Eleven island-like mountain ranges tower thousands of feet above adjacent basins in southeastern Arizona. In the order that they are presented in this book, these island ranges are: Santa Catalina, Rincón, Tortolita, Santa Rita, Tucson, Galiuro, Pinaleño, Chiricahua, Mule, Huachuca, and Whetstone Mountains (Figure 1). These island mountains, together with over a dozen other ranges, that are either lower in elevation or more difficult to access, make up the Basin and Range geologic province of southeastern Arizona. Most of these ranges and intervening basins trend northwest-southeast and are part of the huge geologic Basin and Range Province that extends from southern Oregon to central Mexico. This mountain-vally topography results from a period of extension from about 15 to 5 million years ago that broke the crustal rocks of western North America into blocks, separated by steeply dipping faults. Some of these crustal blocks were uplifted to form ranges; other blocks subsided as much as 2.4 mi (4 km) to form deep basins. Streams cut deep canyons into the rising ranges and transported eroded boulders, cobbles, gravel, sand and clay to aprons of sediment - alluvial fans and bajadas - in nearby subsiding basins. Many of these sediment-filled basins had no drainage outlets to the sea and held shallow lakes (playas) that fluctuated in size with changes in climate. As the Santa Cruz and San Pedro Rivers and other streams integrated the drainage of southeastern Arizona with that of the Gila and Colorado Rivers, their tributaries eroded headward into the alluvial fill of these formerly closed basins. Today, ephemeral or seasonal drainages continue to deeply erode alluvial fans and bajadas on all sides of the ranges (Figure 2).