mass of pebbly, sandy, and muddy sedimentary rocks, devoid of fossil remains. [02]

Close examination of the rocks of the Great Smoky Mountains indicate that they were deposited later than most of the rocks of the Blue Ridge, which are of earlier Precambrian age (formed more than a billion years ago), but before the rocks of the Appalachian Valley, which are of early to middle Paleozoic age (formed 600 million to 300 million years ago). Most of the rocks of the Great Smoky Mountains were formed during some part of later Precambrian time (a billion to 600 million years ago). [02]

Basement Complex
Basement Complex, the rocks of the Blue Ridge Mountains, have a crystalline foundation. They extend along the southeastern side of the Great Smoky Mountains and reappear at several places within the mountains where tectonic forces have pushed them up or have thrust them into contact with younger rocks. [02]

The basement complex consists of a wide variety of gneiss and schist, including layered gneiss from sedimentary or volcanic rocks and non-layered granite-based gneiss. The layered gneiss contains various amounts of biotite, muscovite, quartz and feldspar. They also contain small amounts of mica schist and larger amounts of hornblende. The non-layered gneiss is mostly quartz monzonite and granodiorite whose chief minerals are biotite, epidote and magnetite. [02]

The hornblende gneiss of this area may have been part of a volcanic flow, and the granitic rocks which dominate the northwestern part of the Blue Ridge Mountains may have originated partly as magma that invaded the rocks. Rocks of the basement complex date as far back as one billion years with a scattering of rocks as young as 350 million years. [02]

Ocoee Series

The later Precambrian sedimentary rocks, which form most of the Great Smoky Mountains and large parts of the adjacent foothills, are known as the Ocoee Series. This series extends far beyond the Great Smoky Mountains to the northeast and southwest, along the trend of the ranges–from northeast of Asheville, NC, at least as far as Cartersville, GA, a distance of more than 175 miles. Near the Great Smoky Mountains this series extends across the ranges about 30 miles (wider in some places). [02]

Toward the northwest of the Ocoee Series the clay minerals in the sedimentary rocks have been altered to chlorite; southwestward these minerals have been transformed to biotite and garnet; to the southeast these minerals are represented by staurolite and kyanite. The southeastern rocks with dominant clay minerals have changed from shale to slate, phyllite and schist. [02]

The Ocoee Series can be divided into three groups: Great Smoky Group which forms the main mass of the Great Smoky Mountains; the Snowbird Group occurs in the middle, in the foothills just north of the mountains; and the Walden Creek Group which occurs in the northwest, in the part of the foothills nearest the Appalachian Valley. Characteristic Snowbird outcroppings of Pigeon Siltstone may be seen north of Gatlinburg and characteristics of Roaring Fork Sandstone may be seen southeast of the Great Smoky Mountains park headquarters. [02]

The Great Smoky Group is a thick mass of sedimentary rocks, pebble conglomerate, coarse to find sandstone, and silty rocks, which can be divided into three formations: the fine-grained Elkmont Sandstone below, coarse-grained Thunderhead Sandstone in the middle, and cark sily rocks of the Anakeesta Formation above. Both the Elkmont and Thunderhead Sandstones are gray and composed principally of quartz and potassic feldspar, with a small amount of plagioclase feldspar and light-colored granite and quartizite. The Thunderhead Sandstone may contain blue-tinted quartz grains. The Anakeesta Formation consists mainly of dark silty rocks altered to slate, phyllite or schist. [02]

Sedimentary rocks of the Walden Creek Group form the northern and northwestern parts of the foothills. This group is mostly chale and siltstone, but it includes masses of conglomerate and sandstone, as well as nimor layers of quartzite, limestone and dolomite. [02]


[01] Sculpted by Water. Geology. Great Smoky Mountains. American Park Network. http://www.americanparknetwork.com. accessed 19 April 2007.

[02] Bedrock Geology. Geology of the Great Smoky Mountains National Park, Tennessee and North Carolina. Geological Survey Bulletin 587. USGS. http://www.cr.nps.gov/history/online_books/geology/publications/pp/587/sec1.htm. accessed 19 April 2007.