Water Gaps- worth a voyage across the Atlantic

Water gaps are intriguing and iconic landforms that have long drawn humans to them.  We are all familiar with streams and rivers flowing in valleys; a water gap is dramatically different- it’s a place where a river cuts though a ridge or mountain range.  Thomas Jefferson discusses the Potomac River water gap in his Notes on the State of Virginia (1785), declaring in an often-quoted passage: “This scene is worth a voyage across the Atlantic.

Harpers Ferry

View of the Potomac Watergap from Jefferson Rock by Ferdinand Richardt (1857).
View is to the east, with Harpers Ferry in the foreground. (from- http://www.histarch.uiuc.edu/harper/1859render.html)

For me, the prose that comes earlier in the same paragraph is even more vivid.

The passage of the Patowmac through the Blue ridge is perhaps one of the most stupendous scenes in nature. You stand on a very high point of land. On your right comes up the Shenandoah, having ranged along the foot of the mountain an hundred miles to seek a vent. On your left approaches the Patowmac, in quest of a passage also. In the moment of their junction they rush together against the mountain, rend it asunder, and pass off to the sea. 


Topographic map of the Potomac River water gap. Note the river cutting through two ridges.

The Appalachian Mountains are flush with water gaps (e.g. Delaware water gap, Cumberland gap), but water gaps are common features in many mountain ranges worldwide.  Water gaps are important as they typically form a route of conveyance through steep and mountainous country and they’ve long been utilized as routes for wagon trails, railroads, and highways.  The Potomac River water gap through the Blue Ridge Mountains has been a pivotal place in American history since Colonial times.

Southwest from Harpers Ferry the Blue Ridge Mountains form an unbroken topographic barrier for 240 km (150 miles).  The next water gap is near Lexington, Virginia, where the James River has carved a 10-km (6 mile) gorge through the Blue Ridge, a range with peaks over 1,200 meters (4,000 feet) in elevation.  This is an impressive gap- see for yourself by playing this Google Earth tour through the James River water gap (kmz).

How can a stream cut a path across a mountain ridge or range that lies in its course?

At the Potomac River water gap Jefferson opined:

…this scene hurries our senses into the opinion, that this earth has been created in time, that the mountains were formed first, that the rivers began to flow afterwards, that in this place particularly they have been dammed up by the Blue ridge of mountains, and have formed an ocean which filled the whole valley; that continuing to rise they have at length broken over at this spot, and have torn the mountain down from its summit to its base.

TJ is certainly entitled to his opinion, but he’s not the only one to wax poetic on this topic.

For another take on the landscape consider John Denver’s famous lyrics in the song Country Roads:

Almost heaven, West Virginia
Blue Ridge Mountains, Shenandoah River
Life is old there, older than the trees
Younger than the mountains, blowing like a breeze

So according to John Denver the trees are younger than the life, and both the trees and the life are younger than the mountains (i.e. old mountains).

But what about the Shenandoah River? Is the Shenandoah River older or younger than the Blue Ridge Mountains? In Jefferson’s landscape model, the mountains formed first, creating a topographic barrier that was later breached by the river carving out a water gap. But there is another possibility: what if the rivers were there first and the mountains formed later? In this model, the rivers are older and maintain their courses while the mountains are uplifted. These rivers would be antecedent streams that pre-date the current topography through which they flow.

What do you think? Are the Blue Ridge Mountains older than the rivers (Potomac/Shenandoah and James systems) that flow through these impressive water gaps? Or do these rivers pre-date the formation of the Blue Ridge and Appalachian Mountains?

Let me know and we’ll return to this question in a follow up post.

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