GEOTECTONICS OF ESTANCIA PLAINS 



449 



tacic Age. The andesitic plate is in reality a great sill, from the Ortiz 

 laccolith, 12 miles to the westward, now bent into a broad syncline. 



Some distance beneath the andesitic plate which crowns the Cerro 

 Pelon dark clay-shales are exposed. The valley plain to the eastward 

 is also occupied by soft black shales. At a point several hundred 

 feet below the foot of the Pelon hill a core drill penetrated these shales 

 to a distance of 800 feet without passing through them. They are 

 therefore over 1,000 feet thick. 



The dark shales dip to the westward — the valley plain being worn 

 out on their beveled edges. Six miles to the east of the Cerro Pelon 

 the bottom of the shale formation reaches the surface and then the 

 underlying sandstones continue as the surface rocks to the edge of 

 the Glorietta escarpment, forming the western cliff of the Rio Pecos. 



Fig. II. — Fault bisecting the Sandia and Manzano ranges: displacement about 

 1,000 feet. 



This eastern segment of the section is arching and is the pitching 

 anticline by which the Rocky Mountains are terminated southward. 



The Tijeras, or typical plains cross-section extends from the 

 south end of the Sandia Range, at the upper end of the Tijeras Canyon, 

 southeastwardly for a distance of 50 miles to the Padernal Hills, 

 which form the drainage divide between the Estancia Plains and the 

 Valley of the Pecos River (section E-F of Group, Fig. 10). 



The most characteristic feature of this section is the even surface 

 of the plains worn out on the beveled edges of the strata beneath. 

 Only near the Sandia Range do the strata display any indications of 

 marked dislocation. One of these faults has enabled the Tijeras 

 Canyon to be formed between the Sandia and Manzano ranges. 

 This fault (Fig. 11) has a throw of over 1,000 feet. At the hamlets 

 of Tijeras and San Antonio it is well displayed. At the last-mentioned 



