Phoenix Mountains, part 3

Two small arroyos drain north and northeast from Piestewa Peak. These arroyos occupy small, adjacent valleys of lower relief topography separated by a low divide in the upper reaches, and a bedrock-cored hill further downstream. Well-defined channels are incised between one-half and two meters into the surrounding low-relief surface. The smaller, western arroyo drains northward towards 32nd street, and the larger, eastern arroyo drains northeastward towards 40th street. While the low-relief topography on valley floors appears homogenous, stratigraphy revealed in channel walls indicate different origins for the surface.

Western valley showing low-relief surface.

Eastern valley showing low-relief surface.

Where the Christensen trail crosses arroyos in the western valley, channel banks reveal metamorphic bedrock. In this area, the low relief valley floor is an erosional surface, with approximately one-half meter of soil and regolith atop truncated bedrock. The arroyos are also incised into bedrock. This type of surface is called a pediment.

Arroyo bank in western valley showing erosional surface.

Where the trail crosses arroyos in the eastern valley, channel banks reveal gravel alluvium. This alluvium is composed of mostly subangular, clast-supported gravel, and exhibits strong pedogenic induration. The gravel unit shows a crude stratigraphy that is parallel to the land surface, indicating the alluvium is likely deposited by the modern arroyo system. In this area, the low-relief topography is a depositional surface.

Arroyo bank in eastern valley showing depositional surface.

Likely, these surfaces have a spatial relationship with erosion up slope and deposition down slope. This will be investigated during subsequent visits.

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