12 March 2013
Yesterday, I took five new GigaPans at Thoroughfare Gap, a water gap where Broad Run cuts through Bull Run Mountain, the eastern limb of the Blue Ridge Anticlinorium at my latitude. The rocks here are the Cambrian-aged Chilhowee Group, with bedding tilted moderately to the east during Alleghanian mountain-building in the late Paleozoic. To the west is the crystalline core of this massive regional fold, and to the east is a normal-fault-bounded rift valley, the Culpeper Basin, that formed during the breakup of Pangea in the Triassic.
The first two images show Chapman’s Mill, a historic structure built at this geographical choke-point:
The mill is constructed of quartzite from the Weverton Formation. If you explore it, you can find examples of bedding (primary structure) and plumose structure (the fine-scale anatomy of a joint, a tectonic structure). The ridge of Weverton quartzite that projects south into Bull Run (from which the above image was shot) duplicates in miniature the larger-scale structure and differential weathering of the mountain itself.
Stratigraphically above the Weverton is the Harpers, which is quite metamorphosed here (schistose in texture). It crops out on a railroad cut immediately north of the mill:
Lastly, consider these two looks at the Waterfall Formation, a “fanglomerate” containing Triassic basalts, that built out into the Culpeper Basin when it looked more like the East African Rift.