Alleghenian orogeny

From Freepedia

Image:Appalachian orogeny.jpg The Alleghenian orogeny or Appalachian orogeny is the geological mountain-forming event (orogeny) that formed the Appalachian Mountains and Allegheny Mountains.

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 pressure on what is today the Eastern Seaboard of North America, resulting in an enormous uplift of the entire region, called the Appalachian orogeny. Pressure and heat over millions of years "cooked" the rock, folding, twisting and faulting it. Farther west the collision was gentler and resulted in less faulting and more wrinkling, creating ridges and valleys which became the Appalachian mountains.

Closer to the point of impact, the immense pressure turned igneous and sedimentary rock into metamorphic rock and broke it in numerous places, creating faults which were very susceptible to being worn away by wind and water. Over time these erosive forces wore the landscape down to nearly sea level. This worn-down landscape is now the Piedmont.

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 north to Labrador and 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. 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.

See also: Geology of the Appalachians

External links

• Geology of Manassas National Battlefield Park (public domain source)