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 New England Province, starting around New York City and
stretching up into the Gaspe Peninsula. The rolling hills of eastern
Massachusetts and Connecticut are backed up by the Green Mountains
of Vermont, the White Mountains of New Hampshire, stretching to the
Notre Dame Mountains in Quebec. New England is a patchwork of terranes
from different ages; it was assembled in much the same way as Alaska, pieces
being carried up a San Andreas-like transform fault
to lodge against the White Mountains.
The Piedmont stretching from New Jersey down through Georgia,
more rolling hills carved into highly deformed metamorphic rocks, some
so altered that there is no way to determine how old they are. The eastern
edge of the Piedmont is strongly delineated from the Atlantic Coastal
Plain at the Fall Line, along which many large Eastern cities grew.
The Piedmont peters out somewhere around Talladega, Alabama.
The Blue Ridge which starts in east-central Pennsylvania as
a single narrow ridge called Blue Mountain, and turns southwest through
Maryland as the Catoctin Mountains and South Mountain, through Virginia,
where it widens into the Great Smoky Mountains forming the border between
North Carolina and Tennessee. Clingmans Dome in the Great Smokies
is the highest point in the Appalachian system.
The Ridge and Valley Province which consists of highly eroded
fold mountains separated by flat-bottomed alluvial valleys. The most
prominent feature of the Ridge and Valley province is the Great Valley,
which starts south of Montreal and winds its way behind the Blue Ridge,
finally coming out at Montgomery, Alabama.
Finally, the Appalachian Plateau Region, more gently and evenly
folded than the Ridge and Valley Province, if higher. The eastern
limit of this region is the Allegheny Front, made up of the Allegheny
Mountains in Pennsylvania and Virginia, and the Cumberland Mountains
in Kentucky and Tennessee. The Allegheny Plateau lies in southwestern
New York, northern and western Pennsylvania, and northeastern Ohio; the
Cumberland Plateau stretches through southeastern Ohio, West Virginia,
most of Kentucky, central Tennessee, and northern Alabama. Human
activity in these plateau areas forms the kernel of the popular stereotypes
The Adirondack area of northern New York
is geologically an extension of the Canadian Shield but is arguably part
of the Appalachian system.
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 Rodinia.
Himalaya-sized mountains were formed in this first known orogeny, the
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.