Chapter 3

The name of the Loudoun formation is given on account of the frequent occurrence of all its variations in Loudoun County. Throughout the entire extent of the Catoctin Belt, and especially through its central portions, the Loudoun formation has frequent beds of sandstone, conglomerate, and limestone. The limestones occur as lenses along two lines; one immediately west of Catoctin Mountain, the other three or four miles east of the Blue Ridge. Along the western range the limestone lenses extend only to the Potomac. There they are shown on both sides of the river, and have been worked in either place for agricultural lime. Only the refuse of the limestone now remains, but the outcrops have been extant until recent years. Along the eastern line the limestone lenses extend across the Potomac and into Maryland for about one mile, and it is along this belt that they are the most persistent and valuable. As a rule they are altered from limestone into marble, and at one point they have been worked for commercial purposes. Nearly every outcrop has been opened, however, for agricultural lime. Where Goose Creek crosses this belt a quarry has been opened and good marble taken out, but want of transportation facilities has prevented any considerable development. The relation between marble and schist is very perfectly shown at an old quarry west of Leesburg. The marble occupies two beds in schist, and between the two rocks there is gradation of composition. In none of the western belts are the calcareous beds recrystallized into marbles, but all retain their original character of blue and dove-colored limestone. None of them, however, is of great thickness and none of great linear extent.

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.

_Weverton Sandstone._

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.

_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 broad flat hills or uplands. The rock weathers into a fine gray and brown clay with numerous bowlders of unaltered rock of a marked concentric shape.

While the diabase dikes are most prominent in the Newark rocks, they are also found occasionally in the other terraces. In the Catoctin Belt they appear irregularly in the granite and schist. Rare cases also occur in the rocks of the Piedmont plain. The diabase of the Newark areas is almost exclusively confined to the red sandstone, and the dike at Leesburg cutting the limestone conglomerate is almost the only occurrence of that combination.

The diabase occurs only as an intrusive rock in the vicinity of the Catoctin Belt. Of this form of occurrence, however, there are two types, dikes and massive sheets or masses. The dikes are parallel to the strike of the inclosing sandstone as a rule, and appear to have their courses controlled by it on account of their small bulk. The large masses break at random across the sandstone in the most eccentric fashion. No dislocation can be detected in the sandstones, either in strike or dip, yet of course it must exist by at least the thickness of the intrusive mass. That this thickness is considerable is shown by the coarseness of the larger trap masses, which could occur only in bodies of considerable size, and also by the width of their outcrops in the westward dipping sandstones. The chief mass in point of size is three miles wide. This mass fast decreases in width as it goes north, without losing much of its coarseness, and ends in Leesburg in a hooked curve. The outline of the diabase is suggestive of the flexed trap sheets of more northern regions, but this appearance is deceptive, since the diabase breaks directly across both red sandstone and limestone conglomerate, which have a constant north and south strike. An eastern branch of this mass crosses the Potomac as a small dike and passes north into Pennsylvania. The diabase dikes in the Catoctin Belt are always narrow, and, while many outcrops occur along a given line, it is probable that they are not continuous.

At Leesburg the limestone conglomerate next the diabase is indurated, its iron oxide is driven off, and the limestone partly crystallized into marble.

_Catoctin Schist._

The Catoctin schist is geographically the most important of the volcanic rocks of Loudoun.

Throughout its entire area the schist is singularly uniform in appearance, so that only two divisions can be made with any certainty at all. These are dependent upon a secondary characteristic, viz, the presence of epidote in large or small quantities. The epidote occurs in the form of lenses arranged parallel to the planes of schistosity, reaching as high as five feet in thickness and grading from that down to the size of minute grains. Accompanying this lenticular epidote is a large development of quartz in lenses, which, however, do not attain quite such a size as those of epidote. Both the quartz and epidote are practically insoluble and lie scattered over the surface in blocks of all sizes. In places they form an almost complete carpet and protect the surface from removal. The resulting soil, where not too heavily encumbered with the epidote blocks, is rich and well adapted to farming, on account of the potash and calcium contained in the epidote and feldspar.

Except along the narrow canyons in the Tertiary baselevel the rock is rarely seen unless badly weathered. The light bluish green color of the fresh rock changes on exposure to a dull gray or yellow, and the massive ledges and slabs split up into thin schistose layers. It is quite compact in appearance, and as a rule very few macroscopic crystals can be seen in it.

A general separation can be made into an epidotic division characterized by an abundance of macroscopic epidote and a non-epidotic division with microscopic epidote. These divisions are accented by the general finer texture of the epidotic schist.

The schists can be definitely called volcanic in many cases, from macroscopic characters, such as the component minerals and basaltic arrangement. In most cases, the services of the microscope are necessary to determine their nature. Many varieties have lost all of their original character in the secondary schistosity. None the less, its origin as diabase can definitely be asserted of the whole mass. In view of the fact, however, that most of the formation has a well defined schistosity destroying its diabasic characters, and now is not a diabase but a schist, it seems advisable to speak of it as a schist.

Sections of the finer schist in polarized light show many small areas of quartz and plagioclase and numerous crystals of epidote, magnetite, and chlorite, the whole having a marked parallel arrangement. Only in the coarser varieties is the real nature of the rock apparent. In these the ophitic arrangement of the coarse feldspars is well defined, and in spite of their subsequent alteration the fragments retain the crystal outlines and polarize together. Additional minerals found in the coarse schists are calcite, ilmenite, skeleton oblivine, biotite, and hematite.

