Virginia’s Blue Ridge Mountains were never bluer than the ridges of the Wyoming Valley yesterday when we went to see Black Widow (figure 1). The ridges in question mark the Susquehanna’s meandering passage through them into the valley as it bends twice while flowing south toward the Chesapeake (figure 2).
You probably know this, but geographically a water gap is where, contrary to expectation, a river flows transversely through a mountain ridge, cutting it, as here. Since water flows downhill, it’s hard to understand how it seemingly defied a necessity of flowing along the mountain, or around it, or even away from it.
Sometimes, they say, two rivers, one on each side of a ridge, became one as the larger ‘captured’ the lesser. The theory is that the rivers went their way merrily until erosion, biting into the ridge over time, opened up a gap, and at that point the one river gobbled the other. This does in fact happen and you’ll find this explanation in the official literature about the Delaware water gap, where the Delaware River passes through the Kittatinny Ridge. The idea is that faulting cracked the quartzite—a notoriously hard stone—of the ridge, making it susceptible to erosion by streams running into the two rivers.
This scenario depends upon a constellation of just the right conditions. Yet we find water gaps all throughout the ridge and valley province of the Appalachians. For that matter, pursue the Susquehanna but twenty miles south from the gap in figure 1 and you will find it cutting through three ridges as the river exits the valley at Shickshinny (figure 3). There is a more parsimonious solution to the problem of these and other gaps in the Appalachian ridge and valley province.
The Susquehanna, it transpires, is older than the Appalachians by hundreds of millions of years. It and other rivers and streams had worn down a Himalayas-sized mountain chain where the Piedmont sits now into practically a flat plain. Through this flat region meandered the Susquehanna, Delaware, Potomac, and Hudson rivers: meandering as rivers do when they are running through such a flat zone, like the Mississippi does now (figure 4).
It’s not like these rivers are nailed to the ground: speed up geologic time and they writhe like a snake around their average course as you can infer from figure 4. So when I say the Susquehanna et al. are hundreds of millions of years old, I do not argue that they have maintained identical courses the whole time.
About the time the dinosaurs met their fatal meteor the region that would become the present-day Appalachians was geologically uplifted like a table. Turns out that when you erode a trillion tons of rock away the continental crust bobs up on the mantle like a boat that’s unloaded its cargo.
There is a direct relationship between how high above sea level land is and how rapidly it erodes: the higher, the faster. Water immediately began removing this lifted rock, and as this sediment flowed away the harder layers emerged a little like the buried bones of the landscape and they became the Appalachians’ ridges (and the softer bits the valleys).
Rivers as powerful as the Susquehanna, however, cut down through the harder ridge rock as fast as it could rise. Put another way, the present Appalachians rose around the bed of the Susquehanna. The river’s just doing its two-hundred-million-year-old thing quite in defiance of the parvenu ridges, and so we have our gaps.
And when a river began cutting down but was overwhelmed by the rising land (that happens, too), we find a resultant notch in the mountain ridge: a ‘wind gap’.
Bibliographical moment: Charles B. Reif discusses the Susquehanna’s gaps in the Wyoming Valley and lines up the evidence in his insightful article, “The Water Gaps of the Susquehanna River and Its Tributaries in the Wyoming-Lackawanna Valley of Northeastern Pennsylvania,” Journal of the Pennsylvania Academy of Science , September, 1993, Vol. 67, No. 1 (September, 1993), pp. 42-45.