The San Gabriel Mountains are characterized by steep slopes, deep canyons, and sharp ridges. They are young mountains and have been uplifted from their surroundings by faulting, or earthquake movements. Because of earthquake activity, the mountains are composed of shattered rocks which are eroding rapidly.
Heavy rains cause sand and rocks to be washed down to lowlands and basins.
The deeper levels of the soil remain moist most of the year, permitting plants with deep roots to reach limited quantities of water. There are two types of rocks in Eaton Canyon - Igneous and Metamorphic. Muscovite Mica Schist is silvery from the fine parallel layers of mica contained in its small, round, brown garnet crystals. Schists have the quality of breaking off in thin, wavy layers. Biotite Mica Schist is similar to the Muscovite Mica Schist above, but is very dark and lustrous.