Here's why:

* Formation: The Rockies were formed by the collision of tectonic plates (the North American plate and the Farallon plate). This collision caused the Earth's crust to buckle and fold, creating the towering mountain ranges we see today.

* Characteristics: Folded mountains are characterized by their distinctive peaks, valleys, and ridges that are often arranged in parallel lines. The Rockies perfectly exemplify this with their jagged peaks and long, winding valleys.

While the Rockies are primarily folded mountains, some sections also exhibit characteristics of fault-block mountains. This is due to the additional forces of faulting and uplift that contributed to their formation.