Folding bends many layers of rocks without breaking them. The Appalachian Mountains and Rocky Mountains of the United States, and the Alps of Europe are examples of mountain ranges that were formed by folding. Many of the greatest mountain ranges of the world have formed because of enormous collisions between continents. The Appalachian Mountains in the Eastern United States were formed about 400 million years ago when North America and Africa collided. Mountains sometimes form when many layers of the Earth's crust are moved vertically upward at fault lines by pressures caused by plates colliding.
Fault lines are great cracks in the crust.
The mountains that are formed in this way are called fault-block mountains. The Sierra Nevada mountains in California and Nevada, and the Grand Teton range of Wyoming are examples of fault-block mountains. The Black Hills of South Dakota and Wyoming, and the Adirondack Mountains of New York are low mountains that were formed when the crust was heaved upward without folding or faulting into a rounded dome. These are called Dome Mountains. Dome mountains are much higher in elevation than the surrounding land and because of this erosion occurs at a very fast rate.