Santa Elena Canyon View 14: It's Definitely My Fault | Big Bend National Park, Texas, USA

On the walk out of the canyon, we pause on the Rio Grande floodplain near the confluence of Terlingua Creek to glance east-northeastward. The river itself can been seen as an indistinct dark band winding through the point-bar vegetation.

To me, anyway, this image and its implications are mindboggling. No, it isn't the kind of prettified eye-candy that nature photographers produce in wholesale quantities, and I make no claim to artistry. All I bring to this scene is a sense of awe. There are processes visible here that dwarf all human works.

A cliff this tall (ca. 1500 ft / 457 m) is imposing enough in its own right. But this rampart, Sierra Ponce on the Mexican side of the stream, is also the scarp of the great Terlingua Fault. As such, it's one of the most dramatic and immediately comprehensible demonstrations of the mobility of the Earth's crust I know.

The lowland US block on which we stand—geologists call it the hanging wall—has dropped down roughly 3000 ft (914 m) relative to the lofty mass of rock, the footwall, directly to its south. As a result, older Lower-Cretaceous rock strata now loom high over younger Upper-Cretaceous beds exposed on this side of the fault trace.

This huge normal fault, a magnificent paradigm of large-scale displacement, is my own favorite example of local adjustment to the extensional tectonics (crustal thinning). But, to be fair, there are thousands of others in western North America's Basin and Range Province—including one not far away, in Big Bend's Boquillas Canyon.

The Terlinga Fault began to form approximately 25 Ma ago, and was most active until 2 Ma. For those stratigraphically inclined, here's the sequence of Sierra Ponce's Lower Cretaceous formations. From bottom to top, they're

- the slope-forming Telegraph Canyon,

- the cliff-forming Del Carmen Limestone,

- the slope-forming Sue Peaks, and

- the cliff-forming Santa Elena Limestone.

I tell you what: to take a gander at the other photos and descriptions in this series, y'all mosy on down to my my Natural History: Big Bend Region & All of Trans-Pecos Texas album.

Taken in: United States / Texas / Terlingua Abaja (show map)

Taken on: March 13, 2002

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Camera: MAKER NAME 96660, ISO 50 (more info) (hide info)

Camera:	MAKER NAME 96660
ISO:	50
Flash:	Auto, Did not fire
Exposure Program:	Program AE
Metering Mode:	Average
White Balance:	Auto
Color Space:	Uncalibrated
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