From Persimmon Gap to Boquillas Canyon, Part 11: Approaching the Mouth of Boquillas Canyon | Big Bend National Park, Texas, USA
Along the Boquillas Canyon Trail. Looking east-northeastward.
This is one of my all-time favorite structural-geology photos (of those taken by me, that is).
This visit occurred in the late afternoon, and the walls of this great breach in the Sierra del Carmen were bathed in golden light. I actually like the fact that this slide transfer is distinctly smudgy-impressionist in some places and grainy-pointillist in others. Not a crisp digital image to be sure, but rather painterly.
What's visible here is mostly or entirely the cliff-forming Lower Cretaceous Santa Elena Limestone. Since the late Mesozoic it has been subjected to two episodes of supersized crustal disturbance:
- compression of the Laramide Orogeny (Upper Cretaceous to Lower Tertiary), which build a great monocline of strata warping upwards toward the northeast like a giant stair step; and
- Upper Tertiary to Lower Quaternary extension as part of the Rio Grande Rift / Basin and Range stretching of a huge portion of western North America. This created a series of normal faults and fault blocks—with the down-dropped blocks called grabens (also known as bolsons).
In this vista one doesn't have to be a trained structural geologist to see that the brightly illuminated fault block on the far side of the Rio Grande is tilting quite dramatically to the left (northward). As the angled Santa Elena strata show, it's slumping down toward the graben beyond it. The imposing west-facing fault scarp of the Sierra del Carmen was also produced by this Basin and Range activity.
To see the other photos and descriptions in this set and my other Big Bend series, visit my my From Persimmon Gap to Boquillas Canyon album.
From Persimmon Gap to Boquillas Canyon, Part 11: Approaching the Mouth of Boquillas Canyon | Big Bend National Park, Texas, USA
Along the Boquillas Canyon Trail. Looking east-northeastward.
This is one of my all-time favorite structural-geology photos (of those taken by me, that is).
This visit occurred in the late afternoon, and the walls of this great breach in the Sierra del Carmen were bathed in golden light. I actually like the fact that this slide transfer is distinctly smudgy-impressionist in some places and grainy-pointillist in others. Not a crisp digital image to be sure, but rather painterly.
What's visible here is mostly or entirely the cliff-forming Lower Cretaceous Santa Elena Limestone. Since the late Mesozoic it has been subjected to two episodes of supersized crustal disturbance:
- compression of the Laramide Orogeny (Upper Cretaceous to Lower Tertiary), which build a great monocline of strata warping upwards toward the northeast like a giant stair step; and
- Upper Tertiary to Lower Quaternary extension as part of the Rio Grande Rift / Basin and Range stretching of a huge portion of western North America. This created a series of normal faults and fault blocks—with the down-dropped blocks called grabens (also known as bolsons).
In this vista one doesn't have to be a trained structural geologist to see that the brightly illuminated fault block on the far side of the Rio Grande is tilting quite dramatically to the left (northward). As the angled Santa Elena strata show, it's slumping down toward the graben beyond it. The imposing west-facing fault scarp of the Sierra del Carmen was also produced by this Basin and Range activity.
To see the other photos and descriptions in this set and my other Big Bend series, visit my my From Persimmon Gap to Boquillas Canyon album.