The oldest human activity in the Salt River Valley is agriculture. But the second oldest, in the era since American settlement began in the late 1860s, is land: platting, subdividing, buying, selling, flipping. It's an old-fashioned extraction industry. The remarkable thing is that it remains Phoenix's economic foundation.
With the 1851 Salt and Gila River Meridian, or "baseline," located near today's Phoenix International Raceway, the Americans set in place the point from which land would be surveyed and divided. This is a historic method of American empire, going back to the Northwest Ordinance of 1787. It laid down a template that organized and regularized land to make it fungible.
Initially, the land was divided into farms, the square-mile layout that remains the bones of Phoenix until one gets into the mountains. But as the towns of Phoenix, Tempe, Mesa, Glendale and others grew, increasing amounts were subdivided for houses and businesses. Phoenix's unique location in one of the world's richest river valleys made agriculture a natural source of wealth. But so was the land itself. The 1877 Desert Lands Act expanded the Homestead Act, not only attracting settlers but also speculators.