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Stories in the Rocks

  

To those who have learned to read their language, the Blue Ridge rocks tell of momentous events. But their story as written here is like the story of the Shenandoah mountain people--both are based on educated guesses. Experts study the evidence, then decide, "It must have been like this...." The guesses of geologists are highly educated, and geologists all agree broadly on what happened here. Only when you get down to details, such as why it happened and exactly when, are there differing opinions.


View From Mary's Rock
Photo taken by
Matthew Singer

The oldest rocks in the park are the ancient granites that form the core of the mountains. In most places they lie hidden under more recent rocks. But they are exposed on some of the higher peaks, such as Hogback, Mary's Rock, and Old Rag, and in road cuts along the Drive. Geologists divide this granite core into two categories:

Old Rag granite is exposed on Old Rag and on the crest of Oventop, and on a rather narrow strip of land that connects the two.

Granodiorite of the Pedlar formation, which is far more extensive, is exposed in many places along the main Blue Ridge.

The two differ somewhat in chemical composition. Old Rag granite is a little lighter in color than the granodiorite. Both have been somewhat changed from their original structure by what happened later.

As we've seen, the human history of the Blue Ridge spans eleven thousand years. The history of the rocks is a hundred thousand times that long. The granite that forms the core of these mountains is a fourth as old as the earth itself. It crystallized from molten magma over a billion years ago. Heatwole says, 1,100,000,000 years and notes that the date can be fixed, with a reasonably small percentage of error, by measurement of radioactive elements and their decay products.

There's a certain satisfaction in assigning dates to events, even though we can't appreciate their meaning. Eleven hundred million years, in human terms, is an inconceivable span of time. We might try to grasp its magnitude by analogy. If the age of the oldest Blue Ridge rocks were twelve hours, then all the time that has elapsed since the birth of Christ would be considerably less than a tenth of a second.

Evidence tells us that the ancient rocks cooled and crystallized very slowly, for the different minerals form an interlocking mosaic of moderately large crystals. Such a structure forms only when the rocks solidify slowly, at high temperature and under great pressure. Thus we know that the granite core of the mountain was more than a mile underground when it solidified from molten magma. What lay on top of it we'll never know. Over a period of three hundred million years, the land was slowly lifted upward. And as it rose, the higher rocks weathered, disintegrated, and washed into the sea.

With a little effort we can picture the landscape as it was eight hundred million years ago. Hills and low mountains of bare granite, from a few hundred to perhaps two thousand feet high, covered the land. Streams flowed down the hollows and through the valleys. Here and there on a wet shaded rock, there may have been a green film of algae; but there was no other life outside the oceans. As the granite hilltops weathered, streams carried sand and gravel to the lowlands, and spread them in a layer up to 200 feet thick over the valleys.

Then to this bleak landscape came lava flows. The granite, strained by the forces that were lifting it upward, cracked and split. The lava that surged up through the fissures was so hot that it spread out in the valley and formed a smooth flat sheet before it hardened. Where the eruptions were especially violent, clouds of volcanic dust and ash poured through the fissures and settled on the land, and were covered by lava.

Now the landscape was different. The low mountains of bare granite were still there, but a sheet of lava covered the valley floor. Streams still flowed down the hollows, and began to carve channels through the lava. They carried more sand and pebbles from the eroding granite hills, and spread them here and there over the lava on the valley floor.

The lifting force continued, until at last the rocks split again. More dust and cinders spewed out, and more lava flowed, forming a new and higher floor in the valleys. And this was repeated at least a dozen times, for we find evidence of a dozen separate lava flows in the park. With each new eruption, lava reached higher on the granite hillsides, and eventually covered all but the highest peaks.

The valley sediments that were covered by the first lava flow were cemented by mineral fluids from the lavas, and compacted by heat and pressure. These sediments now form the rocks of the Swift Run formation, which is exposed at several places along the Drive.

The lava flows, collectively, comprise the Catoctin formation, consisting mostly of igneous rocks (which hardened from a molten state). But in places the lava flows are separated by sedimentary rock (formed of sediments laid down by water, and later hardened by pressure). These relatively thin layers of sedimentary rocks were the sand and pebbles that washed down onto each new lava surface before the next flow occurred. In many places, soil that had formed on top of a lava flow was torn up and churned into the base of the next flow. There's a good example of this at Little Stony Man.

Near its upper surface, newly hardened lava is porous and filled with gas bubbles. We can assume that in many places the pores and voids were filled with ground water when a new flow of white-hot lava covered them. Minerals crystallized from the superheated solutions. Bubble cavities filled with concentric shells of minerals such as epidote (bright yellow green), (dark green), feldspar (bone white), and quartz (glassy, milky white to nearly transparent).

The epidote that filled the lava pores served as cement. When this lava was later compacted by the pressure of overlying rocks it became greenstone, which makes up nearly 80 percent of the Catoctin formation within the park. Greenstone caps our highest peaks, and forms nearly all our waterfalls. Where rock surfaces are old and weathered and covered by lichens, the green color may be hidden under shades of gray; on freshly broken rock it's very evident.


An Example Of Columnar Jointing
Photo taken by Linda Lavender

As each flow of lava cooled it contracted, and cracks traced polygons on its surface as they do on the surface of drying mud. As cooling continued the cracks spread downward, forming long prismatic columns of five, six, or seven sides - from a few inches to more than two feet across. This columnar jointing is evident at many points in the park, and examples are pointed out in the log of the Drive.

