Catalina Gneiss (pronounced NICE) is the predominant rock that visitors see in the Rincon Mountains and the Tanque Verde Ridge; exposures of granite and schist are also found. Gneiss is a coarse-grained metamorphic rock resembling granite, having a banded appearance, and consisting of alternating layers of different minerals such as feldspar, quartz, mica, and hornblende. The banding and texture reveal that the gneiss, now exposed by uplift and erosion, was formed from parent rocks deep below the earth’s surface during the Precambrian Era more than a half billion years ago. There the parent rocks, subjected to pressure and heat, melted, flowed, and crystallized, before resolidification. Though the gneiss of the Rincon Mountains is ancient, the uplift that raised it to its present height is thought to have occurred rather recently (within the past 24 million years), during the period when most of the Sonoran Desert ranges apparently were formed. In age, these mountains thus would fall somewhere between the older Rockies and the younger Sierra Nevada. The schist, which underlies much of the Cactus Forest, where it is exposed along washes, was probably formed during the Cretaceous period more than 65 million years ago. Like the gneiss, the schist is a metamorphic rock, having been formed by the transformation of parent rocks at depth under great pressure and heat. Due mainly to differences in the composition of the parent rock, together with its mica content, the rock that resulted was platy and cleavable (accounting for its classification as schist). Granite, which forms Wasson and Amole peaks in the Tucson Mountain Section, is an igneous rock that (like the metamorphic gneiss and schist) originated at depths below the earth’s surface. The granite was formed by the solidification of molten rock material that moved upward en masse from greater depths, rather than by the alteration of ancestral rocks. Erosion of the uplifted land mass has not only stripped away the overburden but has developed valleys deeply incised into the granite itself.