Thoroughfare Gap

Thoroughfare Gap is located in the Bull Run Mountains. This is the beginning, or eastern edge, of the Blue Ridge Mountains and Culpeper Basin is located to the east. The strata in the area are dipping to the east (Fig. 1).


Figure 1. Bedding dipping to the east.


Phyllite with a kinked foliation plane (like stair steps)


Blue quartz conglomerate from the basement complex.


As the supercontinent, Rodinia, was rifting apart, flood basalts flowed into the rift zones. That flood basalt is known as the Catoctin Formation. The sea worked its way into the opening and became a beach environment with conglomeratic sections, hence the quartz arenite from the Weaverton Formation. Then mud was deposited, known as the Harpers Formation.

The Taconian (460 Ma), Acadian (360 Ma), and Alleghanian (300 Ma) orogenies shaped the anticline (fig. ) between Thoroughfare Gap and the Blue Ridge Mountains, where the center has been eroded. The eastward dipping strata on Thoroughfare Gap and the westward dipping beds of the eastern side of the Blue Ridge is evidence that it is an anticline. It is hard to see at first glance because the center has been eroded, but once the measurements and facts are laid out we can come to this conclusion.


Figure . GMU students standing on the eastward dipping beds looking towards the western dipping Blue Ridge, with the eroded valley in between.


Chilhowee Group:

• Weaverton - Beach environment
• Harpers: Cambrian - Lagoonal environment
• Antietam (not exposed here, has been eroded away): Cambrian - Quartz arenite sandstone + Skolithos