The Loudoun sandy loam consists of from 8 to 12 inches of a heavy brown or gray sandy loam, underlain by a heavy yellow or red loam or clay loam. Often the subsoil contains a considerable quantity of coarse sand, making the texture much the same as that of the soil. The sand of the soil and subsoil is composed of very coarse rounded and subangular quartz particles. The surface material is not a light sandy loam, but is more like a loam containing considerable quantities of very coarse quartz fragments. It is generally quite free from stones, but small areas are occasionally covered with from 5 to 20 per cent of angular quartz fragments several inches in diameter.
The Loudoun sandy loam occurs in irregular areas of considerable size in the intermediate valley between the Blue Ridge, Short Hill, and Catoctin mountains. The largest area of the type is found in the vicinity of Round Hill.
The topography of this soil in the valley varies from gently rolling to hilly, the slopes being long and gently undulating, while along the valley walls and in the uplands it is ridgy. Owing to the position which this type occupies, surface drainage is good. The light texture of the soil admits of the easy percolation of water through it, and, except where the subsoil contains considerable sand, the soil moisture is well retained. In dry weather, if the ground is cultivated, a mulch is formed, which prevents the evaporation of the soil moisture and greatly assists the crops to withstand drought.
Nearly the whole of this type is in cultivation. Where the forest still stands the growth consists chiefly of oak. The soil is easy to handle, and can be worked without regard to moisture content. It is considered a good corn land, but is too light-textured for wheat, although a considerable acreage is devoted to this crop. Corn yields at the rate of 40 or 50 bushels per acre, wheat from 12 to 15 bushels and occasionally more, and grass and clover at the rate of 1 or 2 tons per acre. The productiveness of the soil depends greatly on the sand content of the subsoil. If the quantity be large, the soil is porous and requires considerable rain to produce good yields. If the clay content predominates, a moderate amount of rain suffices and good yields are obtained. Apples, pears, and small fruits do well on this soil.
The Penn clay consists of from 6 to 12 inches of a red or reddish-brown loam, resting upon a subsoil of heavy red clay. The soil and subsoil generally have the Indian-red color characteristic of the Triassic red sandstone from which the soil is in part derived. From 1 to 10 per cent of the soil mass is usually made up of small sandstone fragments, while throughout the greater part of the type numerous limestone conglomerate ledges, interbedded with Triassic red sandstone, come to the surface. In other areas of the type numerous limestone conglomerate bowlders, often of great size, cover from 10 to 25 per cent of the surface.
This latter phase occurs in the vicinity of the Potomac River near Point of Rocks, Md., and near the Potomac, 3 miles north of Leesburg, and in these places the heavier phase of the type occurs, the clay often being very near the surface. In other parts of the County, where the limestone conglomerate is not so preponderant, or where it lies deeper and is mostly unexposed, the surface soil is deeper, often consisting of 18 inches of loam. The land is locally termed "limestone land." Near Catoctin Mountain the rocks seem to have weathered to considerable depth, there being no exposures or outcrops. Here the soil has been washed away from some of the more elevated small areas, and the heavy red clay subsoil is exposed.
In a great many places along the base of the mountain the formation of this type is somewhat complicated by the wash from the mountain, which consists principally of subangular quartz fragments, from 1 to 4 inches in diameter. This rock sometimes forms as much as 30 or 40 per cent of the soil mass. This phase is called "gravelly land," and is hard to cultivate on account of its heavy texture and stony condition, although it is inherently productive.
This type occurs in one irregular-shaped area, about 15 miles long, varying from less than 1 mile to 3 or 4 miles in width, being cut by the Potomac River just east of Point of Rocks, Md. It thus lies in the central part of the County, in the Piedmont Plateau, extending from immediately north of Leesburg, and skirting the eastern foot of Catoctin Mountain.
The general surface drainage is good, there being many small streams flowing through the type and emptying into the Potomac River. The stream beds are but little lower than the surface of the surrounding land, while the slopes are long and gentle. Excessive erosion scarcely ever occurs. The heavier phase of the type would undoubtedly be improved by tile draining, as it is usually lower lying than the lighter phase. The heavier phase bakes and cracks in dry weather much the same as the heavy limestone soils of the Shenandoah Valley, but with the lighter phases, where the soil covering is deeper, good tilth is easily maintained throughout the growing season.