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 is entirely within loudoun county and 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.