The fissures through which the lava poured are now filled with dikes of solidified lava. Within the park more than a hundred greenstone dikes in the granite rock have been found and mapped. The best example beside the Drive is at the north portal of the Mary's Rock tunnel, mile 32.2.

Just how long the intermittent volcanic activity lasted, no one can say. It had probably ended entirely by the beginning of the Cambrian period, something less than 600 million years ago. After the last lava flow a few hilltops of granite were still exposed, and it's likely that there were higher granite hills or mountains to the west. New streams cut channels through the lava beds, and deposited sand and pebbles that washed down from the granite mountains. These deposits became the rocks that now constitute the Weverton formation.

Subsequently the land sank. As it neared sea level, streams stopped flowing and became bogs. The sea came nearer. The land that is now the park was covered by shallow lagoons, separated by sand bars from the open ocean. Sandy mud and clay washed into the lagoons and, as the land continued to sink, built up to a depth of hundreds of feet. This material later became the sandstone and shale of the Hampton formation.

The land sank farther. The sea advanced, and the Blue Ridge area became a sandy shore. The white beach sands later became the white quartzite of the Erwin formation.

Then the land sank beneath the sea. The sediments now were carbonates - some precipitated by chemical action in the sea water, and some consisting of shells of marine animals. The land continued sinking for perhaps another eighty million years, while sediments built up limestone and dolomite deposits two or three miles thick. Then the land rose again, and the limestone emerged from the sea. That must have been roughly 450 million years ago.

The story of the rocks during the next 225 million years is somewhat garbled, or maybe it loses something in translation. The details are uncertain. We know that a number of things happened, though we can't put them in exact sequence, or assign exact dates. The rocks we now see in the park were metamorphosed, which means their physical nature was changed by the pressure of rocks above them.

Subsequently, the uplift of the land continued and tremendous forces thrust against the land, pushing toward the northwest. The rocks buckled and broke; fault lines developed; great masses of rock were thrust on top of others; formations in some places were tilted, and in others turned on their sides. But this didn't happen in one great, literally earth-shaking event. It must have resulted from continued thrust, and intermittent slippage along fault lines, spread out over nearly 225 million years.

The final chapter brings us from 225 million years ago to the present. During most of this time uplift continued, and during all of it the land eroded. The miles of sediments that lay above the granite and greenstone of the Blue Ridge crest were washed away. But granite and greenstone are more resistant than the sedimentary rocks, and when they were exposed the erosion slowed. To the west, the softer carbonate rocks had been pushed downward by the buckling of the land that shaped these mountains. The limestone eroded rapidly, forming what is now the Shenandoah Valley.

Uplift of the land may still be going on; if so, it's too slow to detect. Erosion of the Blue Ridge speeded up during the ice ages when, most likely, the talus slopes that we now see along the western slopes of the mountain were formed. Erosion is slower now, but it continues nevertheless.

You might ask why the surface of the earth rises and falls. The earth has a solid outer crust maybe fifty miles thick, floating on hot plastic material. "Plastic" in this sense means capable of flowing. But it's not a liquid. It can flow very slowly, in response to great pressure. A sideways pressure can make the crust buckle, so that it rises in some places and sinks in others. That's a simplified version of one theory. There are others.


A Boulder On Old Rag
Photo taken by Charles A. Thomas

Scientists have determined that in this region there were significant sideways pressures on the earth's crust that caused the "tremendous forces" that pushed the land toward the northwest and made it buckle and break. Again, we have a choice of theories as to specifics. Here's a simplified version of one of them. The earth's crust is not a continuous mass; rather, it consists of a number of separate "plates." Hugh Crandall and Reed Engle, in "Shenandoah, the Story Behind the Scenery," aptly compare the plates to ice floes that are loosely frozen together at their edges. Currents in the sea can break the floes apart and grind one floe against another, or crush two floes together. In much the same way, plates in the earth's crust can be moved by slow but powerful convection currents in the hot plastic material underneath The forces that thrust one plate against another can cause earthquakes and, over a period of time, can build mountains. As you tour the Drive and hike the trails you'll see rocks belonging to all the formations mentioned. You will see how one lava flow rests on another, sometimes separated by the sediments of ancient streams. You will see the prismatic columns into which lava cracked as it cooled, and you will see dikes of cooled lava in the granite. Heatwole hoped you would find satisfaction in knowing how these things came about. What is presented above is a simplified outline of the story of the rocks. To fill in some of the details, look for these books at one of the park's sales outlets or from the Shenandoah Park Association:

  1. Geology along Skyline Drive, by Robert L. Badger. This well illustrated book is a self-guided tour for motorists.
     
  2. A Hikers Guide to the Geology of Old Rag Mountain, by Paul Hackley. An interesting publication that brings the eons of geology down to the scale of a day hike. The geology of 12 specific stops are described in detail.
     
  3. Geology of Shenandoah National Park, by Thomas Gathright II. 93 pages with black and white photos and illustrations and a folded map of the park in a back cover pocket.
     
  4. Peterson First Guide to Rocks and Minerals, by Frederick H. Pough. 128 pages. A concise field guide to more than 250 common gems, ores, and other rocks and minerals.


More in-depth geologic information can be found here.

   
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