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Geology
The geology of Great Smoky Mountains National Park is summarized in King and others (1968), and the geology of the Mount Le Conte quadrangle is discussed by Hadley and Goldsmith (1963). A good layman's guide to the geology of the Park is by Moore (1988), which also contains technical and non-technical references. A regional tectonic synthesis, which includes the area of Great Smoky Mountains National Park, can be found in Hatcher (1989), and an extensive reference list on the technical aspects of the geology in the park accompanies a recent study by Montes (1997). The summary below is taken from these sources.
Great Smoky Mountains National Park lies within the Appalachian Blue Ridge geologic and physiographic province. The highest mountains in eastern North America occur in the Blue Ridge province, and some of the highest peaks in this province are in the Great Smoky Mountains National Park. The oldest rocks in the Blue Ridge province are at least 1 billion years old and consist of metamorphosed sedimentary and igneous rocks. These Proterozoic rocks form the core of the ancient Appalachian Mountains. Sediments deposited over these older rocks form the majority of rocks in the Great Smoky Mountains National Park.
Most of the rocks in Great Smoky Mountains National Park are metamorphosed sedimentary rocks. These sedimentary rocks were formed from approximately 800 to 450 million years ago from deposits of clay, silt, sand, gravel, and lime or calcium carbonate. The oldest sedimentary rocks consist of vast amounts of clastic material that washed down into lowland basins from adjacent highlands. Rocks of the ancient highlands are similar to the ancient granite, gneiss, and schist found in the eastern parts of the Great Smoky Mountains National Park (Hadley and Goldsmith, 1963). Approximately 450 million years ago, the rocks were metamorphosed or "changed" by heat and pressure. For example, sandstone was changed to quartzite and shale to slate.
About 200 to 300 million years ago, the last phase of Appalachian Mountain building, (the Alleghenian orogeny) occurred when the North American continent collided with the African and European continents, closing the early Atlantic Ocean. This process, part of a continuous mountain building cycle known as "plate tectonics," ended sedimentation in the Appalachian region and uplifted the entire Appalachian mountain chain from Newfoundland, Canada to Georgia. These mountains may have been much higher than they are today, with elevations perhaps similar to the present-day Rocky Mountains. As the continents collided, the original horizontal layers were bent or folded and broken by fractures and faults. Tremendous forces caused huge masses of older rocks to be pushed westward, up and over younger rocks, along nearly flat-lying faults. Rocks in the Great Smoky Mountains National Park moved westward on the Great Smoky Mountain fault.
Bedrock in the Mount Le Conte quadrangle consists of a thick sequence of Late Proterozoic (800-600(?) million years old) metamorphosed sandstone, siltstone, shale, and conglomerate. A few metamorphosed igneous dikes of uncertain age occur in the Mount Le Conte area (Hadley and Goldsmith, 1963). The metasedimentary rocks in the Mount Le Conte quadrangle have been divided into two groups, chiefly on the basis of age and rock characteristic.
The oldest rocks exposed in the Mount Le Conte area are
metasandstone and metasiltstone of the Roaring Fork Sandstone.
Named after the rocks exposed along Roaring Fork, the
metasandstone forms ledges and falls along Little Pigeon River at
the entrance to Greenbriar Cove.
The metasandstone is characterized by light green color due to
chlorite and slump folds that occurred before the sediments were
lithified to rock .
Pieces of green metasiltstone similar to the overlying Pigeon
Siltstone occur as ripped-up fragments in the metasandstone
(adjacent to rock hammer).
Overlying the Roaring Fork Sandstone is fine-grained laminated
green metasiltstone called the Pigeon Siltstone. Named after excellent
exposures along the Pigeon and Little Pigeon Rivers, the laminated
metasiltstone displays microfaults and fractures (upper left) and
folds (upper center) that developed before the silt was lithified to rock.
The Pigeon Siltstone and the Roaring Fork Sandstone are part of the older Snowbird Group and are overlain across the Greenbrier fault by the younger Elkmont Sandstone, Thunderhead Formation, and Anakeesta Formation of the Great Smoky Group (Hadley and Goldsmith, 1963).
Gray and brown metasandstone and dark phyllitic metasiltstone
of the Elkmont Sandstone overly the rocks of the Roaring Fork
Sandstone along the Greenbriar fault.
The dark metasiltstone is sheared along the Greenbriar fault.
Overlying the Elkmont Sandstone are massive thick beds of
light gray metasandstone and conglomeratic metasandstone
interbedded with dark metasiltstone that comprise the Thunderhead
Sandstone. Named after the rocks exposed on Thunderhead
Mountain to the west, the massive metasandstone forms high cliffs
on the north-facing slope of Mount Le Conte .
The metasandstone is both thick bedded and medium bedded with
calcareous concretions that weather out to form ovoids (near the
brunton compass).
These are turbidite deposits with graded beds. The clastic
sedimentary rock is composed of coarse grains (1mm) of white
feldspar and blue and gray quartz that were weathered from an
older granitic rock.
Dark metasiltstone is interbedded with the metasandstone and
locally there are folded vein quartz showing deformation within the
formation . These dark slaty rocks are the precursor to the similar
overlying rocks of the Anakeesta Formation.
Named after Anakeesta Ridge south of Mount Le Conte, rocks
of the Anakeesta Formation include dark, light, and variegated
metasiltstone, phyllite, and slate, gray metasandstone similar to the
Thunderhead Sandstone, and dark sandy dolomite. Dark slaty rocks
of the Anakeesta Formation exposed along Newfound Gap Road
contain bodies of metasandstone and sandy dolomite.
Dolomite is recognized by the pitted dissolution surface.
Along the Boulevard Trail are laminated metasiltstone with
garnet crystals.
