GEOTECTONICS OF ESTANCIA PLAINS 



435 



with Fig. 2). As in the case of the latter plain the general aspect is 

 that of a wide shallow valley which has a tendency to impart some- 

 thing of a synchnal character to the substructure. This apparent 

 general relief feature, however, bears no relationship to the arrange- 



Sandia. Mt-i. 



7'vuento.) Alts. 



Cerro Psion, 



Fig. I. — General structure of Estancia Plains. 



ment of the formations beneath. The surface of the plains is not a 

 stratum-plane, as is naturally at first inferred, but a plane worn out 

 in part at least on the beveled edges of the strata below, which lie 

 at many different angles and dip in many different directions. 



This general structure of the plains appears to have been the 

 characteristic feature of the region before their continuity was inter- 

 rupted by the faulting which gave rise to the block mountains, and by 

 the laccolithic intrusions. 



In most of the parallel bolsons and valleys which are found in 

 the high tableland region of New Mexico the bounding mountain 

 blocks are so tilted that across some plains great fault-scarps face 

 each other, as in the case of the Jarilla bolson immediately south of 

 the Estancia district. Here the abrupt fault-scarp of the Sierra San 



3ieFra.Se las Co.bQ.lioi 



■5o.crarTx.en.to /l/)f.s. 



Fig. 2. — Structure of the basin ranges in central New Mexico. 



Andreas, rising over 3,000 feet above the level of the plains, faces the 

 equally abrupt scarp of the Sacramento Range to the east which rises 

 even higher. In other instances, as in the Jornada to the southwest 

 of the Estancia Plains, the main portion of the bolson is flanked on 

 either side by the gentle backslopes of the monoclinal blocks. The 

 fundamental plan of geologic structure is best indicated by diagram 



