Major North American mountain range stretching from Quebec to Alabama. They posed a major barrier to westward expansion in the early years of white settlement.

Like the Rocky Mountains, the Appalachians were created by the collision of large tectonic plates in a series of orogenies or mountain building episodes. Unlike the Rockies, the Appalachians are old and eroded. The east coast of the United States and Canada has been a passive margin for the last 200 million years. During this time no tectonic activity has been taking place on the margin of the Appalachians and the only geologic events to occur were erosion and sedimentation as rivers carried sediment to be deposited on the continental margin.

The Appalachians feature rounded, tree-topped peaks and isolated green humid valleys often called 'hollows,' or 'hollers' in the local dialect. Major mountain groups within the Appalachian chain are the Smokey Mountains in Tennessee, the White Mountains in New Hampshire, the Green Mountains in Vermont, and the Adirondacs in New York.

A large region of the Appalachians, stretching from the Carolinas through Eastern Pennsylvania, is known for its unique geographic and cultural characteristics, and is often referred to as Appalachia, where a full discussion is available.

The Adirondacks, geologically speaking, are not part of the Appalachian mountain chain though they are often grouped with it because of their location. While the Catskills are part of the Appalachians, the Mohawk and Hudson river valleys separate them from the Adirondacks.

The other key difference between the Adirondacks and the Appalachians is that the Adirondacks were formed by an uplift, and the mountains in that range are still growing.

An extremely complex system of rocks, fault zones, terranes, and folded mountains stretching through eastern North America from Quebec to Alabama.  Although the rocks that make up the mountains are extremely old, the mountains themselves are of relatively recent origin, formed 40 million years or so ago.  We'll get to that later.

The Appalachian system can be divided into five (or six) basic subregions:

The formation of the Appalachians is quite complicated:

  • The story begins about 1.2 billion years ago, when something collided with the Canadian Shield craton to form the supercontinent RodiniaHimalaya-sized mountains were formed in this first known orogeny, the Grenville Orogeny.
  • The Grenville Mountains were eventually eroded away as Rodinia broke up during the Cambrian period, but the collision also warped the southeast corner of the Canadian shield upward, so that remnants of the orogeny include the Adirondacks, and the Blue Ridge. As the Iapetus Ocean opened up, sediments formed a coastal plain that would later be warped into the folded rocks of the Ridge and Valley province.
  • During the Ordovician Period, a Japan-like volcanic island arc collided with eastern Laurentia, starting a second round of mountain building, the Taconic Orogeny. The original Precambrian batholiths were welded to North America, forming the Piedmont gneiss domes that surround Baltimore. The mountains created by this orogeny were also completely eroded away, but underneath them, the collision warped earlier sediments into the Taconic Mountains of New York, the Green Mountains of Vermont.
  • Towards the end of the Silurian period, Baltica (Scandinavia and the North European Plain) collided with Laurentia from a different direction, forming the supercontinent Laurussia. The Caledonian Orogeny built the mountains of Norway, eastern Greenland, Labrador, northwestern Newfoundland, northern Ireland, and the Highlands of Scotland. Further south, sediments continued to be laid over the shallow sea that covered the peneplain where the Taconic mountains used to be.
  • A little later in the Devonian, another island arc, Avalonia, collided with Laurussia. The Iapetus Ocean closed up for good, starting another period of mountain building, the Acadian Orogeny. Avalonia lies mostly under the Atlantic Coastal Plain and continental shelf, but it pokes up in southeastern New England, Down East Maine, Nova Scotia, southeastern Newfoundland, and southern Ireland and Great Britain. Under the mountains, the rocks of the Piedmont were squeezed and metamorphosed. On the west side of the moutains, a foreland basin formed in the shallow Kaskaskia Sea, laying down sediments that would form the Catskill Mountains, the Pocono Mountains and the base of the plateaus
  • For about 100 million years these mountains eroded away. During the Mississippian Period, limestone formed in the Kaskaskia Sea, and the Avalonian foreland basin filled with the sand that would form the Pocono Mountains. This sea became shallower and shallower, so that by the Pennsylvanian period, coastal swamps on the western side of the mountains laid down the coal beds of the plateau provinces.
  • At the end of the Pennsylvanian period, Gondwana collided with Laurussia and overrode its eastern edge, forming Pangaea. The Alleghenian Orogeny saw African rocks forming Himalaya-sized mountains.  Underneath of them, the older rocks east of the Allegheny front were faulted, deformed and metamorphosed.
  • The Ridge and Valley Province was made extremely complicated by overthrust faults and overturned folds.  A block of Grenville-era rocks were faulted upwards into the Blue Ridge, the Great Smokies, and part of the western Piedmont. Taconic remnants squeezed and metamorphosed into the Eastern Piedmont. West of the Allegheny Front, rocks were crumpled into the regular folds found in the western plateaus.
  • Throughout the Permian period, Appalachian Mountains formed of African rocks formed the backbone of Pangaea.  Then, during the Triassic, Pangaea began to break apart along a rift valley down the middle of these mountains, a rift that would eventually form the Atlantic Ocean. As blocks of the rift valley slid down along the Piedmont, the little triangular half-graben depressions they left were filled with sediments from the still-eroding mountains. Intrusive magma formed dykes in the western Piedmont, and lava flows formed the rocks of the Catoctin Montains and the White Mountains of New Hampshire.
  • By the end of the Cretaceous, even these mountains had been worn smooth, although many volcanoes had replaced them.  The African Appalachian mountains were completely gone; the only evidence of them are the Triassic Lowlands, the sedimentary rocks the dominate the Midwest and a piece that broke off to lie under the coastal plain sediments of southeastern Georgia and Florida.
  • During the mid-Cenozoic Era, a far away piece of Gondwana slid itself into position under the South Pole to form Antarctica.  An ice sheet grew over this continent, locking up much of the Earth's water, resulting in a large drop in sea level. At the same time, sediments deposited on the Atlantic Coastal Plain caused the older Appalachian Plain to warp upwards, and begin eroding again; its detritus add to the Coastal Plain, the remaining rocks formed the Appalachian Mountains we know today.
  • The final force shaping the Appalachians were the ice ages that have been marching back and forth across northern North America ever since.  New England, the Adirondacks, and the Allegheny Plateau were scraped and gouged even further, then buried under glacial till. Climate changes altered the erosion patterns of the mountains further south.

A short geologic history of the northeast United States

Overview - The Geology of Eastern New York

Sixteen Page Summary History of the Geological Evolution of Virginia

USGS - A Tapestry of Time and Terrain

Global Earth History Page (extremely cool maps)

and dozens of others.  If anyone can point me towards good online descriptions of Quebec geology (in English), I'd appreciate it.

Note: The strange voting pattern comes from a bug that ate this writeup a few years back; as part of the cleanup, the writeup was restored and the rep manually reset, but the entries to the vote table were lost forever.

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