Geology and Soils in Deserts of the Southwestern United States - Design with the Desert: Conservation and Sustainable Development - page 39

.130° .120° .110°

OREGON

SOUTH
DAKOTA

IDAHO

WYOMING

NEBRAASKA

NEVADA

UTAH

COLORADO
KANSAS

CALIFORNIA

Pacific
Ocean

OKLAHOMA

NEW
MEXICO

TEXAS

CP

.120° .110°

075150 300 450 600

N

Kilometers

050100 200 300 400 W E

Miles

S

TZ

BR

FIGURE 2.2
Arizona can be divided into three physiographic provinces, each with their own distinctive geological char-
acter. Approximate province boundaries are indicated by black lines: CP, Colorado Plateau Province; TZ,
Transition Zone Province; BR, Basin and Range Province. The black star indicates the location of the state
capitol of Phoenix. Shaded relief base map constructed from the U.S. Geological Survey National Elevation
Dataset (1 arc second). Locator map of the southwestern United States indicates the location of Arizona (gray).

northwest-southeast trending elongated mountain ranges alternating with sediment-
filled valleys. This area formed primarily due to episodes of extension of the Earth's crust
in what is now Arizona from about 15 to 5 million years ago, which resulted in widespread
faulting that caused uplift of mountain ranges and downdropping of adjacent valleys by
fracturing and tilting of large blocks of crust.*
Formation of the oldest bedrock in Arizona took place approximately 1.8-1.7 billion years
ago during a series of mountain-building events related to subduction along the southern
boundary of the ancient core of the North American continent. The oldest of these crystalline
intrusive and metamorphic basement rocks are visible in the bottom of the Grand Canyon.
During the remainder of the Proterozoic Eon (>550 million years ago), the ancient basement
was exposed, eroded, and eventually covered by layers of sedimentary and volcanic rocks
between approximately 1.7 billion years and 550 million years ago. During that interval, the
basement rocks and overlaying sedimentary rocks were again exposed, tilted, and eroded
before being covered by the oldest of the horizontal Paleozoic sedimentary layers.

* For more detail on the geological history of Arizona, see Jenney and Reynolds. 7