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When a Mountain is a Mountain

Recently, we were asked whether the Bull Run Mountains were indeed mountains. To my knowledge there is not one set definition of a mountain. In western New York there is Alleghany State Park, for example, that has many hills over 2,000 feet high up to 3,000 feet high, but none of them are considered mountains. So, in short, I do not know one all encomposing definition that qualifies some physical features as mountains and disqualifies others.

However, I can explain in brief the geology of the Bull Run Mountains and why we consider them indeed mountains.

The Bull Run Mountains stand alone as a narrow, monadnocklike 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 (Fauquier County) for approximately 15 miles to Aldie (Loudoun County), varying from about 0.9 to 2.2 miles wide and rising conspicuously above the surrounding terrain.

While occupying the physiographic Piedmont, the Bull Run Mountains form part of the eastern limb, or flank, of the Blue Ridge anticlinorium, a large fold that presumably resulted from Late Paleozoic Alleghanian orogeny. The Bull Run Mountains 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 well 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, for building stones and flagstones providing a significant input into the local economy.

In short, the Bull Run Mountains stand apart from the rest of the piedmont geologically and form a mountainous terrain in an otherwise rolling hill landscape. So we call them mountains.

Perhaps a more all encomposing definition would be: A mountain is a mountain when the height and diversity of plant life on the ridge top are influenced by physical limitations of water availability and soil developement because it stands well above its surroundings. A hill is a hill because its gentler nature does not have these physical limitations to the same degree, in part because it is not as high above its surroundings.

Michael

Summer Nature Camp enjoying the view at High Point Mountain.