Description

The banded crystalline rocks of Sabino Canyon are a strong aesthetic component of the canyon's dramatic views. Certainly these rocks have attracted much attention from geologists. Until recently they were much-debated but poorly understood. When finally decoded (for example, Davis, 1980), these rocks and those of neighboring canyons became one of the best examples in the world of a "metamorphic core complex" (see also Saguaro National Park East). The main purposes of this trip are to acquaint the traveller with (1) the form of some large intrusions of formerly molten or igneous rock now cooled and solidified, (2) the effects of recrystallization and shearing to form a new rock called "mylonitic gneiss," (3) some faults and folds that formed later, and (4) more recent processes that cut the canyon itself. The intrusions are light-colored granitic rocks of Eocene age (about 50 million years). Granite is a coarse-grained igneous rock consisting mostly of feldspar and quartz. In this area the Eocene granites form sills, i.e. intrusions that parallel the structure of surrounding older rock or host rock. In this canyon, one granite sill, about 250 m (800) feet thick, forms the cliff-like upper canyon walls; two others can also be seen.