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