History and Comprehensive Description of Loudoun County, Virginia. James William Head
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.
Newark System.
An epoch of which a sedimentary record remains in the region of the Catoctin Belt is one of submergence and deposition, the Newark or Juratrias. The formation, though developed in the Piedmont plain, bears upon the history of the Catoctin Belt by throwing light on the periods of degradation, deposition, igneous injection, and deformation that have involved them both.
At the Potomac River it is about 4 miles in width, at the latitude of Leesburg about 10 miles in width, and thence it spreads towards the east until its maximum width is, perhaps, 15 miles. The area of the Newark formation is, of course, a feature of erosion, as far as its present form is concerned. In regard to its former extent little can be said, except what can be deduced from the materials of the formation itself. Three miles southeast of Aldie and the end of Bull Run Mountain a ridge of Newark sandstone rises to 500 feet. The same ridge at its northern end, near Goose Creek, attains 500 feet and carries a gravel cap. One mile south of the Potomac River a granite ridge rises from the soluble Newark rocks to the same elevation.
As a whole the formation is a large body of red calcareous and argillaceous sandstone and shale. Into this, along the northern portion of the Catoctin Belt, are intercalated considerable wedges or lenses of limestone conglomerate. At many places also gray feldspathic sandstones and basal conglomerates appear.
The limestone conglomerate is best developed from the Potomac to Leesburg, and from that region southward rapidly diminishes until it is barely represented at the south end of Catoctin Mountain.
The conglomerate is made up of pebbles of limestone of varying sizes, reaching in some cases a foot in diameter, but, as a rule, averaging about 2 or 3 inches. The pebbles are usually well rounded, but sometimes show considerable angles. The pebbles of limestone range in color from gray to blue and dark blue, and occasionally pebbles of a fine white marble are seen; with rare exceptions also pebbles of Catoctin schist and quartz occur. They are embedded in a red calcareous matrix, sometimes with a slight admixture of sand. As a rule the entire mass is calcareous.
The conglomerate occurs, as has been said, in lenses or wedges in the sandstone ranging from 1 foot to 500 feet in thickness, or possibly even greater. They disappear through complete replacement by sandstone at the same horizon. The wedge may thin out to a feather edge or may be bodily replaced upon its strike by sandstone; one method is perhaps as common as the other. The arrangement of the wedges is very instructive indeed. The general strike of the Newark rocks is a little to the west of north, while the strike of the Catoctin Belt is a little to the east of north. The two series, therefore, if extended, would cross each other at an angle of 20 to 30 degrees. The conglomerate wedges are collected along the west side of the Newark Belt and in contact usually with the Weverton sandstone. The thick ends of the wedges along the line of contact usually touch each other. Going south by east the proportion of the sandstone increases with rapid extermination of the conglomerate. The thin ends of the wedges, therefore, resemble a series of spines projecting outward from the Catoctin Belt.
The result of weathering upon the conglomerate is a very uneven and rugged series of outcrops projecting above the rolling surface of the soil.
The ledges show little definite stratification and very little dip. The topography of the conglomerate is inconspicuous and consists of a slightly rolling valley without particular features. It approaches nearer to the level of the present drainage than any other formation, and decay by solution has gone on to a very considerable extent. Where the draining streams have approached their baselevel, scarcely an outcrop of conglomerate is seen. Where the areas of conglomerate lie near faster falling streams, the irregular masses of unweathered rocks appear.
When but slightly weathered the conglomerate forms an effective decorative stone and has been extensively used as a marble with the name "Potomac marble," from the quarries on the Potomac east of Point of Rocks, Maryland. While it is in no sense a marble, yet the different reds and browns produced by unequal weathering of the limestone pebbles have a very beautiful effect.
The thickness of the Newark formation is most uncertain. The rocks dip at a light angle to the west with hardly an exception, and the sections all appear to be continuous. Even with liberal deductions for frequent faults, nothing less than 3,000 feet will account for the observed areas and dips.
Newark Diabase.
Description of the lithified deposits would be far from complete without reference to the later diabase which is associated with the Newark rocks.
These diabases, as they will be called generically, are usually composed of plagioclase feldspar, and diallage or augite; additional and rarer minerals are quartz, olivine, hypersthene, magnetite, ilmenite, and hornblende. Their structure is ophitic in the finer varieties, and to some extent in the coarser kinds as well. They are holocrystalline in form and true glassy bases are rare, rendering the term diabase more appropriate than basalt.
There is greater variety in texture, from fine aphanitic traps up to coarse grained dolerites with feldspars one-third of an inch long. The coarser varieties are easily quarried and are often used for building stone under the name of granite.
These forms are retained to the present day with no material change except that of immediate weathering, but to alterations of this kind they are an easy prey, and yield the most characteristic forms. The narrow dikes produce ridges between slight valleys of sandstone or shale, the wide bodies produce