Michael J. Kieffer
A Geologic History of the Bull Run Mountains
As the leaves continue to drop from the trees, the landscape seems to change from an ocean of green vegetation to one of grays and browns and of rocks and wood. It is the time of year that old home sites are clearly visible along with the scars of past land use. As we look at these human-altered landscapes that are now forested over, it is almost impossible not to reflect on how the underlying geology is not only the shaper of the woods today, but also the determiner of how the land was used by humans in the past.
It’s a billion years ago, and there are mountains at least 25,000 ft. high (Grenville Range) located about where the present Blue Ridge Mountains are today. There is no life on land, and beautiful white sands are pouring off the mountains and piling up where the Bull Run Mountains (BRM) sit today. It is like Clearwater Beach. Eventually, the sand is buried by more material from that ancient mountain range, and the pressure becomes great enough to turn it into sandstone. Flash-forward to 550-600 million years ago (mya) and volcanoes are erupting and belching out basalt as the continental plates shift forming the Iapetus Ocean, the pre-Atlantic. Some of this molten rock will squeeze between the layers of sandstone forming the beautiful igneous rock we know as quartz, the veins of which we can still see in the rock outcrops on top of High Point. Move forward in time to 500-550 mya and the beautiful white sandstone is being metamorphosed, due to heat and pressure of the material above it, into quartzite, a metamorphic sandstone (metasandstone). Jump now to 230 mya. The pre-Atlantic has disappeared. Africa is colliding with North America, and the present day Appalachian Mountains are being created. The Bull Run Mountains mark the eastern edge of this mountain range. The oldest rock, the thickest bedded metasandstone (Weaverton quartzite) lies on the western edges, the younger thinner bedded quartzite (flaggy-quartzite) lies on the eastern slopes, with the now metamorphic basalt (greenstone) exposed on the valley floors within the BRM and forming the hills to the west of the BRM.
This is an oversimplification of the geologic processes that created the BRM, but gives us an idea of how they formed. It also speaks to the human history. If the vast majority of the BRM is a metamorphic sandstone, the soils it creates will be dominated by sand. Thus the plants that grow on it will be able to live with little nutrients and dry conditions. It also means that farming would be impossible except in the valley floors where greenstone is present. The mountains would be left to hunting for the American Indians, to timbering by the early land barons, and eventually to squatting by those who worked for landowners whose estates surrounded the mountains. See below for a more technical geology of the BRM.
The technical geology of BRM
Bull Run Mountains (BRM) stand alone as a narrow, monadnock-like series of ridges, and comprise a prominent, isolated area of rugged highland terrain within the gentle, lower-lying Piedmont. This highland complex extends north from New Baltimore in Fauquier County for approximately 15 miles to Aldie in Loudoun County, varying from about 0.9 to 2.2 miles wide and rising conspicuously above the surrounding terrain (Fleming, 2001).
While occupying the physiographic Piedmont, BRM form part of the eastern limb, or flank, of the Blue Ridge anticlinorium, a large fold that presumably resulted from Late Paleozoic Alleghanian orogeny. BRM are largely underlain by metasedimentary rocks of the Chilhowee Group. Massive, thick-bedded quartzite, from the Weaverton Formation, is erosion resistant and well exposed as it sloughs off the upper west slope of High Point Mountain north of Thoroughfare Gap, and at White Rock north of Hopewell Gap. The strongly eastward-dipping quartzite strata are powerful ridge-forming features. The eastern dip slopes of the ridges are underlain by thin-bedded quartzite with local interbeds of muscovite schist and pyllite. The Catoctin Formation underlies the lower to middle western slope of the Bull Run Mountains, but is generally covered by quartzite-based colluvium (Leahy, M.J. and S.Y. Erdle, 2003).
The Bull Run Mountains are remnants of an earlier erosion cycle that occurred when the whole Piedmont was level with their summits. The Piedmont presently is a series of rolling hills dissected by streams and rivers. A plain that is not yet worn entirely smooth by the agents of erosion is geologically called a peneplain. Classified as monadnocks (isolated hills of resistant rock rising above the peneplain) , the resistant metasedimentary rock at its highest point is 1,369 feet in elevation, with the peneplain on the western and eastern flanks at approximately 600 feet and 400 feet, respectively. The high, nearly vertical cliffs at Thoroughfare Gap, High Point, and White Rock are unique Piedmont features. In addition, many small quarry operations exploited the geologically younger, thinner bedded “flaggy” quartzite found on the eastern slopes that were used for building stones and flagstones, providing a significant input into the local economy.
Fleming, G.P. 2002. Ecological communities of the Bull Run Mountains, Virginia: Baseline vegetation and floristic data for conservation planning and natural area stewardship. Natural Heritage Technical Report 02-12. Virginia Department of Conservation and Recreation, Division of Natural Heritage, Richmond, Virginia. Unpublished report. 276 pp plus appendices.
Leahy, M. J. and S.Y. Erdle. 2004. Management Plan for Bull Run Mountains Natural Area Preserve. Natural Heritage Technical Report #04-09. Virginia Department of Conservation and Recreation, Division of Natural Heritage. Richmond, Virginia. 71 pp. plus appendices.