_Rocks of the Piedmont Plain._

The Piedmont plain, where it borders upon the Catoctin Belt, is composed in the main of the previously described Newark strata, red sandstone, and limestone conglomerate. East of the Newark areas lies a broad belt of old crystalline rocks, whose relations to the Catoctin Belt are unknown.

The rocks, in a transverse line, beginning a little to the east of Dranesville, in Fairfax County, and extending to the Catoctin Mountain, near Leesburg, occur in the following order, viz: Red sandstone, red shale, greenstone, trap, reddish slate, and conglomerate limestone.

Heavy dykes of trap rock extend across the lower end of the County, from near the mouth of Goose Creek to the Prince William line. "These, being intrusive rocks, have in some places displaced the shale and risen above it, while in other places a thin coat of shale remains above the trappean matter, but much altered and changed in character."[7] A large mass of trap rock presents itself boldly above the shale at the eastern abutment of the Broad Run bridge, on the Leesburg and Alexandria turnpike. Not far to the east the shale is changed to a black or blackish brown color, while at the foot of the next hill still farther eastward the red shale appears unchanged. The summits of many of these dykes are "covered with a whitish or yellowish compact shale, highly indurated and changed into a rock very difficult to decompose."[8]

[Footnote 7: Taylor's _Memoir_.]

[Footnote 8: Ibid.]

_Lafayette Formation._

A great class of variations due to rock character are those of surface form. The rocks have been exposed to the action of erosion during many epochs, and have yielded differently according to their natures. Different stages in the process of erosion can be distinguished and to some extent correlated with the time scale of the rocks in other regions. One such stage is particularly manifest in the Catoctin Belt and furnishes the datum by which to place other stages. It is also best adapted for study, because it is connected directly with the usual time scale by its associated deposits. This stage is the Tertiary baselevel, and the deposit is the Lafayette formation, a deposit of coarse gravel and sand lying horizontally upon the edges of the hard rocks. Over the Coastal plain and the eastern part of the Piedmont plain it is conspicuously developed, and composes a large proportion of their surfaces. As the formation is followed westward it is more and more dissected by erosion and finally removed. Near the area of the Catoctin Belt it occurs in several places, all of them being small in area. One is three miles northeast of Aldie. Here, a Newark sandstone hill is capped with gravel. This gravel is much disturbed by recent erosion and consists rather of scattered fragments than of a bedded deposit.

The materials of the Lafayette gravel are chiefly pebbles and grains of quartz, with a considerable admixture of quartzite and sandstone. The large quartz pebbles were probably derived from the large lenses of quartz in the Catoctin schist, for no other formation above water at the time contained quartz in large enough masses to furnish such pebbles. On the hypothesis that they were of local origin and merely worked over during submergence, they might be connected with the quartz veins of the Piedmont plain. That theory, however, with difficulty accounts for their well-rounded condition, which shows either beach action or long carriage. The quartz sand may well have been derived from the granitic quartzes, but that is an uncertain matter. The sandstones and quartzites are usually massive and pure white, of the variety found along Catoctin and Bull Run mountains. Other varieties of sandstone--the blue-banded type, for instance--are derived from the Weverton sandstone on the Blue Ridge. The white sandstone pebbles in the terraces along Bull Run Mountain can be traced from the ledges to the deposits. In this region, therefore, an absolute shore can be seen. In other areas along Catoctin Mountain a shore can be inferred, because the mountain projects above the baselevel plane and contains no gravel deposits. In fact, only a few points at the stream gaps are cut down to the baselevel.

_Metamorphism._

Dynamic metamorphism has produced great rearrangement of the minerals along the eastern side of the Catoctin Belt, and results at times in complete obliteration of the characters of the granite. The first step in the change was the cracking of the quartz and feldspar crystals and development of muscovite and chlorite in the cracks. This was accompanied by a growth of muscovite and quartz in the unbroken feldspar. The aspect of the rock at this stage is that of a gneiss with rather indefinite banding. Further action reduced the rock to a collection of angular and rounded fragments of granite, quartz, and feldspar in a matrix of quartz and mica, the mica lapping around the fragments and rudely parallel to their surfaces. The last stage was complete pulverization of the fragments and elongation into lenses, the feldspathic material entirely recomposing into muscovite, chlorite, and quartz, and the whole mass receiving a strong schistosity, due to the arrangement of the mica plates parallel to the elongation. This final stage is macroscopically nothing more than a siliceous slate or schist, and is barely distinguishable from the end products of similar metamorphism in the more feldspathic schists and the Loudoun sandy slates. The different steps can readily be traced, however, both in the hand specimen and under the microscope.

The Weverton sandstone has suffered less from metamorphism than any of the sediments. In the Blue Ridge it has undergone no greater change than a slight elongation of its particles and development of a little mica. Along Catoctin Mountain, from the Potomac River south, however, increased alteration appears together with the diminution in thickness. What little feldspar there was is reduced to quartz and mica, and the quartz pebbles are drawn out into lenses. Deposition of secondary quartz becomes prominent, amounting in the latitude of Goose Creek to almost entire recrystallization of the mass. A marked schistosity accompanies this alteration, and most of the schistose planes are coated with silvery muscovite. Almost without exception these planes are parallel to the dip of the formation.