alleghenian

Alleghenian orogeny

The Alleghenian orogeny or Appalachian orogeny is one of the geological mountain-forming events (orogeny) that formed the Appalachian Mountains and Allegheny Mountains. The term and spelling "Alleghany Orogeny" (sic) originally proposed by H.P. Woodward (1957, 1958) is preferred usage. Approximately 350 million to 300 million years ago, in the Carboniferous period, the combined continents of Europe and Africa (Gondwana) collided with North America to form the supercontinent of Pangaea. This collision exerted massive stress on what is today the Eastern Seaboard of North America, resulting in a large-scale uplift of the entire region. Closer to the boundary between the colliding plates, tectonic stresses contributed to the metamorphosizing of the rock (i.e. the transformation of igneous and sedimentary rock into metamorphic rock). These stresses concurrently caused faults (mostly thrust faults and some strike-slip faults) as well as folding. The immense region involved in the continental collision, the vast temporal length of the orogeny and the thickness of the pile of sediments and igneous rocks known to have been involved are evidence that at the peak of the mountain-building process, the Appalachians could have risen as high or perhaps even higher than the present-day Himalaya.

The Appalachian Orogeny is responsible for the creation of the mountains themselves and is not responsible for the topography that now typifies the Piedmont and coastal plain regions east of the mountain chain. The heavily-eroded hills of Piedmont are remnants of the sizeable mountain chain, while the coastal plain is made up of the material that was washed away in that process. Thus, the coastal plain and Piedmont are largely the byproducts of erosion that took place from 150+ million years ago to the present.

Evidence for the Appalachian orogeny stretches for many hundreds of miles on the surface from Alabama to New Jersey and can be traced further subsurface to the southwest. In the north it enters a region of confused topography associated with earlier orogenies, but clearly the Applachian deformation extends northeast to Newfoundland.

The mountains were once rugged and high, but in our time are now eroded into only a small remnant. Sediments that were carried eastward form part of the continental shelf. Sediments that were carried westward form the Allegheny and Cumberland Plateau, which in some areas are popularly called mountains, but are actually simply uplifted and eroded plateaus. Carbonates and fine sediments from these mountains were carried farther to form limey rocks in a shallow sea that was later uplifted and forms the bulk of Tennessee, Kentucky, Ohio, and Indiana.

A portion of the Alleghenian mountain system departed with Africa when Pangaea broke up and the Atlantic Ocean began to form. Today, this forms the Anti-Atlas mountains of Morocco. The Anti-Atlas have been geologically uplifted in relatively recent times, and are today much more rugged than their Alleghenian relatives.