The Granite Dells: One Point Four Billion Years of Rock
Near Prescott, Arizona
A few miles north of downtown Prescott, the landscape does something unexpected. The ponderosa pines and rolling terrain give way to a jumble of rounded granite boulders piled against each other in formations that look too deliberate to be accidental and too large to belong to any human scale. This is the Granite Dells, one of the more striking geological formations in Arizona, and what makes the rock so remarkable is its age.
The granite here dates to roughly one point four billion years ago, the Proterozoic era, long before animals had appeared on Earth. The boulders were shaped through a process called spheroidal weathering, in which water penetrates the cracks and joints in the granite mass and slowly rounds the corners of each block until it becomes a sphere-like form. Over millions of years, the angular granite was transformed into the smooth, rounded boulders that pile on each other today in configurations that seem almost playful.
Two reservoirs, Watson Lake and Willow Lake, sit within the Dells formation and add water to the landscape. The combination of the ancient granite, the lakes, and the ponderosa forest makes the Granite Dells a somewhat disorienting place. You are looking at rock that formed long before animals existed on this planet, carved into shapes by water, surrounded by a forest of trees that are ancient by human reckoning but younger by a factor of many millions than the ground they root in.
