Brief note on the geology of the Appalachians

The Appalachian chain extends for about 1,500 miles (2,400 km) from Newfoundland to Georgia. It has a complex history (as you might guess) that various mountain-building episodes that vary along the range. For our purposes, it is useful to differentiate: • The Northern Appalachians (New England and Canada) which largely resulted from earlier (Ordovician-Silurian) deformational episodes; and • The Central and Southern Appalachians (southern New York to Georgia) which were strongly influenced by later deformation (Silurian to Carboniferous). These include what is termed the “Sedimentary Appalachians” – including the Valley and Ridge province (a classic fold-and-fault belt) and the Appalachian Plateau (a high plateau capped by relatively flat-lying late Paleozoic strata).

Figure source: USGS (2003)

For the purposes of this course, the Central Appalachians is a critical because this is where the focus of the Rogers brothers worked on fold belts in the 1830s-40s. The basic arrangement are a series of zones arranged east to west: • Coastal Plain: Cenozoic cover over metamorphosed units • Piedmont: Crystalline rocks- mostly metamorphic rock and some igneous • Blue Ridge Mountains: Folded Late Proterozoic volcanics and metasediments • Valley and Ridge: fold-and-fault belt of Paleozoic sediments • Appalachian Plateau: elevated and partly dessicated plateau underlain by nearly flat- flying Late Paleozoic beds including Carboniferous coal beds. • Cratonic interior: basins, arches and domes

The structural sections illustrate two interpretations of the structure of the range (Dott and Prothero 1994). The upper panel is taken from King (1950) and shows the classic interpretation. The lower panel illustrates a more modern interpretation which includes a large “nappe” structure analogous to what was inferred for the Alps (modified from F A. Cook 1979). The width of the cross section is about 325 miles.

Finally, as you probably recall from historical geology, the Appalachians were a passive margin in the early Paleozoic so the thickness of sediments varies immensely from east to west. Here is a general cross section from Colton (1970).