English: This is an oblique aerial view of the Appalachians in eastern America. The Appalachian Mountains consist of three physiographic provinces. From west to east, they are: 1) the Valley & Ridge; 2) the Blue Ridge; and 3) the Piedmont.

The prominent, narrow ridge at far-left in this photo (not snow-covered) is Little Mountain in Washington County, western Virginia. It represents the westernmost edge of the Valley & Ridge province and is thus part of the Appalachian Front.

The Valley and Ridge consists of folded sedimentary rocks of Paleozoic age that have been eroded into long, sublinear ridges separated by ~flat-floored valleys. To the east of here is the Blue Ridge province, which is mostly composed of Precambrian-aged basement rocks (igneous & metamorphics). The mountains of the Blue Ridge are generally rounded and not very tall.

The Appalachians extend from Quebec to Alabama, go underground in the Mississippi River area, and re-emerge in the Arkansas-Oklahoma-Texas area as the Ouachita Mountains. The overall mountain chain formed as a result of three separate tectonic collision events during the Paleozoic. The earliest was the Taconic Orogeny (Late Ordovician to Early Silurian) - a volcanic island arc collided with what is now the New England area. Next was the Acadian Orogeny (Late Silurian to Devonian) - a microcontinent called Avalonia collided with eastern North America. The third and most significant mountain building event was the Allegheny Orogeny (Pennsylvanian) - Africa collided with eastern North America. This was a Pangaea supercontinent formation event.

The Appalachians mostly lack the sharp-peaked mountains common to western America's Cordillera, the Andes of South America, the Alps of Europe, or the Himalayas of Asia. Compared with those geologically young mountain chains, the Appalachians are relatively old - they have been subjected to long-term erosion for about one-third of a billion years.

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See info. at:

en.wikipedia.org/wiki/Appalachian_Mountains