This morning I took two of my kitties to the vet. The thirteen mile trip takes me from my home in Letcher County Kentucky to the town of Pound in Wise County, Virginia. It was a very pretty spring morning -- lots of mist and lots of fresh greening forests. However, best part of trips to the vet (or any trip over into Wise county) is traveling through the geological marvel of Pound Gap.
Depending upon which source one consults some time between 275 million and 230 million years ago, what is now the north American continent collided with what is now the northwestern edge of Africa. The enormous forces unleashed by this collision caused the earth's surface to buckle, push up, and even break in places. When breaks occurred, pieces of the earth's crust would push up over other pieces forming what are called "thrust" or "over-thrust" faults.
Imagine a thick, flaky, homemade, pie crust lying on a smooth, but not slick surface. Imagine pushing in from one side on that pie crust, causing it to develop ridges and ripples. If you keep pushing some will push up over other, and (if your pie crust is flakey enough) some of the ridges will split. That is what happened to the earth's crust along what is now the Appalachian Mountain chain. The whole of the Appalachian mountain range was created by a series of parallel folds and thrust faults.
The western-most thrust fault created by the pressure of Africa against north America forms part of the border of what is now Kentucky and Virginia. The southeastern piece of crust (the Virginia piece) moved west and slid up and over the northwest rock (the Kentucky piece). The cracking and pushing upward of this western most fault created a great, long mountain ridge line called Pine Mountain that runs from Tennessee through Kentucky all the way to the Breaks Interstate Park on the border between Kentucky and Virginia. Pound Gap is a natural break in that ridge, that provided access (first by trail and wagon track, then by highway) from Kentucky into Virginia (and vice versa).
About 12 years ago, the Virginia and Kentucky highway departments cooperated to expand to four lanes U.S. route 23 connecting Virginia and Kentucky. The road cut created for this construction has been a great boon to students of geology, because it laid bare the thrust fault that created Pine Mountain for all to see. The photo to the right (taken by me few years ago) shows the fault itself -- the left handside of the photo is the Virginia (southeast) side of the fault, the right is the Kentucky (northwest) side. You can see how enormous the geological forces involved must have been, to have compressed and bent under the strata on the Kentucky (northwest side) pressing them under the crust rising up from the Virginia (southeast) side.
Not only did the earth's crust on the Virginia side tilt upward as it slid up and over the Kentucky side crust, but the downward pressure exerted on the Kentucky portion of the crust caused it to tilt also. The effect of the tilting is very visible since the highway construction was completed. The photo to the left shows the Kentucky portion of the crust (to the right or northeast of the fault shown above) that was tilted by downward pressure at the fault. The photo is from the Kentucky Geological Society.