Historical Geology Understanding Our Planet\'s Past

A region from eastern Utah to the Texas panhandle that deformed during the

late Paleozoic Ancestral Rockies orogeny was completely eroded by the time

of the Laramide.The Rocky Mountain foreland region subsided as much as 2

miles between 85 million and 65 million years ago and then rose well above

sea level, acquiring its present elevation around 30 million years ago.

To the west of the Rockies, numerous parallel faults sliced through the

Basin and Range Province between the Sierra Nevada of California and the

Wasatch Mountains of Utah.This resulted in a series of 20 north-south trend-

ing fault block mountain ranges.The Basin and Range covers southern Ore-

gon, Nevada, western Utah, southeastern California, and southern Arizona

and New Mexico. The crust bounded by faults is literally broken into hun-

dreds of steeply tilted blocks and raised nearly a mile above the basin, form-

ing nearly parallel mountain ranges up to 50 miles long.

Death Valley (Fig. 166), at 280 feet below sea level, is the lowest place on

the North American continent. The region was originally elevated several

thousand feet higher during the Cretaceous.The area collapsed when the con-

tinental crust thinned from extensive block faulting, with one block of crust

lying below another. The Great Basin area is a remnant of a broad belt of

mountains and high plateaus that subsequently collapsed after the crust was

pulled apart following the Laramide.

The rising Wasatch Range of north-central Utah (Fig. 167) is an excel-

lent example of a north-trending series of faults, one below the other. The

fault blocks extend for 80 miles, with a probable net slip along the west side

of 18,000 feet.The Tetons of western Wyoming were upfaulted along the east-

ern flank and downfaulted to the west. The rest of the Rocky Mountains

Figure 166Death Valley
showing salt pan,alluvial
fans,and fault scarp,I ny o
County, California.
(Photo by H.Drewes,
courtesy USGS)

Historical Geology