R. T. HILL — THE TEXAS-NEW MEXICAN REGION. 95 



from the Rio Grande embayment was accentuated, after having already begun in 

 late Cretaceous time, by the loading down of the embayment during Pliocene and 

 Pleistocene time with coastal sediments, thereby breaking the present hysometric 

 continuity of the ancient Llano Estacado baselevel to the coast. 



Basin Deposits of the Trans-Pecos Region. 



( 'Intruder of the Basins. — In addition to the vast sheet of Llano Estacado deposits 

 in Texas, Kansas. Colorado, Nebraska and northern New Mexico, which are sur- 

 rounded more or less upon every side by descending escarpments of erosion, there 

 are many large areas of a somewhat similar but newer formation occurring in valleys 

 eroded in the plains or enclosed by mountain blocks occurring as flats or basins 

 between the mountains, often many hundred miles in length. These so-called 

 basins laying between the mountains constitute nearly all of the irrigable and table- 

 lands of the region west of the Pecos. 



The Rio Grande flows most of the way in basins for five hundred miles south of 

 Albuquerque to a point near the Quitman mountains, except at the mountain passes 

 at the southern ends of the Mesilla and the Jornado basins respectively. The river 

 lias cut far into and below the latest level of the basins. Below- El Paso, near Fort 

 Hancock, the depth of the lacustral deposit cut through is twelve hundred feet, and 

 the river has almost reached the ancient hard-rock floor. 



lite ITueco-Organ Basin. — One of the most extensive and characteristic of these 

 great inter-mountain basins of post-Tertiary sediments is that lying between the 

 Organ-Franklin and Hueco-Sacrarnento ranges in extreme western Texas and south- 

 ern New Mexico. This vast expanse of apparently "deadlevel " plain, extending 

 from the Rio Grande northward some 150 miles, is from 90 miles in width at its 

 southern end to 40 at its northern. The Rio Grande cuts through its southern end, 

 exposing a grand section of the structure from El Paso on its western side to Etholen 

 station on the east. The basin, although apparently level, slopes southward, accord- 

 ing to the Whiteoaks railway profile, from 4,500 feet at its northern end to .'1,500 feet 

 at its southern end. 



< )n all sides this flat or basin (locally called " The Mesa '* at El Paso) is surrounded 

 by high mountain blocks, including the Juarez, Franklin-Organ and San Andres 

 blocks on the west and the Sierra Blanca, Hueco and Sacramento blocks on the 

 east, all composed of hard, impervious, metamorphosed limestones, quartzite, gran- 

 ite, porphyry and basalts, the stratified rocks being of all ayes, from the Silurian to 

 the < !retaceous. 



The soil of the basins resembles that of the Llano Estacado, and is the residuum 

 of the substructure of loose or unconsolidated sands (grits), " tierra blanca." clays 

 and water- worn gravel. Around the margin of t he basin near the mountains there 

 are greal fen-shaped benches of debris from the mountains, distributed by the 

 torrential streams running down the slopes and covered with sotol and yucca, the 

 foothill Mora of the region. These marginal deposits constitute extensive terraces 

 in places and are composed of boulders of mountain rock of all sizes and shapes. 



The structure of this basin formation is beautifully shown in the escarpments or 

 mesas of the Rio Grande valley east of El Paso, where the " tierra blanca," or cal- 

 careous conglomerate, can be seen capping the scarp, and in the bluffsialong the 

 railroad bet ween Etholen and Fori Hancock, where the soft, disintegrating escarp- 

 n lent has every aspeel of the typical " bad land " formations of the arid regions. 



