Alleghanian orogeny

The Alleghanian orogeny or Appalachian orogeny is one of the geological events that helped form the Appalachian Mountains and Allegheny Mountains. It happened a very long time ago, between about 325 million and 260 million years ago, during the Carboniferous to Permian period.

This mountain-building event was caused by the continent of Africa moving and crashing into North America. Back then, these landmasses looked very different than they do today. North America was part of a big landmass called Euramerica, and Africa was part of another called Gondwana. When these two pieces bumped together, they created an even bigger land called Pangaea, which included almost all the land on Earth at the time.

The crash put huge pressure on what is now the Eastern Seaboard of North America, pushing up tall mountains. You can still see signs of this old mountain-making event from Alabama all the way to New Jersey, and even further underground to the southwest. In the northern parts, the effects reach up to Newfoundland. Over millions of years, these mountains were worn down by wind, rain, and other natural forces, spreading sediments far and wide.

Continental collision

The Alleghanian orogeny was a time when huge landmasses, or continents, crashed into each other. This collision pushed rocks upward, forming tall mountains. The Appalachian Mountains in eastern North America grew very high during this time, maybe as tall as the Alps or the Rocky Mountains before they wore down over millions of years.

When the continents bumped together, rocks caught in the middle were squeezed and folded. Near the edges where the continents met, the rocks changed shape due to pressure. In the southern parts of the Appalachian Mountains, like North Carolina, Tennessee, Virginia, and West Virginia, the rocks were bent and broken into big cracks called faults. These changes made the land shorter by about 300 kilometres in some places. The effects of this mountain-building slowly faded as you move north toward New York.

Subsequent erosion

The mountains formed by the Alleghanian orogeny were once tall and rugged, but over time they have worn down. Today, they are mostly gentle hills in the Piedmont. Bits of rock and dirt moved east to create the flat areas called the coastal plain and part of the continental shelf. The bits that moved west formed flat areas known as the Allegheny and Cumberland plateaux.

When the supercontinent Pangaea split apart, part of the Alleghanian mountains moved with Africa, becoming the Anti-Atlas mountains in Morocco. These mountains rose up more recently and are still quite rugged compared to the older Appalachian hills.