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.
Metamorphism of the Loudoun formation is quite general. It commonly
appears in the production of phyllites from the argillaceous members
of the formation, but all of the fragmental varieties show some
elongation and production of secondary mica. The limestone beds are
often metamorphosed to marble, but only in the eastern belt. The
recrystallization is not very extensive, and none of the marbles are
coarse grained.
The metamorphism of the igneous rocks is regional in nature and has
the same increase from west to east as the sediments.
In the granite it consists of various stages of change in form,
attended by some chemical rearrangement. The process consisted of
progressive fracture and reduction of the crystals of quartz and
feldspar, and was facilitated by the frequent cleavage cracks of the
large feldspars. It produced effects varying from granite with a rude
gneissoid appearance, through a banded fine gneiss, into a fine quartz
schist or slate. These slaty and gneissoid planes are seen to be
parallel to the direction and attitude of the sediments, wherever they
are near enough for comparison.
Dynamic alteration of the Catoctin diabase is pronounced and
wide-spread. Macroscopically it is evident in the strong schistosity,
which is parallel to the structural planes of the sediments when the
two are in contact. In most areas this alteration is mainly chemical
and has not affected the original proportions of the rock to a marked
extent. Its prevalence is due to the unstable composition of the
original minerals of the rock, such as olivine, hypersthene, and
pyroxene. Along Catoctin Mountain, however, both chemical and
mechanical deformation have taken place, so that the original rock
structure is completely merged into pronounced schistosity. This was
materially assisted by the weak lath shapes of the feldspar and the
mobility of the micas.
The average dip of the schistose planes is about 60°; from this they
vary up to 90° and down to 20°. In all cases they are closely parallel
to the planes on which the sediments moved in adjustment to folding,
namely, the bedding planes. In regions where no sediments occur, the
relation of the schistose planes to the folds can not be discovered.