Loudoun County is preeminently a diversified region; its surface bearing many marked peculiarities, many grand distinctive features. The broken ranges of hills and mountains, abounding in Piedmont Virginia, here present themselves in softly rounded outline, gradually sinking down into the plains, giving great diversity and picturesqueness to the landscape. They are remarkable for their parallelism, regularity, rectilineal direction and evenness of outline, and constitute what is by far the most conspicuous feature in the topography of Loudoun. Neither snow-capped nor barren, they are clothed with vegetation from base to summit and afford fine range and pasturage for sheep and cattle. The main valleys are longitudinal and those running transversely few and comparatively unimportant. The far-famed Loudoun valley, reposing peacefully between the Blue Ridge and Catoctin mountains, presents all the many varied topographic aspects peculiar to a territory abounding in foothills. The Blue Ridge, the southeasternmost range of the Alleghanies or Appalachian System presents here that uniformity and general appearance which characterizes it throughout the State, having gaps or depressions every eight or ten miles, through which the public roads pass. The most important of these are the Potomac Gap at 500 feet and Snickers and Ashby's Gap, both at 1,100 feet. The altitude of this range in Loudoun varies from 1,000 to 1,600 feet above tide-water, and from 300 to 900 feet above the adjacent country. It falls from 1,100 to 1,000 feet in 4 miles south of the river, and then, rising sharply to 1,600 feet, continues at the higher series of elevations. The Blue Ridge borders the county on the west, its course being about south southwest, or nearly parallel with the Atlantic Coast-line, and divides Loudoun from Clarke County, Virginia, and Jefferson County, West Virginia, the line running along the summit. Of nearly equal height and similar features are the Short Hills, another range commencing at the Potomac River about four miles below Harpers Ferry and extending parallel to the Blue Ridge, at a distance of nearly four miles from summit to summit, for about twelve miles into the County, where it is broken by a branch of Catoctin Creek. Beyond this stream it immediately rises again and extends about three miles further, at which point it abruptly terminates. A third range, called "Catoctin Mountain," has its inception in Pennsylvania, traverses Maryland, is interrupted by the Potomac, reappears in Virginia at the river margin, opposite Point of Rocks, and extends through Loudoun County for a distance of twenty or more miles, when it is again interrupted. Elevations on Catoctin Mountain progressively diminish southward from the Potomac River to Aldie, although the rocks remain the same, and the Tertiary drainage, which might be supposed to determine their elevations, becomes less effective in that direction.