History and Comprehensive Description of Loudoun County, Virginia | Page 9

features from pre-Cambrian time till
the present day it furnishes a remarkable and unique example of the
permanence of continental form.
With certain qualifications, a summary of the leading events that have

left their impress on the region is as follows:
1. Surface eruption of diabase.
2. Injection of granite.
3. Erosion.
4. Surface eruption of quartz-porphyry, rhyolite, and andesite.
5. Surface eruption of diabase.
6. Erosion.
7. Submergence, deposition of Cambrian formations; slight oscillations
during their deposition; reduction of land to baselevel.
8. Eastward tilting and deposition of Martinsburg shale; oscillations
during later Paleozoic time.
9. Uplift, post-Carboniferous deformation and erosion.
10. Depression and Newark deposition; diabase intrusion.
11. Uplift, Newark deformation; and erosion to Catoctin baselevel.
12. Depression and deposition of Potomac, Magothy, and Severn.
13. Uplift southwestward and erosion to baselevel.
14. Uplift, warping and degradation to Tertiary baselevel; deposition of
Pamunkey and Chesapeake.
15. Depression and deposition of Lafayette.
16. Uplift and erosion to lower Tertiary baselevel.
17. Uplift, warping and erosion to Pleistocene baselevel; deposition of
high-level Columbia.

18. Uplift and erosion to lower Pleistocene baselevel; deposition of
low-level Columbia.
19. Uplift and present erosion.
Along the Coastal plain reduction to baselevel was followed by
depression and deposition of Lafayette gravels; elevation followed and
erosion of minor baselevels; second depression followed and deposition
of Columbia gravels; again comes elevation and excavation of narrow
valleys; then depression and deposition of low-level Columbia; last,
elevation and channeling, which is proceeding at present. Along the
Catoctin Belt denudation to baselevel was followed by depression and
deposition of gravels; elevation followed and erosion of minor
baselevels among the softer rocks; second depression followed, with
possible gravel deposits; elevation came next with excavation of broad
bottoms; last, elevation and channeling, at present in progress.
The general structure of the Catoctin Belt is anticlinal. On its core
appear the oldest rocks; on its borders, those of medium age; and in
adjacent provinces the younger rocks. In the location of its system of
faulting, also, it faithfully follows the Appalachian law that faults lie
upon the steep side of anticlines.
After the initial location of the folds along these lines, compression and
deformation continued. Yielding took place in the different rocks
according to their constitution.
Into this system of folds the drainage lines carved their way. On the
anticlines were developed the chief streams, and the synclines were left
till the last. The initial tendency to synclinal ridges was obviated in
places by the weakness of the rocks situated in the synclines, but even
then the tendency to retain elevation is apt to cause low ridges. The
drainage of the belt as a whole is anticlinal to a marked degree, for the
three main synclinal lines are lines of great elevation, and the anticlines
are invariably valleys.
In order of solubility the rocks of the Catoctin Belt, within the limits of
Loudoun County, to which section all subsequent geologic data will be

confined, stand as follows:
1. Newark limestone conglomerate; calcareous.
2. Newark sandstone and shale; calcareous and feldspathic.
3. Newark diabase; feldspathic.
4. Granite; feldspathic.
5. Loudoun formation; feldspathic.
6. Granite and schist; feldspathic.
7. Catoctin schist; epidotic and feldspathic.
8. Weverton sandstone; siliceous.
All of these formations are in places reduced to baselevel. The first
three invariably are, unless protected by a harder rock; the next three
usually are; the Catoctin schist only in small parts of its area; the
Weverton only along a small part of Catoctin Mountain.
The Catoctin Belt itself may be described as a broad area of igneous
rocks bordered by two lines of Lower Cambrian sandstones and slates.
Over the surface of the igneous rocks are scattered occasional outliers
of the Lower Cambrian slate; but far the greater part of the surface of
the belt is covered by the igneous rocks. The belt as a whole may be
regarded as an anticline, the igneous rocks constituting the core, the
Lower Cambrian the flanks, and the Silurian and Newark the adjoining
zones. The outcrops of the Lower Cambrian rocks are in synclines, as a
rule, and are complicated by many faults. The igneous rocks have also
been much folded and crumpled, but on account of their lack of
distinctive beds the details of folds can not well be traced among them.
They are the oldest rocks in the Catoctin Belt and occupy most of its
area. They are also prominent from their unusual character and rarity.
An important class of rocks occurring in the Catoctin Belt is the

sedimentary series. It is all included in the Cambrian period and
consists of limestone, shale, sandstone and conglomerate. The two
border zones of the Catoctin Belt, however, contain also rocks of the
Silurian and Juratrias periods. In general, the sediments are sandy and
calcareous in the Juratrias area, and sandy in the