Alleghanian

Alleghanian is an adjective that can refer to the Allegheny Mountains or the broader Alleghany region of the eastern United States, and most often to the Alleghanian orogeny, a major late Paleozoic mountain-building event that helped shape the Appalachian Mountains.

The name derives from the Allegheny Mountains, which themselves take their name from a Native American toponym associated with the region. In geology, the term is widely used to designate the suite of structural, metamorphic, and stratigraphic features linked to this event.

Geologic significance: The Alleghanian orogeny occurred roughly 325 to 260 million years ago, during the late Paleozoic, as the continents were assembling into the supercontinent Pangaea. It resulted from the collision of what is now Africa with North America, closing the Iapetus Ocean. The collision produced extensive thrust faulting, recumbent folds, and large-scale deformation that uplifted and reshaped much of the Appalachian Mountain belt, particularly the southern and central regions. Metamorphism and tectonic transport formed complex structural stacks and contributed to long-wavelength topography, including the development of the Allegheny Plateaus and the Appalachian highlands.

Geographic distribution and usage: In modern usage, Alleghanian refers to rocks, structures, and fossils from this late Paleozoic phase of the Appalachian region. The term is common in geology and regional stratigraphy in states such as Pennsylvania, West Virginia, Virginia, and neighboring areas, and is often used interchangeably with the spelling Alleghenian in scholarly literature.