The Casa Grande ruins lie one and one-half miles south of the Gila River, about 50 miles above its junction with the Salt River. These two streams, the principal drainages of southern Arizona, head in mountainous regions to the northeast and have cut their shallow valleys, across the plains of their lower courses. Their flow is intermittent and interrupted, but there is evidence of a steadier flow, and of less violent flood runoffs, before the 20th century.
Physiographically, this area is part of the Basin and Range Provincegreatly eroded roots of mountain ranges standing a few hundred feet above wide, almost level, plains. The ranges seen from Casa Grande National Monument are pre-Cambrian granites and schists; some are cut by younger granitic rocks, and flanked by Tertiary lava flows (Vandiver 1935). The inter-range plains are basins filled with alluvial debris, the accumulation of millions of years of erosion from higher elevations; near the basin centers the fill may be 2,000 feet deep, and so nearly leveled that the desert floor often shows elevation variations of only a few feet.
This is part of the Sonora Desert, an area of mild winters, high summer temperatures, and low annual rainfall. At the Monument cooperative Weather Bureau station, the annual precipitation was averaged for a recent 15-year period at 9.41 inches (with annual extremes of 4.63 inches and 19.22 inches).
With irrigation, this desert is capable of producing crops of surprising variety and quantity. There is a frost-free period of 263 days. With modern machinery, irrigation, and farming methods, crops have become one of the 3 top sources of income for the state of Arizona. The greater portion of the nation's supply of lettuce is grown in southern Arizona deserts; the irrigated land of Arizona produces more cotton per acre than any comparable area in the world.
Geologically, the formation most affecting the lives of the prehistoric inhabitants here was caliche, the limy hardpan occurring in this area 2 to 4 feet below the ground surface. Caliche is formed when calcium carbonate-bearing ground waters lose either moisture or carbon dioxide; the limy precipitate may occur in almost pure form, or it may cement together sands and gravels at the level of the deposition (Breazeale and Smith 1930). Caliche varies in hardness and in density; it may be impervious to water and result in eventual puddling of irrigated top soil (a condition which is corrected in modern farming by deep-plowing to break up the hardpan); it can be very useful in construction, as shown by the strong and weather-resistant walls of Casa Grande.
To the prehistoric farmers of this area, caliche was probably a mixed blessing. It provided, in a land without suitable building stone, material for lasting and massive house construction. It may also, as an impervious underground layer, have forced the Indians to abandon waterlogged, unproductive, farms and extend their canal system to new lands, until an imbalance between crop production and canal efficiency was reachedand the caliche-walled towns had to be abandoned.
Last Updated: 02-Nov-2009