Grus, sometimes referred as pea gravel because the average grain size is roughly that of a pea, is an accumulation of angular, medium- to coarse-grained rock fragments frequently of granitic composition. This image shows grus from weathering of the Late Cretaceous Amole pluton.
For more on the Tucson Mountains, see Pete Lipman's 'Geologic Map of the Tucson Mountain Caldera, Southern Arizona. https://ngmdb.usgs.gov/Prodesc/proddesc_10186.htm
Photographer: Mike Conway
Photo Date: October 2010