Free ebooks ==> http://www.Ebook777.com
Arizona north into Utah and east into Colorado and New Mexico. Initially,
the area surrounding the canyon was practically flat. Over the last 2 billion
years, heat and pressure buckled the land into mountains that were later flat-
tened by erosion. Again, mountains formed and eroded, and the region
flooded with shallow seas.The land was uplifted another time during the ris-
ing of the Rocky Mountains. Between 10 and 20 million years ago, the Col-
orado River began eroding layers of sediment, exposing the raw basement
rock below. Its present course is less than 6 million years old. Substantial por-
tions of the eastern Grand Canyon are geologically young, having been
eroded within only the past million years.
About 30 million years ago, the North American continent approached
the East Pacific Rise spreading center, the counterpart of the Mid-Atlantic
Ridge. The first portion of the continent to override the axis of seafloor
spreading was the coast of southern California and northwest Mexico.When
the rift system and subduction zone converged, the intervening oceanic
plate dove into a deep trench.The sediments in the trench were compressed
and thrust upward to form California’s Coast Ranges.A system of faults asso-
ciated with the 650-mile-long San Andreas Fault (Fig. 184) crisscross the
mountain belt. The Sierra Nevada to the east rose about 7,000 feet over the
Figure 182A view of
the Grand Canyon from
Mojave Point.
(Photo courtesy National
Park Service)247TERTIARY MAMMALS