Basement Complex of the Rockies

The Rocky Mountains were formed over millions of years, but began their largest transformation some 75 million years ago during the late Cretaceous. An oceanic plate, called the Farallon, had been subducting beneath the North American plate for roughly 80 million years until it began to subduct beneath the North American plate at a very shallow angle. The shallowness of the angle allowed the plate to travel over 1000 miles inland before causing tremendous amounts of friction in the continental plate. All of the friction deep within the crust caused widespread faulting and uplifting throughout the states of Montana, Colorodo, Wyoming, and South Dakota. 

 

The faulting was so large that the bedrock was included in the fracturing. As the faulting occurred, compression caused large blocks of basement rock to be uplifted thousands of feet. After the blocks were uplifted, erosion ate away at the top layers of weaker rock until the outcrops of highly metamorphosed basement rock was all that remained.