The southwestern portion of the United States has stretched in an east-west direction over the past three million years to twice its width. The Basin-Range, as geologists call it, is now about five hundred miles wide. The stretching caused cracks along which mountains rose and basins sunk. And erosion of the mountains filled the basins with sediment, producing what looks from space like mountain-ships floating on a sea of desert.
A dozen of these of these mountain ranges are near Tucson, and I have visited several of them. But today I ascended the largest—the Santa Catalina Mountains.
As I drove up the twisting road from the desert floor, rising to nine thousand feet at Mt. Lemmon, I questioned this stretching theory and wondered what happened to 250 miles of land that theoretically disappeared during the process. Maybe some of it is under your feet if you read this from Southern California on the Pacific Plate. Meanwhile, resting on the North American Plate, I recede slowly away from you each moment that the stretching continues.
Of course, if what I just said is true, then the idea that the San Andreas Fault, which separates the two plates, is primarily moving you north relative to me, cannot be entirely true. I often think that geology is as close to understanding the earth as physics is to understanding dark matter.
Anyway, what you are seeing in these pictures is granite that appears much like the sandstone hoodoos and arches of southeast Utah—not as spectacular, but weathered in similar ways through long exposure.
At the top, all sign of desert has given way to ponderosa pine and rock crags not unlike the Sierra Nevada Mountains of California.