Everything old is new again
The inter-related geologic and geographic heritage of the Blue Ridge Province is a complex but fascinating and rewarding subject to consider. As part of the introductory portion of my natural history workshops, I give a presentation called “Where Are We?” This allows me to touch upon the basics of the region’s geologic history as well as its present day geography — thereby laying the groundwork for subsequent field trips, during which methods for identifying the habitats, plants, and animals of the Blue Ridge are taught. That presentation goes something like this.
The Appalachians were uplifted between 300 and 250 million years ago — when the massive super continent known as Pangea was being formed via continental drift and plate tectonics.
Today, the Appalachians extend more than 2,000 miles from the Gaspe Peninsula in Canada to north Georgia and north Alabama. How high was this range when first uplifted as a young and rugged mountain range? Based on discussions with various geologists, my ballpark figure had been 22,000 or so feet.
But one of the foremost Appalachian geologists — Dr. Robert D. Hatcher Jr. of the University of Tennessee — one of the co-authors of Introduction to the Environment and Vegetation of the Southern Blue Ridge Province (1998) concluded that, “Around 280-260 [million years ago] the collision of Africa with North America was complete and so was the Appalachian mountain-building process, culminating in a chain as lofty as any modern mountain chain.”
That would imply that the original elevation of the young Appalachians approached 30,000 feet. One of the foremost researchers in regard to the topography and natural areas of the Blue Ridge Province — Dr. J. Dan Pittillo, now retired from Western Carolina University — recently advised me that, “What it boils down to is the fact that there’s no hard evidence in regard to the original elevation of these mountains. No one knows. It may be that both erosion and uplift have been more or less the same so that the original mountain chain was about what it is today in regard to elevation.”
For many years, I envisioned the aging of the Appalachians and its Blue Ridge component as an ongoing process in which they would pass through the current phase by gradually eroding into foothills, such as the adjacent Piedmont Province, and then level terrain, such as the Coastal Plain Province. But David Prowell of the U.S. Geological Survey in Atlanta and other geologists are presently considering the possibility that the Appalachians actually started “getting taller” about 100 million years ago. In an interview with journalist Carlos Santos, Prowell and other geologists discussed this theory.
David Prowell ... tells the Richmond Times-Dispatch that the Blue Ridge Mountains are barely rising — a little over 100 feet every million years — but it’s enough to offset natural erosion.
“They should have eroded away 100 million years ago ... but somehow they are renewing themselves,” said Prowell, whose claims have slowly gained support in the scientific community.
“I think it’s a new and unique theory,’ said Scott Southworth, a geologist with the U.S. Geological Survey in Reston, [Virginia]. “Mountains don’t persist very long ... But you look out now and see some pretty high ground from the Smoky Mountains to the Shenandoah. People still refer to the Appalachians as very old mountains, but it’s just not true, at least in geological terms.”
“Erosion can reduce a mountain chain by 90 percent of its height in 20 million years,” Prowell said.
He said that, about 250 million years ago, the African continent pulled away from North America, leaving “gashes” in the earth that filled with tens of thousands of feet of sediment from the eroding mountains. Then our familiar Appalachian Mountains and the Blue Ridge began forming about 140 million years ago, when a westward push of the continental plate started at the mid-Atlantic ridge. That compression is still forcing the mountains to rise, though at such a slight rate that it’s not easy to discern.
John Dennison, a geologist at the University of North Carolina, who agrees with Prowell’s theory, said the movement of the continental plate is ‘as fast as a fingernail grows.’
Prowell’s theory, of course, would mean the Appalachians are relatively young.
“The mountains that now exist aren’t the ones that were there in the Paleozoic,” he said. “These are new mountains.”
Even without the possible intervention of Prowell’s westward-crawling continental plate, geologists have contended for years that as mountains erode downward they also, paradoxically, rise upward. Rock is less dense than material lower in the mantle. When mighty mountain ranges are uplifted, the underlying layers of the earth are depressed. As described by Mark W. Carter and the other compilers of A Geologic Adventure Along the Blue Ridge Parkway in North Carolina (1999), a “root” develops that helps “buoy up” the less dense mountains. As the mountains erode, pressure is decreased so that simultaneous uplift also occurs. The name for the process is “isostatic rebound.”
It’s probable that factors other than erosion, isostatic rebound, and tectonic plate movement are involved in the ongoing creation of the Blue Ridge Province and adjacent mountainous regions. Out of this sort of complexity, these wondrous mountains are created anew each day. I wholeheartedly agree with Merrill Gilfillan — one of the most rigorous describers of modern American landscapes and a lifelong student of Appalachian topography and ways — in his recent assessment put forth in Burnt House to Paw Paw: Appalachian Notes (1997) that, when we venture into the Appalachians, “we have officially entered what geologists call ‘the most elegant mountain chain in the world.’”
(Editor’s note: This column is part of the opening section of George Ellison’s new book, A Blue Ridge Nature Journal: Geography, Flora, Fauna & Lore, which will be published by The History Press (Charleston, S.C.) in September 2006. In addition to 30 essays the volume will feature 40 full-color paintings and 30 decorative chapter illustrations by artist Elizabeth Ellison.)