the Lower Pecos Valley, New Mexico. 419 



San Andreas. 4 The San Andreas limestone of the Pecos 

 Valley, extending across the Sacramento Mountain dips 

 gently to the east and finally disappears under the Red- 

 beds just west of the Pecos River. The tributaries of the 

 Pecos have extensively dissected this limestone area and 

 in the case of the Rio Hondo the San Andreas limestone 

 has been completely cut through and the underlying Yeso 

 exposed near Lincoln. 



Red -beds. — Above the San Andreas there occurs an 

 extensive series, 250 to 300 feet in thickness, of massive 

 white and red crystalline gypsum with some interstrati- 

 fied red shales and sandstones. This east-dipping series 

 of gypsum beds gives rise to a belt of striking solution 

 topography characterized by extensive slumping and 

 actual flowing of gypsum, sink-holes, caverns, and "bot- 

 tomless" lakes, which crosses the Pecos River diagonally 

 in the central portion of the area. 



Beyond the gypsum belt the upper Permian red sand- 

 stone and shales form the surface eastward to the bluffs 

 of the Staked Plains and there disappear beneath the 

 cover of caliche. The normal dip is approximately 30 

 feet to the mile in a direction just south of east. The 

 thickness of this upper sandstone phase of the Red-beds 

 is difficult to determine but there are at least 700 feet of 

 sediments represented. 



Cretaceous Series. — In the Sierra Blanca coal field in 

 Lincoln County the Cretaceous formations according to 

 "Wegemann 5 consists of a coal-bearing series 630 feet in 

 thickness comprising shales, sandstones, and occasional 

 thin limestones throughout which occur several seams of 

 bituminous coal. Beneath the coal-bearing strata occur 

 shales, sandstones, and limestones, shales predominating, 

 which aggregate 1,000 feet in thickness and which in the 

 lower part have been correlated with the Benton. These 

 formations are underlain by massive coarse-grained buff 

 sandstones, 175 feet thick, often forming prominent 

 escarpments, which has been tentatively correlated with 

 the Dakota. Underlying the Dakota there occurs a series 

 of variegated shales with thin beds of limestone, white 



4 The San Andreas limestone of the Alamogordo section has been actually 

 traced across the Sacramento Mountains and definitely correlated with the 

 limestone west of Roswell by Mr. Dorsey Hager of New York City. 



5 Carroll H. Wegemann : Geology and Coal Resources of the Sierra Blanca 

 Coal Field, Lincoln and Otero Counties, New Mexico, IT. S. Geol. Survey, 

 Bull. 541, 1914. pp. 418-452. 



