March 23. Appalachian basin

When blocks of the earth’s crust collide, several different things can happen. When good dense oceanic crust impinges on relatively light continental crust, the denser one, oceanic crust, usually goes down under the lighter one. This process is called subduction, and it’s going on all over the world today. The continental crust above isn’t immune to effects – it can get uplifted, depressed, scrunched and broken, and volcanoes can and do pop up through the continental crust. Probably the best example of this today is the Andes Mountains along the west coast of South America, where the oceanic plate underlying the Pacific Ocean is diving down beneath the South American continental plate.


If two relatively low density blocks – two continents, or a continent and something like an island arc – collide, then neither is likely to really descend beneath the other. This is a true head-on collision, and it can make some of the highest mountain ranges. This is what’s happening to day where the Indian continental plate collides with the Eurasian Plate. The Himalayas form.

An obvious consequence of uplifted mountains is erosion – it starts as soon as rocks are above sea level. The Queenston Delta that we talked about the other day is the evidence of that kind of erosion from an uplifting mountain range. Sometimes there is so much erosion that the weight of the sediment is enough to bow down the crust itself, starting a trough-like depression along the mountain front. It can become a self-perpetuating thing, a depression, a basin, into which more and more sediment pours, and all that sediment keeps pushing the crust further and further down…. And so on. The weight of the stuff that’s colliding and being pushed up over the edge of the continent helps, too – all adding up to a physically low area to receive sediments. It’s called a foreland basin, because it’s in the foreland, adjacent to a rising mountain uplift.

That’s what happened in eastern North America, where the Appalachian Mountains are today. We’ll be talking about the Appalachian Mountains for months – millions of years – as various things happen over time to contribute to their formation. But that’s getting started now, toward the end of the Ordovician, as the Taconic Orogeny gets started.

During the Cambrian and early Ordovician, most of eastern North America was pretty stable, accumulating relatively thin uniform packages of sediment over pretty large areas. In the late Ordovician, as collisions began to bow down the crust, and lift up mountains to be eroded, changes began. Packages of sediment thicken noticeably to the east or southeast (see cross section above - the colored part is the Upper Ordovician), closer to the source area. More sediment is dumped close to the mountains than is carried hundreds of miles away from the mountain front, even though those far-flung sediments are part of the process as well. The mountains may have been in New England, extending down to Tennessee and Georgia, but the eroded mud made its way as far northwest as Ohio and beyond.

The foreland basin that began during late Ordovician time is called the Appalachian Basin, and like I said, it will be months before we’ve heard the end of it.

—Richard I. Gibson

See also this excellent SmartFigure by Callan Bentley. One of the best visualizations of the tectonic development of the Appalachians that I've seen.

Cross section from Harris & Milici, 1977, USGS Prof. Paper 1018.
Map from USGS