Intermontane Plateaus

The Intermontane Plateaus of the Western United States is one of eight U.S. Physiographic regions (divisions) of the physical geography of the contiguous United States. The region is composed of intermontane plateaus and mountain ranges. It is subdivided into physiographic provinces, which are each subdivided into physiographic sections.

The Columbia Plateau Province is a large igneous province of flood basalts erupted in Miocene and early Pliocence epochs across the states of Washington, Oregon, Idaho, Nevada, and California.

The province was uplifted and divided into great blocks by faults or monoclinal flexures and thus exposed to long-lasting denudation in a mid-Tertiary cycle of erosion. They were then broadly elevated again with renewed movement on some of the fault lines. The current erosion cycle was introduced in late Tertiary time during which the deep canyons of the region have been trenched. The results of the first cycle of erosion are seen in the widespread exposure of the resistant Carboniferous limestone as a broad platform in the south-western area of greater uplift through central Arizona where the higher formations were worn away. They are also seen in the development of a series of huge, south-facing, retreating escarpments of irregular outline on the edges of the higher formations farther north. Each escarpment stands forth where a resistant formation overlies a weaker one. Each escarpment is separated from the next higher one by a broad step of weaker strata.

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