A marked change in the thickness of the Weverton sandstone occurs along Catoctin Mountain, the formation diminishing from 1,000 to 200 feet in a few miles.  This plainly indicates shore conditions, and the nature of the accompanying 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 deposition.

Aside from this marked change in thickness, none of unusual 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 outcrops range around 500 feet, and those of the Catoctin line are in the vicinity of 300.  This permanent difference in thickness 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, deposition, 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 considerable wedges or lenses of limestone conglomerate.  At many places also gray feldspathic sandstones and basal conglomerates appear.

The limestone conglomerate is best developed from the Potomac to Leesburg, and from that region southward rapidly diminishes until it is barely represented at the south end of Catoctin Mountain.