The region of Loudoun County that has karst geology is located in an area roughly one mile on either side of State Route 15 from south of Leesburg and north to the Potomac River bridge. The region is bounded sharply to the west by the Bull Run Fault, which runs at the base of Catoctin Mountain through Loudoun County.
Limestone Geology refers to areas that are underlain by carbonate rocks such as limestone, dolomite, and marble. Carbonate rocks will dissolve when exposed to acid and since normal rainwater is slightly acidic, it will dissolve these rocks over time. This slow process will eventually produce a landscape characterized by fluted and pitted:
Subsurface Drainage Systems
These features are called “karst features” and the resulting landscape is called "karst terrain." Karst terrain is also characterized by abundant rock outcrops, closed depressions, and limited surface drainage.