Friday, May 27, 2011

The Allegheny Mountains - Geology

When you look at a topographical map of the central-eastern United States, in western Virginia and eastern West Virginia, there is a large range of mountains that are part of the Appalachian Mountain system. The entire Appalachian range was formed in three distinct geological events over hundreds of millions of years. Around 1.2 billion years ago, the Eurasian and North American plates came together to form the Grenville Mountains as the crusts of the two plates buckled. Heat generated by the collision melted crustal rocks into magma that now forms the backbone of the Appalachian Mountains. Over the next several hundred million years, these mountains eroded quickly, as there was no vegetation to slow the process. Approximately 750 million years ago (Ma), the supercontinent formed by this collision broke apart, leaving the Iapetus Ocean (vide infra) in its wake. About 450 Ma, in the Ordovician period of the mid-Paleozoic era, the Eurasian and North American plates collided again, and this time the Eurasian plate was subducted. The sediments formed by the erosion of the Grenville Mountains were lifted up, forming the northern Appalachian Mountains. This event was known as the Acadian orogeny (from the Greek: oros = mountain; genesis = formation). About 250 Ma, during the late-Permian period at the end of the Paleozoic era, the African plate collided with the combined Eurasian/North American plates in an event known as the Allegheny orogeny, which formed the southern Appalachian Mountains, of which the Allegheny Mountains are part. This was a temporary collision, and as the plates drifted to their present positions, the Atlantic Ocean was formed. [Iapetus was the father of Atlantis in Greek mythology.]

It is easy to find fossils of early molluscs from the Cambrian period of the Paleozoic era (525 Ma) in limestone deposits on our property. The newer rocks that would have held fossils of higher animals have been eroded. We have found a nice specimen of a fossilized Lepidodendron tree, common during the Carboniferous period (305 Ma). Lepidodendron provided more than 90% of the carbon of coal deposits, and was part of what is known as the coal forest flora. They were not truly trees, but are classified as club mosses.

Perhaps the best visual evidence of these collisions can be seen in the road cut-through for the newest portion of US-33 north of Elkins, WV. The layers of rock from the North American plate have been pushed up from their original horizontal orientation by as much as 45-60°. Most of the Allegheny Mountains have eroded from heights greater than the Rocky Mountains to their present 4000-6000' heights, and virtually all seismic activity from mountain formation and plate tectonics has subsided. The largest earthquake recorded in West Virginia was in 1969, and it measured a measly 4.3 on the Richter Scale. It cracked some plaster in a few houses.


  1. Didn't know you were interested in geology. The seismic non-activity of WV was not reassuring to me when I was a kid, when for some reason I was afraid a volcano might pop up nearby. Now it seems remarkably safe, as long as you're not in a holler where you could get flooded.

    I have a chunk of slate with a couple of fern(?) fossils. Too bad there aren't any really interesting fossils in WV!

  2. Thank you for an interesting essay on the Appalachian mountains.