Volume V - Nos. 2 & 3
UP FROM THE SEA
A Geologic Biography of the Great Smoky Mountains
BY GEOFFREY W. CRICKMAY,
ASSISTANT PROFESSOR OF GEOLOGY,
UNIVERSITY OF GEORGIA
"Once upon a time, many millions of years ago" begins the story of the Great Smokies in the book of the mountains themselves. Yet, despite the fairy tale opening, the story is one of scientific exactness, the summation of many careful and accurate observations, checked by detailed microscopic study. There are still blank pages of geological problems that have never yet been solved; and pages blurred or even erased by the long, long times that have come and gone since they were written.
To most of us, the phrase "many millions of years ago" refers to a time in the vague past, beyond the reach of the known and the familiar, beyond historical time, into geological time. The book of the history of the earth is of such immense antiquity, that if each word in this article represented half a million years, there would be just about enough words to carry the reader back to the beginning of the first chapter of the story of the Great Smoky Mountains.
When we inquire into the age of the mountains, distinction must be made between three great events: the formation of the rocks, their folding and uplift, and their erosion into the forms seen today. The rocks of the Great Smokies are for the most part younger than those of the Blue Ridge to the east, but older than those of the Appalachian Valley to the west. Again, the rocks of the Great Smokies are younger than some of those in the Rocky Mountains, but the Appalachians were uplifted long before the western cordillera. The moderate slopes and rounded crests of the Appalachians are not a mark of great age, but are a natural result of the geologic processes by which they have been formed. These processes are operative today, and thus the mountains are only as old as yesterday. Tomorrow new forms will have been shaped.
The visitor who, from some high vantage point, views a sea of clouds below, with peaks upthrust like islands, has looked upon a scene from the past. A real sea once covered this entire area. For proof one needs only to examine exposures on the roadside from Newfound Gap to Forney Ridge. There the rocks are found in distinct layers; and some strata contain water worn pebbles and concretions. A close study shows that the rocks were originally sands and clays, much like the sediments along our present shore.
Literally thousands of feet of sediments were deposited in this ancient sea. The oldest strata underlie the Great Smokies, but to the west in the Appalachian Valley area younger layers were deposited. Geologists have shown that these sediments must have been washed into the sea from an ancient continent, called Appalachia, which rose to unknown heights to the east. Primitive animals and plants undoubtedly lived during these early times, but their remains have not been found in the rocks of the mountains. Some strata in the Appalachian Valley contain abundant skeletons of corals, shell-fish, clams, and other marine animals; some layers contain the remains of land plants. Quite evidently then this sea was not part of the deep ocean, but occupied a broad, shallow depression alternately filled and drained of its marine waters.
The visitor, from his vantage point, looks upon a changing scene; the sea of clouds is wafted away, and the bold mountains appear to be thrust up into view. The same change affected the ancient Appalachian sea. As though squeezed in the jaws of some gigantic vise, the horizontal strata were folded, twisted, and torn apart. Partly because of these compressive forces, the sediments were changed to hard rock. Sand became sandstone and then quartzite; clay became shale and then slate. No rock was strong enough to withstand the tremendous pressure, which seems to have been a great shove from the east. Without violent volcanic activity or catastrophic disturbance, a new land slowly arose from the sea, a land which was to be carved later into the Appalachian Mountains.
Two exposures will serve to demonstrate this chapter of geologic history. At a parking area about midway between Newfound Gap and Forney Ridge, quartzite layers are found inclined to the southeast. This angle of dip, which persists along the road, is a rough measure of the deformation of once horizontal beds. All the rocks are broken by cracks, called joints, some of which are regularly spaced and filled with white quartz, a mineral deposited there by seeping water. These joints correspond rather closely to cracks in a plastered wall resulting from sagging foundations.
