THE NARROWS WATER GAP
The Narrows in western Virginia is a classic example of a water gap (see photo). A water gap is a "deep, narrow, low-level pass penetrating to the base of and across a mountain ridge, and through which a stream flows, especially a narrow gorge or ravine cut through resistant rocks by an antecedent stream" (Glossary of Geology). The Appalachian Mountains of eastern North America have quite a few long, tall, linear ridges. Travel-wise, rivers are traditionally the best way to get across the Appalachian's ridges. At the Narrows, the New River has punched through one of these ridges (called Peters Mt. on the eastern side & called East River Mt. on the western side). This particular ridge's bedrock has an interval of very hard quartzite. The rock is so hard that even this old, large river has rapids developed here.
The Narrows Water Gap (see map) - rapids in the north-flowing New River, running over hard, resistant beds of the Silurian-aged Clinch Quartzite. Looking ~SW.
How did the Narrows Water Gap form? The two traditional explanations are:
1) the New River is an antecedent stream
2) the New River is a superimposed stream
Antecedent streams refer to rivers that existed before the mountains formed. Well, the Appalachian Mountains formed principally in the Pennsylvanian (Late Paleozoic), during the assembly of the supercontinent Pangaea. The New River existed before the Pennsylvanian, and the erosive power of the river was strong enough to maintain a channel as the mountains were uplifted.
This means that this very water gap is >300 million years old! Is this believable?
Some orogenic deformation has defeated some river channels in the Appalachians - there are examples of abandoned channels in this part of the world.
If the New River is a superimposed stream, then the following must have happened:
The rocks of the Appalachians were deformed (folded & faulted & uplifted), and that landscape got buried in flat-lying sediments. Regional drainage atop these flat-lying sediments resulted in dendritic stream patterns. The flat-lying sediments were eventually eroded away, re-exposing the underlying deformed rocks, and the stream pattern got superimposed on that landscape.
Which is correct? Well, there isn't any preserved evidence for any cover of flat-lying sedimentary rocks. The antecedent stream idea seems to apply here.
The rapids in the New River at the Narrows Water Gap run over the Silurian Clinch Quartzite, a very hard quartzose sandstone. "Quartzite" is traditionally supposed to refer to crystalline-textured, quartzose metamorphic rocks, the result of metamorphism of sandstones. Many non-metamorphosed sandstones have the outcrop characteristics of true quartzites, and so are also called quartzites. This is a bit of a problem. The same term shouldn't refer to some sedimentary rocks and some metamorphic rocks. Many geologists now use the term metaquartzite to refer to metamorphosed sandstones, to avoid confusion with the sedimentary use of "quartzite".
The Clinch Quartzite (aka Tuscarora Quartzite, Clinch/Tuscarora Sandstone; Clinch/Tuscarora Formation) generally consists of hard, well-cemented, quartzose sandstones and some quartz-pebble conglomerates. Commonly seen sedimentary features include cross-bedding and burrows. The quartz grains in the Clinch appear to have been recycled, probably several times during the Precambrian and Early Paleozoic. These sediments were deposited near the end of the Taconic Orogeny.
Stratigraphy & Age: Clinch Quartzite, Llandoverian Series, lower Lower Silurian.
Clinch Quartzite - Rt. 460 roadcut at the Narrows Water Gap. Looking E.