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The state comprises three major geographic zones. In the east, most of interior Washington is made up of the Columbia Plateau and the valleys of the Columbia River and its tributaries. Central Washington is dominated, and the state is divided, by the north-south Cascade Range. To the west of the Cascades lie coastal lowlands in the Puget Trough, Puget Sound and its many arms, and to their west the Coast Ranges, which in part form the backbone of the Olympic Peninsula.
The Interior
Washington's interior is a region of hard volcanic substructure, in many places scoured by glacial and river action, that is left largely dry by the shield the Cascades form against the Pacific winds; in some areas, as in the southeastern Palouse hills, loess deposits provide a basis for irrigated agriculture. The Blue Mts., an offshoot of the Rockies in the state's southeast corner, are one of the interior's few forested sections. The Columbia River enters the state from British Columbia in the northeast. After recver from the east, it turns westward across the state and swings south at the foot of the Cascades, enclosing the Big Bend country. Near Washington's southern border, it receives the Yakima (from the west) and Snake (from the east), then bends westward again, forming the boundary with Oregon as it cuts through the Cascades on its way to the sea.
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