In some places the folding was so sharp that the strata were torn apart along great fractures, called faults. There are probably hundreds of such breaks in the rocks of the park. They can be seen in the cliff of Alum Cave where thin quartzite layers in slate have been buckled into S-shaped folds and torn apart. But nowhere in this area is there a large fault displayed as well as in road-side exposures on Tennessee State Highway No. 73, about two and a half miles southeast of Townsend, in Tuckaleechee Cove.1 Here the dark colored slates of the mountains rest on top of blue-grey limestone. The limestone is known to be younger than the slate. The cove is everywhere underlain by limestone but around its margin the old slate is found resting on top of the younger rock. The old mountain rocks must have been shoved up and over the younger valley rocks along a great fault. Geologists have estimated the displacement on the fault to be as much as 35 miles. Erosion has cut through the overthrust mass leaving Tuckaleechee Cove as a window through which the structure of rocks beneath the mountains can be seen. No man knows the cause of these great deforming forces, but when they were spent the area became solid land subject to slow but repeated uplifts and continuous erosion.
The visitor from his vantage point looks out over the panorama of the Great Smoky Mountains; the clouds of the past have cleared away; below he sees the tortuous channel of Little River, the cascading waters of Roaring Fork, the sharp cleft of West Prong, the Oconaluftee, the Cataloochee, and a hundred more rivers, streams, and brooks. Running water, the sculptor of the mountains, is at work, grinding with the sand and gravel in the stream bed, cutting valleys ever deeper. If it were not for repeated uplifts of the land, these streams long since would have reduced the entire area to a lowland. If it were not for erosion, the area would be a high plateau. Erosion has dug its deep trenches through which land waste is carried back toward the sea. The Mississippi delta can be thought of as the other half of the Great Smokies.
The erosional part of the story starts when the rain first falls on the ground, on mountain top and valley walls. Part of the water soaks into the ground, following cracks and crevices, later to emerge as mountain springs, but in its underground course this water has effected an alteration of the rocks. At Alum Cave, on one of the trails to Mount LeConte, a white to yellow powder fills crevices in slate, deposited there by ground water. This material is mainly of sulphates of aluminum, iron, and magnesium. Veins of white quartz in rocks along the road from Newfound Gap to Forney Ridge likewise have been deposited by ground water in a past age. Here it is clearly seen that seeping water both precipitates and dissolves minerals. Rock cracks have been wedged open, partly by frost, partly by growing tree roots, and these cracks are filled with spongy "rotten" rock. From the hard firm rock at the base of these road cuts, gradations can be found into the loose, friable soils above.
All rocks exposed to the weather are thus loosened and altered, but all are not equally susceptible. Hard quartzite is extremely resistant to weathering and thus areas underlain by this rock make prominent ridges and high peaks, like Forney Ridge and Clingmans Done. The slates of the area form lower but sharper peaks, such as the Chimneys and Charlies Bunion. Limestone, the most soluble of all common rocks, has come to occupy the lowest ground and is found only in the coves, as near Townsend, Tennessee. The form and position of most of the mountains in the park are actually a reflection of the character of underlying rocks.
Once time foundation rocks have become loosened by weathering, they start their relentless downward journey to the sea. True enough, erosion in a heavily, forested park is slow --- slow by the human clock. Does not the ancient spruce deny that the mountain slopes are being washed away? It is slow when compared even to the life span of forest trees. But every year literally millions of tons of soil and rock are carried down the slopes, some by sudden landslides (they scar the sides of Mount LeConte), some by slow hillside creep.
The Great Smoky Mountains National Park is young, one of the youngest of the park system, but the mountains themselves are old, older than the hills. The keenest visitor will see from his vantage point more than a three-dimensional panorama; he will see a fourth dimension of time and a kaleidoscopic change of scene that starts "millions of years ago."
1H. S. Ladd, "To Move a Mountain and Make a Window", The Regional Review, Vol. II, No. 1, pp. 13-15.
|<<< Previous||> Contents <||Next >>>|