The McDowell Mountains and the surrounding smaller peaks and desert are the result of billions of years of geological processes.
The visible landscape of the Preserve is the product of relatively recent geological events, but the rocks that make up the landscape are much, much older than the events that created what we see today. The basic rock underlying the Preserve is ancient granite and metamorphic rock formed between 1.4 and 1.7 billion years ago. This rock was mostly underwater for more than a billion years, during which time it became covered with a thick layer of sediment that became sedimentary rock. The geological processes that created the Rocky Mountains about 65 – 75 million years ago (MYA) also lifted central Arizona above sea level, which caused this thick layer of sedimentary rocks on top of the granite and metamorphic rock to begin to erode.
The major geological process that shaped the local landscape was the formation of the basins and ranges. Between 8 and 15 MYA, volcanic activity beneath the crust stretched the surface of central and southwestern Arizona. The surface began to crack in many places, forming parallel north-south cracks. Earthquakes and continuing volcanism caused some of the regions between cracks to slide downward by as much as four miles, while other areas did not. The lower areas are now called basins and the higher areas are called ranges. The McDowell Mountains, the Phoenix Mountains and the Sierra Estrellas are parallel, roughly north-south ranges created during this period. The valleys in between, including metropolitan Phoenix, are examples of basins. The basins have mostly filled with sediment which eroded from the ranges in the millions of years since they were formed. In the McDowells, most of the sedimentary rocks have eroded away, leaving mostly granite and metamorphic rock.
