Why does North Carolina have so many, and so many kinds of, monadnocks?

Figure 1. By the beginning of Pliocene time, streams had remodeled the land surface, leaving the ancient Schooley Peneplain as mountaintops in the Blue Ridge and monadnocks in the Piedmont. Click to enlarge. (From Fred Beyer, North Carolina: The Years Before Man (Durham, N.C.: Carolina Academic Press, 1991), figure 15-6. More about the illustration)

North Carolina has more than a dozen monadnocks scattered among its Blue Ridge mountains, and another ten or more on its Piedmont Plateau. These monadnocks formed during dramatic and diverse events that occurred as the state’s crust formed.

Most of these geologic events occurred during collisions between continents that thrust up the Appalachian mountains over 300 million years ago. Collisions between Africa and North America compressed the crustal rocks of what is now North Carolina, and the energy of compression heated and melted some of them. Some of the molten and metamorphosed rock moved upwards, then cooled to form domes and other shapes that “intruded” into the overlying rock layers. Later stages of the compression led to rock layers breaking and being thrust up over one another. The folding, fracturing, and melting that took place in and under these “overthrusts” led to more upward intrusions of molten rock. In Appalachian Mountains, there is evidence that some sedimentary rocks were moved laterally more than 100 kilometers (66 miles), although this almost certainly was accomplished by repeated movements of relatively short distances.

These and other great crustal events produced many different rock types under the Piedmont. The crust was further modified when crustal forces reversed about 200 million years ago. The compression stage stopped as the colliding continents ground to a stop beside each other and began to move apart again to begin the opening of the Atlantic Ocean. As the forces of compression stopped, those of tension began. Tensional stretching of crustal rocks layers led to cracks and fractures that were filled from below by yet more types of molten material. This material, like those formed during the compression period, had erosion resistance’s different from those of the layers around them. Thus was the stage set for the erosional formation of today’s monadnocks.

When we see the ridges and peaks of our monadnocks our first reaction is to think how high they are. If we were a monadnock, though, we would probably be impressed with how low the surrounding land had become since we first appeared at the land’s surface. In fact, we and the monadnock would both be right. The monadnocks have risen upwards as rock has eroded from around them. The resultant reduction of weight on the crust caused it to be elevated by forces from below. It is also true, though, that most of the difference in elevation between the Piedmont surface and the top of the monadnocks has been caused by material having been eroded from the former, but not from the latter. Figure 1 illustrates the old land surfaces, the areas removed by erosion, and the mountains, monadnocks, and new land surfaces that resulted.

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