History and Comprehensive Description of Loudoun County, Virginia Part 2

The geology of more than half the area of Loudoun County has received thorough and intelligent treatment at the hands of Arthur Keith in his most excellent work ent.i.tled "_Geology of the Catoctin Belt_,"

authorized and published by the United States Geological Survey.[5]

[Footnote 5: Credit for many important disclosures and much of the detail appearing in this department is unreservedly accorded Mr. Keith and his a.s.sistants.]

Mr. Keith's a.n.a.lysis covered the whole of Bull Run Mountain, the Catoctin in its course through Virginia and Maryland to its termination in southern Pennsylvania, the Blue Ridge and South Mountain for a corresponding distance, all intermediate ridges and valleys and contiguous territory lying outside this zone and paralleling the two flanking ranges.[6]

[Footnote 6: The name "Catoctin Belt" is applied to this region because it is separated by Catoctin Mountain from the Piedmont plain as a geographic unit more distinctly than in any other area, and because its geological unity is completed by Catoctin more fully and compactly than elsewhere.]

In this important work the Catoctin Belt is shown to be an epitome of the leading events of geologic history in the Appalachian region. It contains the earliest formations whose original character can be certified; it contains almost the latest known formations; and the record is unusually full, with the exception of the later Paleozoic rocks. Its structures embrace nearly every known type of deformation.

It furnishes examples of every process of erosion, of topography derived from rocks of nearly every variety of composition, and of topography derived from all types of structure except the flat plateau type. In the recurrence of its main geographic 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 const.i.tution.

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 const.i.tuting 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 cla.s.s 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 Catoctin Belt.

They have been the theme of considerable literature, owing to their great extent and prominence in the topography.

_Granite._

The granite in the southern portion of the County is very important in point of extent, almost as much so as the diabase in the same section.

The areas of granite are, as a rule, long narrow belts, and vary greatly in width.

The mineralogical composition of the granite is quite constant over large areas. Six varieties can be distinguished, however, each with a considerable areal extent. The essential const.i.tuents are quartz, orthoclase and plagioclase, and by the addition to these of biot.i.te, garnet, epidote, blue quartz, and hornblende, five types are formed.

All these types are holocrystalline, and range in texture from coa.r.s.e granite with augen an inch long down to a fine epidote granite with scarcely visible crystals.