in width from 8 to 12 miles, and in altitude from 350
to 730 feet above sea level. Allusion to the physiography of this
valley--so called only by reason of its relation to the mountains on
either side--has been made elsewhere in this department.

Immediately south of Aldie, on Little River, near the point of
interruption of Catoctin Mountain, another range commences and extends
into Fauquier County. It is known as "Bull Run Mountain," but might
rightly be considered an indirect continuation of the elevation of the
Catoctin, its course and some of its features corresponding very
nearly with that mountain save only that it is higher than any of the
ranges of the latter, excepting the western.

East of the Catoctin the tumultuous continuity of mountains subsides
into gentle undulations, an almost unbroken succession of sloping
elevations and depressions presenting an as yet unimpaired variety and
charm of landscape. However, on the extreme eastern edge of this
section, level stretches of considerable extent are a conspicuous
feature of the topography.

Three or four detached hills, rising to an elevation of 150 or 200
feet above the adjacent country, are the only ones of consequence met
with in this section.

COMPARATIVE ALTITUDES.

The hilly character of Loudoun is clearly shown by the following
exhibit of the elevation of points and places above tide-water. The
variations of altitude noted in this schedule are based upon
conflicting estimates and distinct measurements made at two or more
points within a given circumference and slightly removed one from the
other.

Feet.
Sterling 415
Ashburn 320
Leesburg 321 to 337
Clarke's Gap 578 to 634
Hamilton 454 to 521
Purcellville 546 to 553
Round Hill 558
Bluemont 680 to 730
Snicker's Gap 1,085
Neersville 626
Hillsborough 550
Waterford 360
Mount Gilead 600
Oatlands 270
Little River, near Aldie 299
Middleburg 480
Potomac River, near Seneca Dam 188
Potomac River, at Point of