They were formed during a period when the western North American continent was being stretched, cracking into blocks, bordered by large, steep faults. The Catalinas were probably only small hills at that time. However, as the continent stretched and blocks around the Catalinas sank forming valleys, the mountain range was left as a high point of land. No sooner were they formed than wind, ice, and water began to erode them. Tucson sits atop thousands of feet of sediments already eroded from the Santa Catalinas and other surrounding mountain ranges.
Current theory has it that the gneiss was formed nearly a billion and a half years ago.
At that time, a mass of molten rock cooled deep underground, forming a large layer of granite. Another mass of molten rock invaded the deeply buried granite about 45 million years ago. This second rock pooled in the granite in great fingers and sheets. Where they are visible today, these rock layers were stretched before they cooled and hardened, so that they layers are thin. The dark rock layers that we see today are the remains of the ancient granite, the light layers are the younger rock that invaded it 45 million years ago.