Rocks of the Piedmont Plain.

The Piedmont plain, where it borders upon the Catoctin Belt, is composed in the main of the previously described Newark strata, red sandstone, and limestone conglomerate.  East of the Newark areas lies a broad belt of old crystalline rocks, whose relations to the Catoctin Belt are unknown.

The rocks, in a transverse line, beginning a little to the east of Dranesville, in Fairfax County, and extending to the Catoctin Mountain, near Leesburg, occur in the following order, viz:  Red sandstone, red shale, greenstone, trap, reddish slate, and conglomerate limestone.

Heavy dykes of trap rock extend across the lower end of the County, from near the mouth of Goose Creek to the Prince William line.  “These, being intrusive rocks, have in some places displaced the shale and risen above it, while in other places a thin coat of shale remains above the trappean matter, but much altered and changed in character."[7] A large mass of trap rock presents itself boldly above the shale at the eastern abutment of the Broad Run bridge, on the Leesburg and Alexandria turnpike.  Not far to the east the shale is changed to a black or blackish brown color, while at the foot of the next hill still farther eastward the red shale appears unchanged.  The summits of many of these dykes are “covered with a whitish or yellowish compact shale, highly indurated and changed into a rock very difficult to decompose."[8]

[Footnote 7:  Taylor’s Memoir.]

[Footnote 8:  Ibid.]

Lafayette Formation.

A great class of variations due to rock character are those of surface form.  The rocks have been exposed to the action of erosion during many epochs, and have yielded differently according to their natures.  Different stages in the process of erosion can be distinguished and to some extent correlated with the time scale of the rocks in other regions.  One such stage is particularly manifest in the Catoctin Belt and furnishes the datum by which to place other stages.  It is also best adapted for study, because it is connected directly with the usual time scale by its associated deposits.  This stage is the Tertiary baselevel, and the deposit is the Lafayette formation, a deposit of coarse gravel and sand lying horizontally upon the edges of the hard rocks.  Over the Coastal plain and the eastern part of the Piedmont plain it is conspicuously developed, and composes a large proportion of their surfaces.  As the formation is followed westward it is more and more dissected by erosion and finally removed.  Near the area of the Catoctin Belt it occurs in several places, all of them being small in area.  One is three miles northeast of Aldie.  Here, a Newark sandstone hill is capped with gravel.  This gravel is much disturbed by recent erosion and consists rather of scattered fragments than of a bedded deposit.