University of Virginia Library

Geology.

FOWEVER interesting the details of the geological structure
of this county may be to the scientific person, the
scope of this publication only permits a general description
of the formations recognized.

There are but two ages certainly represented—the Archæan
and Mesozoic. Some sandstone west of the Southwest Mountains
is so much like the Potsdam sandstone, found west of
the Blue Ridge in adjoining counties, that it is not improbable
that it belongs to that epoch. Of the two formations certainly
represented the Archæan is the most extensive; the

Mesozoic being confined to an area roughly bounded, on the
west by an irregular line beginning at Howardsville, extending
north-easterly, and gradually diverging from the eastern foot
of the Greene Mountains to Hardware river, which it crosses
at the plantations of Messrs. Moon and Barksdale, quickly
retreats towards the south, by a very roughly defined line to
near the north-western part of the town of Scottsville, from
thence south-westerly to the James.

Its western border is marked by a very coarse conglomerate,
which most likely rests upon the underlying Archæan rocks
throughout the Mesozoic area, for it comes to the surface at
many different points throughout this region. This conglomerate
is very properly called a "bowlder conglomerate," being
made up of fragments of the common Blue Ridge rocks, large
rounded, or angular, such as epidote and hornblende schists',
green quartzites and fragments of the Potsdam sandstone. It
is extremely hard and difficulty is experienced in attempting
to secure specimens, even with a sledge hammer. Besides
this conglomerate there is a finer-grained one in which are
white particles of quartz, feldspar or calcite embedded in a
brown matrix. A brown sandstone of varying degrees of
induration and texture is also found; in some localities it possessess
a texture and appearance that make it valuable for
building purposes.

At several points, within this Mesozoic area, Diorite of igneous
origin lifts itself to the surface, always at the axis of the
anticlinal folds. Near Glendower Mills there is a ledge of
slates, variegated in color, but not cleaving well enough to be
valuable. A mile west of Glendower, on the Porter's Precinct
road, an impure limestone is found. The strike of the Mesozoic
is about N. 25° E., varying but little locally, the dip is
most commonly towards the south-east, and amounts to about
25°. No fossils have been reported from this formation.

At the period when the James was breaking its way through
the Blue Ridge, the locality now occupied by this Mesozoic
area, was most evidently a bay, into which the river carried
and deposited the debris, produced by erosion, coarse at first,

afterwards becoming finer, and these, by consolidation and induration,
were converted into the conglomerates and sandstones
of the formation.

The remainder of the rocks of the country belong to the
Archæan. Two periods being recognized — the Laurentian
and Huronian. There being no uncomformability nor fossils
in the strata, the division is based entirely upon the differences
in the rocks. The granitic rocks and gneisses, diorite (bedded)
and mica schists are considered as Laurentian, while the slates
and other schists are most usually considered to be Huronian.
The strata of this age have been frequently and so closely
flexed, and their tops so much eroded as to render it almost
impossible to accurately determine their connection and relative
superposition. The strike of these rocks is about W. 32°
E., varying locally to a slight amount. The dip is always towards
the S. E., except along an east and west section passing
through Dudley's Mountain, where there is an anticlinal fold
in the Mica schist.

The Blue Ridge is mainly made up of a series of metamorphic
schists, greenish, blue and black in color. Concretions
of epidote and other minerals are frequently found in the
schists, giving them a curious mottled appearance.

The Ragged Mountains is composed of a species of granulite,
the quartz possessing a waxy lustre and bluish color, due
to being filled with microscopic crystals of Titanium oxide.
This rock varies much in composition, weathers unequally,
and hence gives to the mountains the appearance which justifies
their name.

The range east of the Virginia Midland railway, in which
are Dudley's, Appleberry, and Lead Mine Mountains is made
up of mica schist and some bedded Diorite.

The Southwest Mountains are are made up of metamorphic
schists, like the Blue Ridge, and some granitic rocks and
quartzites. The Greene Mountains, towards their southern
part, become very slaty, and in some localities the slates are
so highly impregnated with iron oxide as to appear to be fairly
good micaceous hematite.

East of these mountains there is a succession of slates,
sometimes hydro-micaceous in character and usually soft.

In the debris of Moorman's river, which rises in the Blue
Ridge, Potsdam bowlders are so frequently met with as to
lead to the belief that Potsdam S. S. is to be found in situ, on
the eastern slopes of the Blue Ridge; and in Black Rock gap,
in which a tributary of Moorman's river rises, there are
found, near the summit, masses of Potsdam embedded in the
soil as if they occupied their original place. These masses
are usually detached from each other, their strike and dip being
different for each mass, points to the belief that they have
been moved down the mountain from their original position.
Their position may be accounted for by the following facts:

The Potsdam formation lies upon the western slope of the
Blue Ridge, forming a range of mountains which are usually
of a lower height than the crest of the Blue Ridge. But,
north-west of Black Rock Gap, in Rockingham county, there
is a peak, surmounted by the Potsdam sandstone, higher than
the crest of the Blue Ridge at the Gap; this peak is connected
with the crest by a long slope, and it would be only a matter
of time for the Potsdam sandstones, propelled by action of
frost, &c., to work their way down and over the crest to the
eastern slope of the Blue Ridge.