The Loudoun formation, of course, followed a period of erosion of the Catoctin Belt, since it is the first subaqueous deposit. It is especially developed with respect to thickness and coarseness to the west of Catoctin Mountain. Elsewhere the outcrops are almost entirely black slate. This is true along the Blue Ridge, through almost its entire length, and also through the entire length of the Catoctin Mountain. On the latter range it is doubtful if this formation exceeds 200 feet in thickness at any point. Along the Blue Ridge it may, and probably does, in places, reach 500 feet in thickness.
The distribution of the coarse varieties coincides closely with the areas of greatest thickness and also with the synclines in which no Weverton sandstone appears. The conglomerates of the Loudoun formation are composed of epidotic schist, andesite, quartz, granite, epidote, and jasper pebbles embedded in a matrix of black slate and are very limited in extent.
The formation next succeeding the Loudoun formation is the Weverton sandstone. It is so named on account of its prominent outcrops in South Mountain, near Weverton, Maryland, and consists entirely of siliceous fragments, mainly quartz and feldspar. Its texture varies from a very fine, pure sandstone to a moderately coarse conglomerate, but, in general, it is a sandstone. As a whole, its color is white and varies but little; the coarse beds have a grayish color in most places. Frequent bands and streaks of bluish black and black are added to the white sandstones, especially along the southern portion of the Blue Ridge. The appearance of the rock is not modified by the amount of feldspar which it contains.
From the distribution of these various fragments, inconspicuous as they are, considerable can be deduced in regard to the environment of the Weverton sandstone.
The submergence of the Catoctin Belt was practically complete, because the Weverton sandstone nowhere touches the crystalline rocks. Perhaps it were better stated that submergence was complete in the basins in which Weverton sandstone now appears. Beyond these basins, however, it is questionable if the submergence was complete, because in the Weverton sandstone itself are numerous fragments which could have been derived only from the granite masses. These fragments consist of blue quartz, white quartz, and feldspar. The blue quartz fragments are confined almost exclusively to the outcrops of the Weverton sandstone in the Blue Ridge south of the Potomac, and are rarely found on Catoctin.
The general grouping of the Loudoun formation into two classes of deposit (1), the fine slates associated with the Weverton sandstone, and (2), the course sandstones occurring in deep synclines with no Weverton, raises the question of the unity of that formation. The evidence on this point is manifold and apparently conclusive. The general composition of the two is the same—i. e., beds of feldspathic, siliceous material derived from crystalline rocks. They are similarly metamorphosed in different localities. The upper parts of the thicker series are slates identical in appearance with the slates under the Weverton, which presumably represent the upper Loudoun.
A marked change in the thickness of the Weverton sandstone occurs along Catoctin Mountain, the formation diminishing from 1,000 to 200 feet in a few miles. This plainly indicates shore conditions, and the nature of the accompanying change of constituent material locates the direction of the shore. This change is a decrease of the feldspar amounting to elimination at the Potomac. As the feldspar, which is granular at the shore, is soon reduced to fine clay and washed away, the direction of its disappearance is the direction of deep water. Thus the constitution and thickness of the Weverton sandstone unite in showing the existence of land not far northeast of Catoctin Mountain during Weverton deposition.
Aside from this marked change in thickness, none of unusual extent appears in the Weverton sandstone over the remainder of the Catoctin Belt. While this is partly due to lack of complete sections, yet such as are complete show a substantial uniformity. The sections of the Blue Ridge outcrops range around 500 feet, and those of the Catoctin line are in the vicinity of 300. This permanent difference in thickness along the two lines can be attributed to an eastward thinning of the formation, thus, however, implying a shore to the west of the Blue Ridge line. It can also be attributed to the existence of a barrier between the two, and this agrees with the deductions from the constituent fragments.