A number of major faults cut through the basement. In the belt, there are numerous thrust faults, and layers of rock between the faults are folded. Their formation accommodates shortening of the crust. In Late Mesozoic and Early Cenozoic time, the angle of the subducting plate decreased, and it began to grind along the base of North America.
Basement is pushed up on a reverse fault, and overlying strata are warped into a monoclinal fold.
Eventually, the monoclinal fold erodes away, so that at the crest of the mountains, you have exposed basement.