remotely possible. The surface of the relict fan is resistant to erosion, and parts of the surface may be flooded by local runoff. This does not, by itself, mean that this area is subject to alluvial fan flooding, however.

The Tortolita Mountain piedmont lies 20 km (12.4 mi) northwest of downtown Tucson, Arizona. The region is semiarid, with 280-mm (1.1-in) average annual precipitation. Average annual temperature is 20.3 degrees Celsius. Summer monsoonal precipitation results in intense floods. The area is characterized by sloping alluvial surfaces extending approximately 10 km (6.2 mi) from the front of the Tortolita Mountains to the floodplain of the Santa Cruz River (Figure 4-7). The highest elevation in the Tortolita Mountains is 1,533 m (5,030 feet). Elevation at its mountain front is approximately 900 m (2,953 feet), and the Santa Cruz valley floor elevation is approximately 600 m (197 feet). The Tortolita Mountain front was originally a steep fault scarp, but this range has been tectonically inactive since the late Tertiary. There is no modern fault scarp, and the mountain front is highly sinuous. Because this mountain front is inactive, deposition on the piedmont during the Quaternary has been controlled primarily by climate change (Fuller, 1990).

The Tortolita piedmont consists mainly of the dissected remnants of ancient fans. These do not meet the committee's definition of an alluvial fan. However, there are subunits of the piedmont that do display the criteria. An example is Cottonwood fan (see box in Figure 4-7).

The piedmont is mantled by alluvial sediments. The dissected fan remnants close to the Tortolita Mountain front are composed of gravel and boulders. The surfaces of these remnants are weathered to form desert pavements and soils. The latter include accumulations of fine material blown in by wind and the alteration products of chemical weathering over long periods of geologic time. These coarse-grained deposits decrease in grain size distally from the mountain front.

The alluvial fans that are inset into these dissected remnants are composed mainly of sand. This is because the fan apexes are not at the mountain front. The apexes are in the active stream channels that are incised through the old piedmont surfaces. The fine sediment is derived largely from the old soils that characterize the dissected fan remnants, which make up the divides between active stream channels cut into the piedmont.

The Tortolita piedmont consists of old alluvial surfaces that slope away from the mountain front. The surfaces were formed by ancient coalescing alluvial fans but are not considered fans today under the committee's definition because they lack the characteristic fan-shape.