Generalized east-to-west cross section
through the central Hudson Valley region.
The geology of the Appalachians
dates back to more
than 480 million years ago. A look at rocks exposed in today's Appalachian
Mountains reveals elongate belts of folded and thrust faulted
marine sedimentary rocks, volcanic rocks and slivers of ancient ocean floor - strong evidence that these
rocks were deformed during plate
collision. The birth of the Appalachian ranges marks the
first of several mountain building
plate collisions that culminated in the construction of the
supercontinent Pangaea with the Appalachians and neighboring
Anti-Atlas (now in Morocco) near the center.
mountain ranges were once higher than today's Himalaya mountain range
, which was
also formed by continental collision.
During the earliest Paleozoic
continent that would later become North
straddled the equator
Appalachian region was a passive
, not unlike today's Atlantic Coastal Plain
During this interval, the region was periodically submerged beneath
shallow seas. Thick layers of sediment and carbonate rock
were deposited on the shallow
sea bottom when the region was submerged. When seas receded,
terrestrial sedimentary deposits and erosion dominated.
During the middle Ordovician
(about 480-440 million years ago), a change in plate motions set
the stage for the first Paleozoic mountain building event (Taconic orogeny
) in North America. The once
quiet Appalachian passive margin changed to a very active plate boundary
when a neighboring oceanic plate
, the Iapetus
, collided with and began sinking
beneath the North American craton
. With the
birth of this new subduction zone
the early Appalachians were born.
Along the continental margin
grew, coincident with the
initiation of subduction. Thrust faulting uplifted and warped older
sedimentary rock laid down on the passive margin. As mountains
began to wear them down.
Streams carried rock debris downslope to be deposited in nearby
This was just the first of a series of mountain building plate
collisions that contributed to the formation of the Appalachians.
Mountain building continued periodically throughout the next 250
million years (Caledonian
, and Allegheny
orogenies). Continent after
continent was thrust and sutured onto the North American craton as
the Pangean supercontinent began to take shape. Microplates
, smaller bits of crust, too small to be
called continents, were swept in, one by one, to be welded to the
By about 300 million years ago (Pennsylvanian
was approaching North American craton.
collisional belt spread into the Ozark-Ouachita region and through the Marathon Mountains area of Texas.
vs. continent collision raised the Appalachian-Ouachita chain to
lofty, Himalayan-scale ranges.
The massive bulk of Pangea was
completed near the end of the Paleozoic Era (Permian
Period ) when Africa (Gondwana
) plowed into the continental
agglomeration, with the Appalachian-Ouachita
near the core.
Mesozoic Era and later
Pangea began to break up about 220 million years ago, in the Early
Era (Late Triassic
Period). As Pangea rifted apart a new
passive tectonic margin was born and the forces that created the
Appalachian, Ouachita, and Marathon Mountains were stilled.
Weathering and erosion prevailed, and the mountains began to wear
By the end of the Mesozoic
Appalachian Mountains had been eroded to an almost flat plain. It
was not until the region was uplifted during the Cenozoic
Era that the distinctive topography of the
present formed. Uplift rejuvenated
the streams, which rapidly
responded by cutting downward into the ancient bedrock. Some
streams flowed along weak layers that define the folds and faults
created many millions of years earlier. Other streams downcut
so rapidly that they cut right across
the resistant folded rocks of the mountain core, carving canyons
across rock layers and geologic structures.