Integrative Natural History of Organ Pipe Cactus National Monument, Part 1: For Now We See through Creosotes, Darkly | Arizona, USA
(Updated on May 10, 2025)
Looking generally northwestward. Taken either at the Pinkley Peak picnic area, or a little south of it. So I was near the beginning of my transit of Puerto Blanco Drive, which for much of its 41 mi (66 km) is just a stony, one-way, high-clearance-only track. I'm proud to say that I negotiated the whole dang loop, without one flat tire and without getting stuck in any washouts, in my little Chevy S-10 2WD pickup. And after that I did the Ajo Mountain Drive, an additional and better graded 21 mi (34 km). For some reason I didn't take photos there.
The Ektachrome slides I took this day along Puerto Blanco Drive are, thirty-three years later, of two types. One group remains quite clear and equitably tone-balanced. The other has the quality of a darkling dream, due to the Creosote Bushes (Larrea tridentata) appearing as Stygian masses below the pea-green Saguaros (Carnegiea gigantea). Perhaps it was the lighting. At this distant remove, I just don't know. But I rather like the effect.
This particular image is obviously one of the second group. What it primarily shows, and what clearly caught my geologist's eye at the time, was the yellow-spotted Pinkley Peak in the background.
Named for an important National Park Service official of bygone years, this prominence is part of the upthrust horst block known as the Puerto Blanco Mountains.
Upthrust ranges, down-dropped basins: that's right, we're smack dab in the Basin and Range Province, that immense world-historical museum of extensional tectonics. A large portion of the western US and Mexico can be likened to a gigantic sheet of saltwater taffy that got stretched apart and badly cracked.
One of my main references for this series is the Bedrock Geology Map of Organ Pipe Cactus National Monument and Vicinity, Southwest Arizona (Thompson et al., Arizona Geological Survey, 2024). It includes an informative booklet.
According to that source, Pinkley Peak is something of a volcanic layer cake. Its dark-toned, nipplelike peak is mapped as Childs Latite. This rock type, the extrusive equivalent of monzonite, contains a roughly equal amount of alkali and plagioclase feldspars, but very little quartz.
Below the latite, however, is the appropriately dubbed Pinkley Peak rhyolite. (Its rock type is not capitalized, the map authors explain, because the name is an informal one.) And the yellower zones on the middle and lower slopes is a lithic-lapili-tuff member within that unit.
All of these are early Miocene in age, and came into being during the heyday of Basin-and-Range magmatic activity. In contrast, the core of the Puerto Blanco Mountains is composed of considerably older metamorphic rocks dating to the Jurassic.
Down here on the Sonoran Desert Floor, however, there's younger, Pliocene-to-Pleistocene alluvium deposits mantled in desert pavement. For a discussion of that amazing feature, see this post of mine in another Flickr series. For now, suffice it to say that in many places the desert itself is better paved than Puerto Blanco Drive.
To see the other photos and descriptions in this set, visit my my Integrative Natural History of Organ Pipe Cactus National Monument album.
Integrative Natural History of Organ Pipe Cactus National Monument, Part 1: For Now We See through Creosotes, Darkly | Arizona, USA
(Updated on May 10, 2025)
Looking generally northwestward. Taken either at the Pinkley Peak picnic area, or a little south of it. So I was near the beginning of my transit of Puerto Blanco Drive, which for much of its 41 mi (66 km) is just a stony, one-way, high-clearance-only track. I'm proud to say that I negotiated the whole dang loop, without one flat tire and without getting stuck in any washouts, in my little Chevy S-10 2WD pickup. And after that I did the Ajo Mountain Drive, an additional and better graded 21 mi (34 km). For some reason I didn't take photos there.
The Ektachrome slides I took this day along Puerto Blanco Drive are, thirty-three years later, of two types. One group remains quite clear and equitably tone-balanced. The other has the quality of a darkling dream, due to the Creosote Bushes (Larrea tridentata) appearing as Stygian masses below the pea-green Saguaros (Carnegiea gigantea). Perhaps it was the lighting. At this distant remove, I just don't know. But I rather like the effect.
This particular image is obviously one of the second group. What it primarily shows, and what clearly caught my geologist's eye at the time, was the yellow-spotted Pinkley Peak in the background.
Named for an important National Park Service official of bygone years, this prominence is part of the upthrust horst block known as the Puerto Blanco Mountains.
Upthrust ranges, down-dropped basins: that's right, we're smack dab in the Basin and Range Province, that immense world-historical museum of extensional tectonics. A large portion of the western US and Mexico can be likened to a gigantic sheet of saltwater taffy that got stretched apart and badly cracked.
One of my main references for this series is the Bedrock Geology Map of Organ Pipe Cactus National Monument and Vicinity, Southwest Arizona (Thompson et al., Arizona Geological Survey, 2024). It includes an informative booklet.
According to that source, Pinkley Peak is something of a volcanic layer cake. Its dark-toned, nipplelike peak is mapped as Childs Latite. This rock type, the extrusive equivalent of monzonite, contains a roughly equal amount of alkali and plagioclase feldspars, but very little quartz.
Below the latite, however, is the appropriately dubbed Pinkley Peak rhyolite. (Its rock type is not capitalized, the map authors explain, because the name is an informal one.) And the yellower zones on the middle and lower slopes is a lithic-lapili-tuff member within that unit.
All of these are early Miocene in age, and came into being during the heyday of Basin-and-Range magmatic activity. In contrast, the core of the Puerto Blanco Mountains is composed of considerably older metamorphic rocks dating to the Jurassic.
Down here on the Sonoran Desert Floor, however, there's younger, Pliocene-to-Pleistocene alluvium deposits mantled in desert pavement. For a discussion of that amazing feature, see this post of mine in another Flickr series. For now, suffice it to say that in many places the desert itself is better paved than Puerto Blanco Drive.
To see the other photos and descriptions in this set, visit my my Integrative Natural History of Organ Pipe Cactus National Monument album.