The Appalachian orogeny is a geological event that formed the Appalachian Mountains.
Approximately 350 million to 300 million years ago, the combined continents of Europe and Africa (Gondwana) collided with North America, to form the super-continent Pangaea. This collision exerted massive pressure on what is today the eastern seaboard of North America. This pressure resulted 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 Appalachian orogeny is also called the Alleghenian orogeny, due to the formation of the Allegheny Mountains at the time.