THE SOUTHERN APPALACHIAN MOUNTAINS
How They Got Where They Are
by Kempton H. Roll
The beautiful Blue Ridge and the majestic Great Smoky Mountains. The southeast's great vacationland, mecca for Fall "leaf lookers," campers and hikers. The summertime escape from the heat of the lowlands. The source of gems and minerals for industry and "rockhounds." And home to increasing numbers of retirees. These are the historical Southern Appalachians shared by the citizens of West Virginia, North and South Carolina, Tennessee and Georgia.
Those who have just discovered them as well as those who have been calling them home for generations, may have wondered how these mountain ranges that add so much beauty and tranquillity to our lives were formed and what this area of the South was like before they existed. Or were they always here? And how about the Ice Age? Did this part of the United States escape the deep freeze? Was the land scraped clean by glaciers like that of our northern neighbors? How high were the tallest peaks before they were worn down to their present height? Or are they still "growing" like the Rocky Mountains out west?
It is quite possible to come up with reasonable answers to these questions. Geology is a well established science and geologic deduction can tell us a great deal without having to be witnesses to an event. It explains the sometimes subtle, sometimes violent, but always powerful forces of nature that are impacting on us today just as they were millenia ago. If you are a if rock hound," you no doubt already know a great deal about these forces and how they can create beautiful minerals and delicate crystals. But let's look at the bigger picture and try to interpret the geological signs we see about us; signs that might offer some clues as to how these mountains were created and what it was like on this part of planet Earth before they arrived.
First, go back in time. Way back. Strangely, this is not an easy thing to do because we all tend to equate time with what we ourselves can understand with the help of clocks and calendars (unless we are astronomers and think in terms of light years - or geologists who think of "ages" and millenia!). Forget years. They are far too short a span to measure geologic time. Start with a century - 100 years - slightly more than the average lifetime. One hundred centuries is 10,000 years. The entire history of mankind stretches back only about 60 or 70 centuries. This is nothing compared to geologic history where centuries, even thousands of them, are a mere drop in an exceedingly large bucket of time. Here is another way to relate to the immense magnitude of geologic time: If we could compress into one year all the centuries that have elapsed since the beginning of the Universe, modern man would have been on Earth for less than a second!
The author's curiosity about the origins of the mountains in which he now resides and his effort as a "rock hound" to understand why the southern Appalachians are so special, and fascinating, geologically speaking, is the reason for this article.
Now transport your thinking back to 46 million centuries ago. This is the calculated age of the oldest rocks we've found on Earth. This was when our planet was beginning to settle down and pull itself together. The age of a layer of rock can be determined by the rate of radioactive decay of its mineral content and often, though less accurately, by its fossil content. Based on this, according to Harry L. Moore in his "Roadside Guide to the Geology of the Great Smoky Mountains National Park," we know that the oldest rocks found in the Blue Ridge mountains are 10 million centuries old. In fact, most of the rocks in the Appalachians are this old. They include metamorphic gneisses, schists, some granitic rocks, as well as sedimentary rocks which are the youngest. They range in age from 3 to 5 million centuries.
That leaves a time gap of at least 36 million centuries before any of the Blue Ridge rocks were even formed.
Try to comprehend what could have been happening during that vast span of time - so many centuries that it boggles the imagination.
In fact, we do have a fairly good idea of what was happening. According to Norman Cutler Smith, a geologist at the University of North CarolinaAsheville (College for Seniors), by studying the types of rocks composing these mountains, geologists have deduced without doubt that this particular portion of Earth was definitely under water - the bottom of a vast ocean, slowly filling up with sediment and sand. Much of the sediment was probably volcanic ash thrown into the atmosphere by ancient volcanos whose vestiges have all but disappeared from this part of North America. The residue was washed off the land by rain water, streams and rivers.
As the sediment became deeper and deeper, the weight of the material itself and the water above created enough pressure to cause the particles at the oldest and thus lowest levels to consolidate into a layered, reasonably solid rock mass, i.e., sandstone, siltstone, limestone; the latter formed from calcium carbonate muds and the skeletons of marine life forms.
THE GREAT COLLISION(S)
But something else was happening at the same time. Huge rock masses (tectonic plates) composed of continents and portions of ocean basins afloat on the earth's semi-molten mantle, were alternately separating and then drifting toward one another. Geologists have long known that at one point the continents were bunched together in a massive proto-continent, dubbed "Pangaea." Recently discovered evidence reveals that the North American "tectonic plate" (continent) was at one time bordered on the east coast by what we now know as South America, on the southwest by Antarctica and the northwest by Australia. The evidence is based on a contiguous rock formation known as the "Grenville Belt" that can be traced from northern Canada and down the eastern seaboard of the United States only to show up once again in Australia as well as on the Antarctica plate. It was like a rocky "ribbon" that once tied all these distant continents together into one huge land mass.
