The range extends in a southwest–northeast orientation for approximately from
Aldie in
Loudoun County, through western
Prince William County to
New Baltimore in
Fauquier County. In the north, from Aldie to the headwaters of
Bull Run Creek, the range serves as the eastern border of the southern
Loudoun Valley. South of Bull Run Creek, the watershed of the range forms the western border of Prince William with Fauquier and thus the eastern slopes are in the former county, while the western slopes are in the latter. The range consists of three closely spaced ridges (with several spurs), which converge in the center of the range. The northern section of the range consists of a solitary ridge, which dies out towards the center of the range, whereupon two ridges to the west gain prominence. In the southern section the central ridge dies out as well and the westernmost ridge solely comprises the range. The section of the Bull Run Mountains south of Thoroughfare Gap is sometimes referred to as the Pond Mountains. The
Broken Hills intersect the range south of I-66 in the southern section. ==Geology==