James W. (James William) Head.

History and comprehensive description of Loudoun County, Virginia [electronic resource] online

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Online LibraryJames W. (James William) HeadHistory and comprehensive description of Loudoun County, Virginia [electronic resource] → online text (page 3 of 15)
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The lyoudoun 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 syn-
clines in which no Weverton sandstone appears. The con-
glomerates of the I^oudoun 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

Weverton Sandstone.

The formation next succeeding the Loudoun formation is the
Weverton sandstone. It is so named on account of its promi-
nent 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 sand-
stone 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, incon-
spicuous as they are, considerable can be deduced in regard
to the environment of the Weverton sandstone.


The submergence of the Catoctin Belt was practically com-
plete, because the Weverton sandstone nowhere touches the
crystalline rocks. Perhaps it were better stated that sub-
mergence 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 I^oudoun formation into two
classes of deposit (1), the fine slates associated with the
Weverton sandstone, and (2), the course sandstones occur-
ring 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 compo-
sition of the two is the same — i. e., beds of feldspathic, sili-
ceous 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 rep-
resent the upper Loudoun.

A marked change in the thickness of the Weverton sand-
stone occurs along Catoctin Mountain, the formation dimin-
ishing from 1,000 to 200 feet in a few miles. This plainly
indicates shore conditions, and the nature of the accompany-
ing 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


Aside from this marked change in thickness, none of un-
usual 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 out-
crops range around 500 feet, and those of the Catoctin line
are in the vicinity of 3OO. This permanent difference in thick-
ness 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, deposi-
tion, 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 con-
siderable wedges or lenses of limestone conglomerate. At
many places also gray feldspathic sandstones and basal con-
glomerates appear.

The limestone conglomerate is best developed from the
Potomac to lycesburg, 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 cal-
careous 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 re-
placed 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 drain-
ing streams have approached their baselevel, scarcely an out-
crop of conglomerate is seen. Where the areas of conglom-
erate lie near faster falling streams, the irregular masses of
un weathered 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 pro-
duced 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 ex-
ception, 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 com-
plete without reference to the later diabase which is asso-
ciated 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 Pied-
mont plain. The diabase of the Newark areas is almost ex-
clusively confined to the red sandstone, and the dike at Lees-
burg 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 ran-
dom 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 sand-
stones. 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 lycesburg the limestone conglomerate next the diabase
is indurated, its iron oxide is driven off, and the limestone
oartly crystallized into marble.

Catoctin Schist.

The Catoctin schist is geographically the most important
of the volcanic rocks of I^oudoun.

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 divi-
sions are accented by the general finer texture of the epidotic

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 second-
ary schistosity. None the less, its origin as diabase can defi-
nitely 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

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 ar-
rangement 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 I^eesburg, occur in the following
order, viz: Red sandstone, red shale, greenstone, trap, red-
dish 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."* 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, "t

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 fur-
nishes 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 hori-
zontally upon the edges of the hard rocks. Over the Coastal

^Taylor's Memoir.
t Ibid.


plain and the eastern part of the Piedmont plain it is con-
spicuously 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 quart-
zite 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 con-
dition, 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 mus-
covite 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 sili-
ceous slate or schist, and is barely distinguishable from the
end products of similar metamorphism in the more feldspathic
schists and the I^oudoun 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 metamor-
phism 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 mus-
covite. 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



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Online LibraryJames W. (James William) HeadHistory and comprehensive description of Loudoun County, Virginia [electronic resource] → online text (page 3 of 15)