To revisit the waning hours of Days 1 & 2 of the 2015 Arizona Trip #1, in the final miles of Interstate 20 in west Texas, the Apache Mountains (due West) and the Davis Mountains (due South) are direct indications of significant changes from the subtle roadside Geology of the southern Great Plains.
The Apache Mountains and the Davis Mountains mark the easternmost extent of the rugged landscapes of the Basin and Range Province (and the Davis Mountains represent the eastern parts of the Trans-Pecos Volcanic Field).
Upon leaving the nostalgia of the Eagle Mts., after passing through Sierra Blanca, Texas, Interstate 10 begins a noticeable topographic drop into the Rio Grande Valley (aka Hueco Bolson). To the north, are the Malone Mountains, composed of Jurassic(?)/Early Cretaceous limestones, considered part of the Chihuahua Tectonic Belt. To the south are the Quitman Mountains, composed of caldera-related Oligocene volcanics and intrusives.
Malone Mountains
Figure 1.
Quitman Mountains
Figure 2.
Immediately west of both mountain ranges (foregrounds of both photos) are coalescing alluvial fan sediments, derived from the weathering and erosion of these mountain ranges, overlying and interfingering with the Fort Hancock Formation and overlying the older Camp Rice Fm basinal sediments associated with the local history of the Basin and Range Province and the Rio Grande River (as well as the Rio Grande Rift).
A little further downslope on both sides of I-10 are 3 or 4 enigmatic "basalt knobs" of uncertain age and origin (unless interpretations have changed since I left the area in 1991). There is one knob south (left) of the freeway and 2 or 3 north.
Are they exposed shallow basalt intrusions or dissected shield volcanoes or dissected cinder cones? Are they associated with the Oligocene Quitman Mts. or with the younger Rio Grande Rift? (In the background of "South Knob" are the Sierra de Los Frailes, in Mexico.)
"South Knob"
Figure 3.
"North Knob" (closest to I-10)
Figure 4.
As all of this land visible is presumably privately-owned, accessibility for geologic study is uncertain. I remain hopeful that someone will study these enigmatic features and determine "where they fit" in the regional scheme of things.
Along the northwestward I-10 corridor, they are the last post-Proterozoic igneous rocks until the Eocene Campus Andesite and the Cerro de Cristo Rey Andesite intrusions, immediately northwest of downtown El Paso. [A side detail of these photos is that they show the flora (vegetation) characteristic of gradual slopes in the Chihuahuan Desert, dominated by Creosote Bush, Mesquite, and Ocotillo.]
[At the time when I needed to find a new Thesis project in 1985, inquiring about the accessibility of these sites did not cross my mind. My 1985 (starting date) Thesis turned out to be interesting, but still, it is one of my regrets that I didn't find out more about these "basalt knobs", west of the Quitman Mountains.]
At the bottom of the long westward descent, I-10 "bottoms out" near the edge of the Rio Grande floodplain between Mile Markers 84 to 82, then begins a long ascent that brought us into the eastern edge of El Paso.
At exit 72 is the sign for Fort Hancock, Texas, the place in the movie The Shawshank Redemption - where Andy Dufresne and later "Red" Redding, crossed the Rio Grande into Mexico (and freedom) on a bus to the Mexican Pacific coast.
The trouble is, from the Mexican village of El Porvenir (across from Fort Hancock), there is no paved highway into the interior of the country, due to the mountain ranges. One would either have to go upstream to El Paso/Juarez (closer) or substantially downstream to Presidio, Texas, and Ojinaga, Chihuahua to catch a bus into the interior.
As we got closer to El Paso, I made some cellphone calls to arrange brief meetings with a longtime friend (Jeniffer), my old girlfriend (Janet), and the Best Man in my 1984 wedding (Doug), as well as a local beer can collector (Wiley). As we arrived at the truck stop on the east side of El Paso, to meet Jeniffer for breakfast, I noticed something stuck to my left shoe, i.e., impaled by an Ocotillo stem. This is why I wear closed-toe shoes when I drive, in case I stop for photos, as I did at the Eagle Mts. exit.
Ocotillo Stem
Figure 5.
After an enjoyable breakfast with Jeniffer, we said "Adios" and resumed our trek. Next, Neal, Sarah (the dog), and I met Janet at a Starbucks near the airport on I-10.
Janet was the reason for the breakup in 1979 that brought things "crashing down". I was a "late bloomer" and took things too hard when we broke up. Anyway, we had reconciled many years later and had revived a friendship that lasted until after the 2016 elections, when some unfortunate, rude things were said on Facebook.
Janet was the reason for the breakup in 1979 that brought things "crashing down". I was a "late bloomer" and took things too hard when we broke up. Anyway, we had reconciled many years later and had revived a friendship that lasted until after the 2016 elections, when some unfortunate, rude things were said on Facebook.
I don't mind honest, polite disagreements, but some name-calling resulted in things "getting put on ice" (again). I try to value friendships over politics, but to too many folks, it is a one-way street. So it goes.
Next, we met Wiley on the west side of El Paso, near Trans-Mountain Road and I-10, to swap some full craft beer cans and "shoot the breeze" for a bit. No politics, just a friendly conversation.
Next, we met Wiley on the west side of El Paso, near Trans-Mountain Road and I-10, to swap some full craft beer cans and "shoot the breeze" for a bit. No politics, just a friendly conversation.
We then went a few miles more to meet Doug, my Best Man at my first wedding. Another case of strained relations due to political differences. After Marla's passing, I was hoping for a reset, but as Thomas Wolfe said: "You can't go home again". The meeting was cordial, but the effect was brief.
With the socializing done, the only thing remaining to be done was to drive the final 400 (+/-) miles to Phoenix and get there before dark. (Which we did drained though we were.)
We made a photo stop at the I-10 West rest area in the Little Dragoon Mts., Cochise County, Arizona. It's a good place to see examples of "Spheroidal Weathering".
With the socializing done, the only thing remaining to be done was to drive the final 400 (+/-) miles to Phoenix and get there before dark. (Which we did drained though we were.)
We made a photo stop at the I-10 West rest area in the Little Dragoon Mts., Cochise County, Arizona. It's a good place to see examples of "Spheroidal Weathering".
By sundown, we were rejuvenated enough to enjoy the sunset from my daughter's rental home.
Figure 7.
Resources:
Physiographic features, Trans-Pecos region, Underwood, James R. Jr., 1980 in: Trans Pecos Region (West Texas); Dickerson, P.W.; Hoffer, J.M.; Callendar, J.F. [eds.], New Mexico Geological Society 31st Annual Fall Field Conference Guidebook, 308 pp.
Pleistocene rocks in El Paso and Hudspeth Counties, Texas adjacent to Interstate Highway 10, Strain, W.S., 1980 in: Trans Pecos Region (West Texas); Dickerson, P.W.; Hoffer, J.M.; Callendar, J.F. [eds.], New Mexico Geological Society 31st Annual Fall Field Conference Guidebook, 308 pp.
[More resources to be added...]
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