Mark P Betts
Sierra del Carmen, Cuesta Carlotta, and Tornillo Flat
The Sierra del Carmen mountains just across the Rio Grande in Mexico dominate the Eastern half of Big Bend National Park. They are majestic in the afternoon sun and spectacular at sundown.
They also represent the "normal" elevation on this side of the park.
Although they look like high mountains, they really are relatively unmoved. The relief is due to the next to the last step in the geological history of the park.
About 15 million years ago, the extension or spreading apart that has dominated the Western United States started to work its way through the Big Bend area. This spreading created what we call the Basin and Range province of the Western US.
Death Valley in Califonia is one of the western-most examples of this and Big Bend is one of the eastern-most examples in the US.
Like the Grand Tetons in Wyoming, the Sierra del Carmens owe their relief to a large fault. The del Carmens remained at their normal elevation while where this picture was taken began to sink forming the basin in the basin and range while the Sierra del Carmens formed the range. The opposite side of the basin is a similar fault at Santa Elena Canyon on the west side of the park. The basin here is not blessed with a colorful Chihuahuan name - some geologist who was a little short on imagination called it: The Sunken Block.
The Chisos Mountains form an unusual high mountainous oasis in this basin. The cooled magma stocks are thought to have prevented them from sinking with the rest of the basin.
This new basin rapidly filled with sediment eroded off of the Chisos Mountains and to a lesser extent, the sediments also eroded off of the Larmide compressional anticlines or swells such as Dagger, San Vincinte, and Mariscal Mountains.
Eventually, the Rio Grande River worked its way through this area, spilling from filled basin to filled basin.
This is the last chapter in the geologic story of Big Bend - erosion.
As the Rio Grande "spilled" into The Sunken Block, it and the basin sediments formed a lake until it reached the height of the southeastern rim of the basin.
Once it began spilling over, it began cutting what would later become Boquillas Canyon. As it cut lower and lower, it began to empty the sediments that had partially filled The Sunken Block.
This erosion began only 5 to 3 million years ago and is still continuing although right now, the decreased flow of the river after the last ice age means that this area has entered a pause in the erosion and is actually slightly filling again along the river.
Tornillo Creek runs in the mid-distance along the low hills and has emptied hundreds of feet of Tertiary sediments into the Rio Grande forming Tornillo flats as it lowers the surface.
If you look closely at the surface near the prickly pear cactus, you will see that it is very gravely. We call this a deflation surface and it is caused by the creek selectively removing the clays, silts, and sands while leaving the larger gravels. As this process continues, more gravels are concentrated at the surface forming a gravel lag deposit.
D0A0021
Sierra del Carmen, Cuesta Carlotta, and Tornillo Flat
The Sierra del Carmen mountains just across the Rio Grande in Mexico dominate the Eastern half of Big Bend National Park. They are majestic in the afternoon sun and spectacular at sundown.
They also represent the "normal" elevation on this side of the park.
Although they look like high mountains, they really are relatively unmoved. The relief is due to the next to the last step in the geological history of the park.
About 15 million years ago, the extension or spreading apart that has dominated the Western United States started to work its way through the Big Bend area. This spreading created what we call the Basin and Range province of the Western US.
Death Valley in Califonia is one of the western-most examples of this and Big Bend is one of the eastern-most examples in the US.
Like the Grand Tetons in Wyoming, the Sierra del Carmens owe their relief to a large fault. The del Carmens remained at their normal elevation while where this picture was taken began to sink forming the basin in the basin and range while the Sierra del Carmens formed the range. The opposite side of the basin is a similar fault at Santa Elena Canyon on the west side of the park. The basin here is not blessed with a colorful Chihuahuan name - some geologist who was a little short on imagination called it: The Sunken Block.
The Chisos Mountains form an unusual high mountainous oasis in this basin. The cooled magma stocks are thought to have prevented them from sinking with the rest of the basin.
This new basin rapidly filled with sediment eroded off of the Chisos Mountains and to a lesser extent, the sediments also eroded off of the Larmide compressional anticlines or swells such as Dagger, San Vincinte, and Mariscal Mountains.
Eventually, the Rio Grande River worked its way through this area, spilling from filled basin to filled basin.
This is the last chapter in the geologic story of Big Bend - erosion.
As the Rio Grande "spilled" into The Sunken Block, it and the basin sediments formed a lake until it reached the height of the southeastern rim of the basin.
Once it began spilling over, it began cutting what would later become Boquillas Canyon. As it cut lower and lower, it began to empty the sediments that had partially filled The Sunken Block.
This erosion began only 5 to 3 million years ago and is still continuing although right now, the decreased flow of the river after the last ice age means that this area has entered a pause in the erosion and is actually slightly filling again along the river.
Tornillo Creek runs in the mid-distance along the low hills and has emptied hundreds of feet of Tertiary sediments into the Rio Grande forming Tornillo flats as it lowers the surface.
If you look closely at the surface near the prickly pear cactus, you will see that it is very gravely. We call this a deflation surface and it is caused by the creek selectively removing the clays, silts, and sands while leaving the larger gravels. As this process continues, more gravels are concentrated at the surface forming a gravel lag deposit.
D0A0021