The Cenozoic Era began about 70 million years ago and extends to the present. In our book of 1000 pages of Earth history, the Cenozoic represents only the last thirteen pages! Most of these pages fall into the Tertiary Period. The Tertiary represents a critical time in the geologic history of Colorado.
The beginning of the Tertiary coincides with the birth of the Rocky Mountains. The event is known as the Laramide Orogeny (orogeny means "mountain building"). The cause of the Laramide Orogeny reaches back more than 200 million years.
At the end of the Triassic period, the great supercontinent known as Pangea began to break apart, and North America began to separate from Europe. Far to the west, the North American crustal plate began colliding with and over-riding the Pacific-Farallon Plate. The collision between the two plates caused the crust to buckle and fold -- just like the fenders of two cars in a head-on collision! This folding started in California and gradually moved its way eastward, finally reaching Colorado about 60 million years ago.
During the Tertiary, the stresses caused by the colliding plates to the west forced several Precambrian crustal "wedges" upwards, forming the Colorado Front Range and the Southern Rocky Mountains. In some areas, the mountain building was accompanied by volcanic eruptions and magma emplacement.