When continents collide, something has to give. What was happening was not unlike compressing a multi-layered piece of water-softened, thick, soggy cardboard one side toward the other. The material in the middle has to go somewhere so it crumples to form ridges and grooves (mountains and valleys). Increasing the pressure, forces some sections of the cardboard to fold over on itself, crunch up and perhaps even break off in some places.
Like the cardboard, the layers of sediment being steadily squeezed together were now emerging out of the water and forced upward higher and higher by the advancing continents. What had been ocean floor was now becoming mountain top.
"Orogeny" is the technical name assigned by geologists to the effects of the collision of these moving continents that form the crust of our globe. The southern Appalachian mountain-building episode, according to Dr. J. William Miller, Jr., assistant professor of geology at the University of North CarolinaAsheville, consisted of a series of at least three such events which began 15 million centuries ago and ended about 2.3 million centuries ago at the end of the Paleozoic time. It culminated in what is called the "Appalachian Orogeny".
So what were once layers of sedimentary rock thousands of feet thick and comprising the bed of an ocean, began rising up and ultimately formed peaks at least as high as the present day Rocky Mountains. Perhaps higher. Based on the tremendous volume of material eroded from them and deposited along the east coast of the United States, it is estimated that a layer of rock as much as 20 miles in thickness has been removed.
THE WEARING DOWN
Then, after a while, give or take a few million centuries, these same continental plates reversed direction, gave up trying to squash each other and began drifting apart, leaving mountainous piles of scrunched up rock on the edge of the restless continents: the Appalachian Mountains in North America, the Pyrenees in Europe and the Atlas Mountains in Africa. The gap between North America and Afro-Europe became the Atlantic Ocean.
This plate movement is still going on and the Atlantic is still expanding at the rate at which fingernails grow: roughly two centimeters per year!
Now exposed to the atmosphere and the elements, the relentless forces of nature began bringing them down. The effects of erosion no doubt kept pace with the sporadic structural uplift so that the elevations of the highest peaks, though certainly greater than today, may never have been so spectacular as the present day Himalayas. Falling rain, flowing streams and rivers, freezing and thawing that produced frost heaving and ice capable of shattering huge boulders and cracking apart walls of seemingly solid rock did most of the work. Blowing wind and the abrading effect of airborne rock particles also contributed to the process. In regions to the north, moving glaciers of ice would break down and scrape off the mountain tops and fill the valleys with rubble. Everything water, rocks, soil - would work its way inexorably to the ocean once again thanks to gravity, nature's gentlest but most persistent force.
THE HEATING UP
All this pushing, squeezing and crunching of rock layers had a side effect: Heat. The extreme orogenic (mountain-forming) pressures resulting from the force of gravity on huge masses of slowly moving rock, generated frictional heat - many thousands of degrees. This, combined with heat coming from the mantle below, caused some of these sedimentary rocks to change (metamorphose) in form and mineral composition. Sandstone became quartzite. Limestones were converted into marble, and shales into slates. Molten material, including the minerals mica, feldspar and quartz, solidified to form an extremely tough igneous rock called "granite."
Sometimes the molten feldspar along with other minerals was injected by the
pressures from below into the older rocks above to form deposits called "pegmatite dikes." The larger, more spectacular molten intrusions are being mined today for their high purity quartz, feldspar, beryl and mica besides a variety of precious gem crystals including emeralds, acquamarines, and tourmalines.
For visual evidence of what took place way back then, get in your car and drive along any modern Appalachian mountain highway. Notice the appearance of the rock in the walls of the road-cuts. It is such slices through geologic history that provide perhaps the best testimony to the intense forces that created these mountains. In contrast, a road-cut in the plains of the midwestern states reveals flat or gently dipping layers of rock strata, clearly a sign that the rolling landscape is due, largely to erosion glacial or otherwise. But in the southern part of the Appalachian mountains, in the Blue Ridge, including the Great Smokies, we see at every road-cut a striking display of awesome, powerful distortion; graphic evidence of the tremendous forces of nature that were in play so many millions of centuries ago. We see layers, no longer flat but twisted and convoluted, sometimes even curling back on themselves; some shearing portions away completely, the missing piece to be found in some other location. The 1-240 road-cut through Beaucatcher Mountain east of Asheville is a classic illustration of the result of what was happening way back then.
