Why are the rocks and plant communities of Roan Mountain interesting to natural scientists?

The origin of grassy balds, like this one elsewhere in the Blue Ridge, is a mystery to scientists. (Photograph by Dirk Frankenberg. More about the photograph)

The rocks of Roan Mountain are interesting because of their age, their mineralogy, and the evidence they provide about the geological processes that formed them. The plant communities are interesting because they are southern examples of communities usually found much further north, and because the origins of one type, the grassy bald, remains an unsolved scientific mystery.

Rock types

There are two rock types that occur on the crest of Roan Mountain. One was formed 1.2 to 1.5 billion years ago when sediments of an ancient sea were heated to their melting point deep in the earth’s crust and converted (metamorphosed) into a new rock type, called gneiss, that has alternating bands of different mineral. The Roan Moutain Gneiss is made up of alternating bands of quartz and feldspar.

The other major rock type of the Roan summit formed when molten material rich in silicate crystallized to form an igneous rock. Igneous rocks make up about 95 percent of the earth’s crust, but they are usually covered by metamorphic or sedimentary rocks and are therefore rarely seen. The igneous rock at the top of Roan Mountain crystallized after it had been forced into a crack in the gneiss while both were still deep in the crust creating a rock called gabbro (specifically, Bakersville gabbro, after a nearby town). A gabbro is a plutonic rock, i.e. one that solidified below the earth’s crust. The alternate rock type formed by the same minerals is volcanic rock. Volcanic rock, by definition, crystallizes at the earth’s surface rather than within the continental crust.

The Bakersville gabbro formed about 750 million years ago. Examples of the last major rock type on earth, sedimentary rocks, make up the ridge and valley province that stretches north and west of Roan Highlands. The sedimentary sandstone rocks of this province are only about 500 million years old.

Plant communities

The major plant communities of the Roan Highlands are the red spruce/Fraser fir forest, the northern hardwood forest, and the grassy bald. All three are as well or better developed here as anywhere in the southeastern United States. The spruce-fir forest here is healthy and contains few dead trees, making it a pleasant contrast to the more frequently visited examples on Mount Mitchell. The northern hardwood forest here has more examples than any other area in our state of species that normally occur no further south than Pennsylvania. These populations are called disjunct because of their separation from other northern populations, and some of these have evolved into quite different plants than northern specimens. Thus is the process of evolution carried on amongst species that have widely separated populations.

The grassy bald community remains a mystery. These grassy areas occur below the timberline — the elevation above which trees cannot grow — and so it is not obvious why trees don’t sprout and eventually take over the areas of grasslands.

There are at least four theories that attempt to explain how these unique grasslands originated and are maintained. One theory suggests that they are remnants of a grassland type that evolved in the Pleistocene (1.6 million to 10,000 years ago) and which has been perpetuated by the unique (but unspecified) natural processes that occur in high southern mountains. Another suggests that these areas formed when large herbivores such as woolly mammoths cropped off the trees, and their smaller herbivorous successors have managed to keep the trees from invading the resultant grasslands. Another theory suggests that native American populations burned the high mountain habitat and thereby thinned and altered the soils so that trees can no longer grow there. Finally, there is a theory that the harsh combination of hot summers, cold winters, and constant wind exposure makes these habitats unsuitable for trees.

There is no agreement among scientists as to which, if any, of these theories is correct; so, as you take this tour, feel free to develop your own idea of why these areas below the timberline continue to have no timber.

Animals

The animals that live high on Roan Mountain are also unusual. There are Southern pygmy shrews, Carolina Northern Flying squirrels, New England cottontail rabbits, and snow buntings in the winter. Ravens are common, and, if you are lucky, you may see a peregrine falcon, a species that was reintroduced here by the North Carolina Wildlife Resources Commission in 1997. There are more than 160 species of birds, many of which are neotropical migrants that fly north from Mexico, South and Central America, and the Carribbean in the spring.

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