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