Along Boulevard Trail and Mount Le Conte are light fine-grained
quartz rich metasiltstone that contains crystal tablets of the mineral
chloritoid.
At the headwaters of Porters Creek northeast of Mount
Kephart are light colored, fine- to medium- grained metasandstone
interbedded with dark metasiltstone. Similar rocks are interbedded
throughout the Anakeesta Formation.
These rocks have been metamorphosed by differing degrees of temperature and pressure. On the west, the rocks are chlorite grade, which indicates a lower temperature and pressure than that experienced by rocks on the southeast part of the quadrangle, which are garnet grade (Hadley and Goldsmith, 1963). Summaries of available data on the timing of thermal metamorphism of rocks in the park and surrounding areas are found in Drake and others (1989). Most studies indicate that the peak of metamorphism occurred during the Ordovician , around 450 million years ago.
The rocks in the Mount Le Conte quadrangle were deformed by folding, faulting, fracturing, and jointing. Additionally, cleavage, a form of planar "fracture" in the rocks caused by metamorphism and folding, is common in the more shaly rocks (Hadley and Goldsmith, 1963). The largest fault in the quadrangle, the Greenbrier fault, is thought to have formed prior to the regional metamorphism, or prior to about 450 million years ago. Other faults in the quadrangle, such as the Gatlinburg and Mingus faults, occurred during the Alleghanian orogeny, approximately 250 million years ago. Cleavage and folding is thought to have occurred both prior to and during the Alleghanian orogeny (Hadley and Goldsmith, 1963).
The surficial geology in the study area consists of alluvium and terraces along modern low-elevation drainages, and extensive boulder colluvium and boulder debris deposits. Also, modern debris-flow deposits occur in the southern part of the quadrangle.
Terrace deposits and alluvium characteristically contain both coarse and fine material.
The Glades, north of Highway 73 between Gatlinburg and the
Greenbriar area, is an abandoned terrace probably deposited by a
precursory drainage of Dudley Creek. The flat pastures and athletic
fields of the high school have rounded gravel that were deposited
prior to diversion of Dudley Creek westward to Gatlinburg.
Coarse alluvium in the highlands near the Chimneys Picnic area.
The car-sized boulders probably were deposited on the slopes and
have been exposed and reworked as streams erode down into the
older slope deposits.
The very coarse material was derived from reworking of older boulder fans as streams eroded and cut into the older deposits. Finer grained material was derived directly from scour of bedrock, or erosion of soil and weathered rock (saprolite). At lower elevations, cobbles and boulders are well rounded and are in a fine-grained sandy matrix. Alluvium along steep gradient streams at high elevations consists only of poorly rounded coarse boulders and cobbles.
Colluvium consists dominantly of boulders and cobbles derived from weathering of bedrock.
Boulders and cobbles of mostly metasandstone have been
deposited downslope by gravity and some have modern streams
that periodically flow through them and modify the landform and
deposit.
Some of these deposits are within site of the bedrock source
whereas others are found miles away from the originating outcrop.
This rocky material has been transported downslope chiefly by gravity. However, some areas mapped as colluvium also contain boulder streams and boulder fields that may have formed in periglacial environments at the higher elevations during the latter part of the Pleistocene (Delcourt and Delcourt, 1985). In the boulder streams and boulder fields, gravity, solifluction, freeze-thaw ice wedging, and ice rafting may have contributed to downslope movement. In general, colluvium and the periglacial deposits contain little or no matrix or fine-grained material, such as soil, sand, or clay, between the coarse rubble. Additionally, boulder streams, boulder fields, and coarse colluvium often do not have trees and other vegetation growing on them and are usually very well drained. No evidence of present day movement is obvious.
Debris deposits are believed to be the result of ancient, large debris flows and floods that carried large boulders and cobbles down steep-walled valleys and deposited them as fans on the lower slopes. These deposits usually have a finer grained matrix and have soil developed on them.
Large fan-shaped deposits of forest-covered debris and boulders
form extensive deposits on the lower slopes of Mt. Le Conte. Some
of the larger fans extend from Balsam Point into Sugarlands and
can be seen from Campbell Overlook.
Today, the deposits are tree covered, stable on the slope, and are being incised by the present streams. Both the size of the deposits and the contact between debris deposits and colluvium is usually gradational.
Modern debris flows are common in the Park, particularly on the highest slopes underlain by shales of the Anakeesta Formation. The debris flows were the result of heavy rain and consist of weathered and fresh bedrock, soil, and vegetation, including trees, shrubs, bushes, and grass.
There are abundant debris flows in this part of GSMNP and virtually
all of them have occurred on the steep slopes and thin soil underlain
by rocks of the Anakeesta Formation. They have historically
occurred after storms with high rainfall (Bogucki, 1970). They are best
seen from the Chimney Tops, Newfound Gap, and along The
Boulevard Trail.Looking east from Chimney Tops, the scenery of
the highlands is the result of debris flows on steep slopes underlain by rocks of the Anakeesta Formation. The treeless areas are scars of debris flows.
The most spectacular debris flows are found on the east side
of Newfound Gap. These debris flows occurred in 1951, 1984, and
1993. The chutes reveal fresh bedrock and deposits of debris.
The south side of Mount Le Conte has extensive debris flows on
this residuum developed on the light colored chloritoid slate unit
of the Anakeesta Formation. The Boulevard Trail (lower right) has
had to be rebuilt on several occasions.
The map shows both the debris-flow deposit on the lower slopes and the debris flow scar or source area higher on the mountain. These debris flows pose a hazard to trail hikers and motorists in the park. Whenever several days of heavy rain have occurred, debris flows can be expected in this part of the park.
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