In the coming semester, I’m teaching a Tectonics and Orogeny course, which will include a field trip to the Appalachians to check out the closest accessible example of tectonics and orogeny look like up close. A successful field trip comes from visiting good outcrops, so yesterday and today I’ve been visiting the area I’m planning to bring my students to in an attempt to find them. Some are quite easy to find, such as the spectacular road cut syncline at Sidling Hill:
As well as giving an inkling of the impressive scale of folding generated by the continental collision between Laurentia (North America) and Gondwana (Africa) that formed the Appalachians, the range of rock types on display here provide some information on what was going on before the orogeny hit – and provides evidence that whilst mountain building might mess things up, it doesn’t always destroy the evidence of what went before. In the Appalachians, this includes a much older collisional event one billion years ago, rifting to create an ancient ocean (the Iapetus) and a lot of subduction and microcontinent collisions as it closed again. You can’t see all of that in this particular outcrop, of course – but there are places that you can.
The other thing you can see when considering this outcrop is evidence that the Appalachians are an old mountain belt. The fact that we have a syncline at the top of a ridge, rather than an arch-like anticline, tells us that the major controls on the landscape we see today are not the forces that created this fold , but the slower but relentless forces of erosion that are slowly returning the mountains back to sea level. And where erosion dominates, it is the hard rocks – such as the sandstone in the core of this syncline – that survive the longest.
Here’s hoping that I can find the outcrops that tell these stories the best.