In 1742, Prince William County, a part of the stupendous
Culpeper grant, was divided and the county of Fairfax created
and named in honor of its titled proprietor. Commencing at
the confluence of the Potomac and Occoquan rivers, the line
of demarcation followed the latter stream and its tributary,
Bull Run, to its ultimate source in the mountain of that name,
from which point it was continued to the summit of said
mountain, pursuing thereafter a direct course to the thoroughfare
in the Blue Ridge, known as "Ashby's Gap."

In 1757, Fairfax was divided and the territory west of its altered
boundary christened "Loudoun County." The new line followed the stream
called Difficult Run, from its junction with the Potomac to its
highest spring-head, and from that point was continued in a direct
line to the northeast border of Prince William County. This boundary
was afterwards changed and the present line between Loudoun and
Fairfax substituted (see "Boundaries," page 17).

The following are excerpts from the proceedings of the Virginia House
of Burgesses that led to the creation of Loudoun County in May, 1757.
The act authorizing the division of Fairfax and establishment of
Loudoun is given intact:

On April 20, 1757, a "petition of sundry Inhabitants of
_Fairfax_ County, praying a Division of the said County, was
presented to the House and read, and referred to the
Consideration of the next Session of Assembly."