26 February 2011
I first mentioned the Waterfall Formation on this blog in May of 2008, but this was my first time visiting it in situ. Previously, including the May 2008 visit, was memorable time with charismatic boulders of float.
The occasion to see this geologic unit in outcrop was the field trip 6 days ago with the George Mason University GeoClub, when we took a hike to Thoroughfare Gap. After looking at the rift-related volcanism of the Neoproterozoic (breakup of Rodinia –> Catoctin Formation), and the subsequent metamorphosed quartz sandstone of the Weverton Formation (including some lovely fracture morphology), we hiked east, into a Triassic rift basin that accompanying another supercontinent breakup (breakup of Pangea –> Culpeper Basin).
We were checking out a border “fanglomerate” called the Waterfall Formation, which was deposited at the edge of the basin. It’s pretty cool to hike from severely metamorphosed and deformed rocks of the Blue Ridge province, and head a quarter mile east, and what do you find but… unmetamorphosed immature sediments!
The group checks out a few outcrops along the railroad tracks:
Here’s a hodgepodge of images from the area, some outcrop and some float, some zoomed out to show texture, some zoomed in to show individual clasts, kind of like I did last week with another conglomerate of another age, in another part of North America.
Some of these clasts are limestones, and some are basalts, including vesicular basalts like the really big clast in the previous image. Here’s a close-up of one of the limestone clasts, weathering away faster than the matrix:
Zooming in on two clasts just to the right of the knife in the previous photo:
Vesicular basalt clast:
Some inverted graded bedding and calcite veins in this one:
(Note how the veins cut directly across the clasts.)