The Willamette Valley ( ) is a long valley in Oregon, in the Pacific Northwest region of the United States. The Willamette River flows the entire length of the valley and is surrounded by mountains on three sides: the Cascade Range to the east, the Oregon Coast Range to the west, and the Calapooya Mountains to the south. The valley is synonymous with the cultural and political heart of Oregon and is home to approximately 70 percent of its population including the five largest cities in the state: Portland, Eugene, Salem, Gresham, and Hillsboro. The valley's numerous waterways, particularly the Willamette River, are vital to the economy of Oregon, as they continuously deposit highly fertile alluvial soils across its broad, flat plain. A massively productive agricultural area, the valley was widely publicized in the 1820s as a "promised land of flowing milk and honey." Throughout the 19th century, it was the destination of choice for the oxen-drawn wagon trains of emigrants who made the perilous journey along the Oregon Trail. Today, the valley is often considered synonymous with "Oregon Wine Country," as it contains more than of vineyards and 500+ wineries. Geology Much of the Willamette's fertility is derived from a series of massive ice-age floods that came from Lake Missoula in Montana and scoured across Eastern Washington, sweeping its topsoil down the Columbia River Gorge. When floodwaters met log- and ice-jams at Kalama in southwest Washington, the water caused a backup that filled the entire Willamette Valley to a depth of above current sea level. Some geologists suggest that the Willamette Valley flooded in this manner multiple times during the last ice age. If floodwaters of that magnitude covered Portland (elevation ) in 2010, only the tops of the West Hills, Mount Tabor, Rocky Butte, Kelley Butte and Mount Scott would be visible, as would only some of the city's tallest skyscrapers. Elevations for other cities in the valley are Newberg, ; Oregon City, ; McMinnville, ; Salem, ; Corvallis, ; and Eugene, . The lake gradually drained away, leaving layered sedimentary soils on the valley floor to a height of about above current sea level throughout the Tualatin, Yamhill and Willamette valleys. Geologists have come to refer to the resulting lake as Lake Allison, named for Oregon State University geologist Ira S. Allison, who first described Willamette Silt soil in 1953 and noted its similarity to soils on