22 August 2013
Our three-day karst theme wraps up today with a visit to Natural Bridge, Virginia, an impressive sight:
I went to Natural Bridge early last week to give a talk to a group of Road Scholars (an Elderhostel-like program) about the Snowball Earth. Part of my compensation for the talk was a night’s lodging at the Natural Bridge hotel, meals, and tickets to the Natural Bridge’s suite of six tourist attractions, primary among which is the Bridge itself. It’s an impressive feature – taller than Niagara Falls (215′ as compared to 167′), spanning Cedar Creek in Rockbridge County (I wonder where they got that name…).
Visitors can walk a paved path beneath the bridge and beyond:
The area is geologically part of the Shenandoah Valley (Massanutten Synclinorium w/ Cambro-Ordovician limestones exposed at the surface), but it is in the James River watershed. Astonishingly, route 11 (the major along-strike route through the Shenandoah Valley prior to the construction of Interstate 81) runs over the top of Natural Bridge, using it as an actual bridge:
This kind of blows my mind – a feature unique in the state utilized for such a municipal function! In my annotations on the Flash Earth screenshot above, I noted that one of the little tributaries of Cedar Creek has a small waterfall that is a site of active deposition of travertine. This is a common feature along riffles and waterfalls in the carbonates of the Shenandoah Valley. Here’s what it looks like:
If you zoom out a bit, you can see that the carbonates here are likely jointed in a suborthogonal joint set, as Cedar Creek displays a rectangular drainage pattern:
According to Spencer (1968), the Bridge itself is Beekmantown Formation (dolostone), while Cedar Creek itself sits in the Chepultepec Formation in the immediate vicinity of the Bridge.
There is a nearby cave, Natural Bridge Caverns, that offers tours. It’s nowhere near as spectacular as Luray, but it’s a nice contrast, and they claim to be the deepest cave (probably commercial, not wild, cave) on the east coast. Here are some of the muddy stalactites therein:
Though I’d driven over Natural Bridge before on Route 11, I had never actually visited it. I was impressed by the scale of the structure, and figure that like Luray or Shenandoah National Park, it’s one of those places every Virginian should endeavor to visit someday.
Spencer, E.W., 1968, Geology of the Natural Bridge, Sugarloaf Mountain, Buchanan and Arnold Valley quadrangles, Virginia: Virginia Division of Mineral Resources, Report of Investigations 13, scale 1:24,000