TO MOVE A MOUNTAIN -- AND MAKE A WINDOW

By H. S. Ladd,
Regional Geologist.

Geology is a science that has been accused of telling many "tall tales" but few of them exceed the one that I propose to relate -- for it is taller than the Great Smoky Mountains. Many people are awed by a visit to the mountains. The vast distances and the towering cliffs impress them with a feeling of solidity and permanence, leading those who are poetically inclined to speak reverently of the "eternal hills." It is not surprising, therefore, that even college Freshmen assume an incredulous expression when they are informed that the Appalachian Mountains were formerly twice their present height and that they have been elevated and worn down---not once, but several times. People find it difficult to believe such things about mountains. How then, will they react to the contention of geologists that the Great Smokies, in addition to being elevated from the sea, have been shoved horizontally from southeast to northwest for a distance of 15 miles? Let us first try to move the mountain -- later we shall "make the window."

Anyone who has visited the Great Smoky Mountains National Park and has ridden over the sweeping curves of the new road that climbs nearly 4,000 feet from Gatlinburg, in Tennessee, to the divide at Newfound Gap and then descends south eastward to Cherokee, in North Carolina, has seen one of the finest geological sections exposed in this part of the world. In road-cut after road-cut there are layers of rock -- slates, quartzites and conglomerates. In many cuts the layers are tilted at high angles. An Englishman, famed for understatement, would observe that there is a great deal of rock in the Great Smoky Mountains.

Geological cross-section of the Great Smoky Overthrust Fault as exposed in road cut on Little River. The younger rocks (limestone) are represented by the brick-like pattern, the older rocks (slates) by dashed lines. Drawn by Robert H. Griffin, Student Technician in Geology.
(click on the above image for an enlargement in a new window)

The simple part of this tale deals with the formation of these rock layers. They were laid down in the sea as muds and sands and gravels. At one time they probably contained the remains of various animals that lived -- and died -- in the sea where the sediments were accumulating. So far as we know, all traces of such organisms were destroyed by the heat and pressure that changed the muds into dense slates, the sands and gravels into hard quartzites.

Slate in Road Cut at Newfound Gap

In speaking of "pressure" we are using the word in a broad sense to include a simple force like gravity -- that will compact the lower layers of a thick mass of sediments - and certain other forces that we may call mountain-building forces about whose exact nature geologists are not in complete agreement. It is not necessary, however, to understand the causes of the pressure to recognize that it has operated. In talking about such forces the geologist is in a position comparable to that of the biologist when he talks about evolution. All biologists see the results of evolution, they agree that it has occurred and that it is still functioning but they disagree on causes and methods. So, too, the geologist sees many mountains have been built by pressure and are still being built -- some by vertical uplift, others chiefly by horizontal compression. He sees the tilted and folded rocks, the breaks (faults) that are planes along which one mass has moved relative to another. In the case of the Great Smokies both vertical and horizontal pressure functioned. The surface of the earth first wrinkled like the skin of a drying apple; finally the forces of contraction became so great that the skin broke and one section overlapped the other -- a magnificent overlay of fifteen miles!

At the time this event occurred erosion had not etched out the Smokies to make the forms we see today. The mass that moved northwestward was thicker still but the northwestern edge of this huge segment was comparatively thin and it is to this edge that we must go to find evidence to substantiate the "tall tale" that we are telling and to "make our window."

Before doing this we must grow slightly technical for a moment and refer to one of geology's fundamental laws - "The Law of Superposition." This "law" merely calls our attention to the fact that when a series of beds of sediment is accumulating the last-formed bed lies on top and is younger than those below it. By studying undisturbed sections of rock the geologist learns what the "normal" sequence of beds is in a given area. When, in some nearby area, he finds the layers tilted, folded, overturned, or in some other unusual relationship, he can make comparisons and try to discover what has happened.

We return now to the northwest edge of the Great Smokies to look at the evidence of the postulated 15-mile "shove." We must bear in mind that the rocks of the Smokies themselves are highly altered rocks that are known to be very old. By working out normal relationships in other areas, geologists have found out that they are very much older than the belt of fossiliferous limestones and dolomites that borders the Smokies on the northwest. The relations of these younger limestones to the older rocks of the Smokies may be seen clearly in a road cut on Little River -- an exposure that, unfortunately, lies just outside the Park boundary. At this place we see -- as shown in the illustration -- that the law of superposition has been violated. The older rocks lie on top of the younger ones. The plane of contact is exposed to casual observation. The rocks close to it are sheared and broken. A multitude of cracks developed in the limestone by the shoving were filled later with crystalline lime to become a multitude of veins. Looking at this exposure one can visualize the mass of slate overriding the layers of limestone. The slates, in doing this, picked up masses of the younger rock and such masses -- underlain and overlain by slate -- now are exposed in the road cut. Admittedly, however, the visible effects of the movement are very local. Dr. Geoffrey W. Crickmay, of the University of Georgia, and the writer collected identifiable fossil shells in the limestone only 400 feet from the exposed fault plane. Obviously more evidence of the postulated shove would be in order so we turn to the line of "coves" that border the northwest side of the Smokies -- for these are our "windows."

As stated earlier, the edge of the overthrust mass was comparatively thin. Even before it came to rest the mass was being attacked by the forces of erosion. Possibly because of original differences in the thickness of the edge, or because of inherent weakness in the rocks at certain places, this erosion did not progress uniformly. Holes were excavated near the edge and in the bottoms of these holes the younger limestones were uncovered. These are the Coves of the Smokies -- great steep-sided basins, some exceeding 1,000 feet in depth. The Germans call them "fensters" because through these "windows" in the overthrust older rocks we may see the younger rocks beneath.

Structural cross-section from Maryville, Tenn., southeastward to the southeastern corner of the Knoxville quadrangle. Section according to the geologic map of the U. S. Geological Survey Folio No. 16. (Drawn by Chas. W. Wilson, Jr., in the Journal of the Tennessee Academy of Science, Vol. X, No. 1, p. 61).
(click on the above image for an enlargement in a new window)