Thoroughfare Gap is a water gap that cuts through the Bull
Run Mountains. Thoroughfare Gap is located
right between the boundaries of Prince William County and Fauquier County with
Interstate 66, Virginia State Route 55 and the Manassas Gap Railroad running
through it. This is the site for the
civil war battle of Thoroughfare Gap; more commonly known as Chapman’s
Mill. In 1862, a skirmish broke out
between the Confederacy and the Union in the Gap. The confederates won and moved onto winning
the Second Battle of Bull Run.
But the history of Thoroughfare Gap goes back even
further. Thoroughfare Gap was formed
from a series of mountain building and rifting.
Starting with the building of the Appalachian Mountains in the Taconian
orogeny 460 million years ago(Ma).
Africa was slowly making its way towards the area where the Appalachian
Mountains would be. Then Africa slammed
in the other land mass creating Pangea and the Appalachian Mountains. This happened 300-250 Ma. The evidence from this event is the
Appalachian Mountains and the many fractures and joints in the rocks.
Many of these cracks were then filled with Quartz.
210 Ma, Africa slowly started pulling back and created the
many rift basins such as the Culpepper Basin as well as the Valley and Ridge,
the Piedmont and the Coastal Plains in Virginia.
At the top of Thoroughfare Gap or the Bull Run Mountains,
you are able to see the Blue Ridge Mountains from afar.
According to the measurements that were
taken, all of the outcrops on the Bull Run Mountains are dipping to the East
which also means the outcrops on the Blue Ridge Mountains are dipping to the
west. With this information, that would
mean that Thoroughfare Gap and the Blue Ridge Mountains were once attached
until the rocks were eroded away and pulled apart.
At the bottom of the Bull Run Mountains and Thoroughfare Gap
is the Culpepper Basin. In this basin
are the Waterfall Conglomerates, sediments that were eroded from the
Appalachian Mountains and deposited in the basin.
