ing large quantities of carbon dioxide from the atmosphere, angiosperms caused
a drop in global temperatures. Forests of broad-leaf trees and shrubs that were a
favorite food of the dinosaurs apparently disappeared just prior to the ending of
the Cretaceous.This therefore also contributed to the dinosaurs’ downfall.
THE LARAMIDE OROGENY
Beginning about 80 million years ago, a large part of western North America
uplifted.The entire Rocky Mountain Region (Fig. 165) from northern Mex-
ico into Canada rose nearly a mile above sea level. This mountain building
episode called the Laramide orogeny resulted from the subduction of oceanic
crust beneath the West Coast of North America, causing an increase in crustal
buoyancy. The Canadian Rockies consist of slices of sedimentary rock that
were successively detached from the underlying basement rock and thrust
eastward on top of each other. A region between the Sierra Nevada and the
southern Rockies took a spurt of uplift during the past 20 million years, rais-
ing the area more than 3,000 feet.
Since the late Cambrian, the future Rocky Mountain region was near sea
level. Farther west within about 400 miles of the coast, a mountain belt com-
parable to the present Andes formed above a subduction zone during the 80
million years prior to the Laramide. It was apparently responsible for the Cre-
taceous Sevier orogeny that created the Overthrust Belt in Utah and Nevada.
Figure 165The Rocky
Mountains, looking
southwest over Loveland
Pass, north of South Park,
Colorado.
(Photo by T. S. Lovering,
courtesy USGS)CRETACEOUS CORALS