Late-afternoon storm clouds build above Fifty-Mile Mountain over a Navajo Sandstone basin filled with countless numbers of small concretion spheres. Although these golf-ball-sized spheres may be seen scattered all across the Colorado Plateau, this is an especially rich deposit. These nodules were formed by small impurities such as iron particles mixed in with the almost pure-quartz sand which formed the dunes covering this area 190 million years ago. As groundwater percolated down through this sand, these iron particles attracted dissolved minerals in the water and begin to form these nodules. Much harder than the surrounding sandstone matrix, they eventually erode out and are left on the surface of the much softer Navajo sandstone. This black & white vertical photograph was captured with a 6x7 format camera. All photos in these fine art photo galleries may be ordered as fine art black & white framed prints or for B&W stock photography usage.