The angle of the "layering" (foliation) of metamorphic rocks or "bedding planes" of sedimentary rocks also provide proof of the mechanism that created these mountains. Examination of those on the Tennessee side reveal formations identical with those on the North Carolina side, with one notable distinction: Those on the northwestern slopes tilt upward from southwest to northeast; those on the southeastern side also tilt upward but in exactly the opposite direction: from southeast to northwest. This is precisely what one would expect from compressing flat land until it started to bulge upward in the middle.
Continuing westward across Tennessee into Harlan County, Kentucky we find coal deposits among the layers of sedimentary rock. These were the ancient wet land marshes and bogs that bordered the coastline of that same ocean bed we now enjoy in the form of the mountains of eastern Tennessee and western North Carolina. During one of the Appalachian orogenies, these lowlands laden with living organic material were buried under layers of sediment which compacted it into seams of bituminous or "soft" coal. Up around Scranton, Pennsylvania layers of bog land were buried to far greater depths and compressed by the greater weight of the rock above to the point where they metamorphosed into anthracite or "hard" coal.
What role did glaciers play in the formation or shaping of these mountains? None in the southern Appalachians. The southernmost traces of the last Ice Age are well above the Mason-Dixon Line. Moreover, glaciers do not build mountains. Rather, they tend to destroy them by eroding their tops and filling in the valleys with rocky debris. The southern mountain country was spared, although ice fields to the north affected the temperatures and precipitation: longer cool cycles brought more rain and snow. This in itself contributed significantly to the wearing down of the mountainous regions of Western North Carolina. It brought frequent and copious rainfall which set the stage for the normal, constant forces of erosion resulting from the flow of streams and rivers. In winter months, it fostered cracking due to freezing and thawing. And with it came the scouring effect of wind.
These same processes continue today - abetted by the ravages of man. That is why the Appalachian Mountains, being geologically older, are much lower and have gentler slopes compared to the Rockies or the Andes or the Himalayas. We just got a tremendous head start in the erosion process - 2.5 million centuries!
Let's move forward to relatively recent times: 200 to 165 centuries before the present. It is believed that the southern Appalachians had tundra vegetation and had developed permafrost where the temperatures averaged below 32°F. In fact, a permanent snowpack may have persisted throughout the year in some higher hollows or valleys. Intense freeze-thaw activity resulted in the development of "block fields", i.e., areas within the mountains strewn with huge boulders developed from jointed bedrock, much of it granite. Alpine tundra herbs and subarctic shrubs persisted above 5,000 feet in elevation. Forests blanketed the hill slopes and valleys at lower elevations, below the upper limit of stunted trees.
Between 165 and 125 centuries before the present, there was an increase in mean annual temperature and precipitation. Freeze-thaw action reworked sediments down the unstable mountain slopes. With warming climates, forests spread upward to the middle elevations and deciduous tree species (oak, birch and ash) migrated into the valleys, expanding from areas in the coastal plain.
By 100 centuries ago, coniferous forests - dominated, as today, by fraser fir and red spruce - were established on the slopes of the higher ridges; oak forests spread into the low and middle elevations. Today spruce-fir forests are found only along the crests of the highest peaks. However, examples of subarctic plant species are found along cliff faces of the higher elevation mountains including Mt. Mitchel, Mt. Le Conte, and Grandfather Mountain. The block fields, now stabilized, support growths of hemlock and hardwood trees.
In the comparatively warm, comfortable climate we presently enjoy, our mountain slopes abound with flora and fauna: about 2,000 species of plants - 130 varieties of hardwoods alone, 50 species of mammals, 39 species of amphibians, and over 200 species of birds.
Looking out today at the quiet beauty and spectacular vistas offered to all who reside in or visit these lovely mountains, one can appreciate what Nature has done over the millennia to create them. And, as we have also seen, to slowly, imperceptibly bring them back down to what they once were: sand and sediment gradually rinsing off the land and building up the bottom of the Atlantic Ocean.
And still mega-centuries later, the whole dynamic continental collision process will no doubt begin again - if it hasn't already! Let us hope that Man will be there to bear witness to the tortuous travails of the next orogeny of our Appalachian Mountains.
Special thanks to Norman Cutler Smith, Professor of Geology, UNC-A College for Seniors and J. William Miller, Jr., Assistant Professor of Geology, University of North Carolina-Asheville for technical assistance; and to "A Roadside Guide to the Geology of the Great Smoky Mountains National Park," by Harry L. Moore for background data.
Reprinted from Mountain Mineral Monthly Vol.60 & Vol. 61. Used by permission.