Thoroughfare Gap

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. 

http://www.nvcc.edu/home/cbentley/geoblog/2008/05/western-conglomerates-culpeper-basin.html

While on this trip, the big question we had to answer was where is the stress coming from.  Many of us  took out our compasses and measured the strike and dip of the beddings, joints, and the quartz veins.  Here is a stereonet of the bedding.

As we can see, the bedding dips towards the east which means that the stress is coming from the east.