GEOLOGY 121 



THE PUERCO VALLEY. 



To the west, separated from the Rio Grande by a low divide 

 not more than twenty five miles wide, is the Puerco River 

 Valley, a broad, open, gently sloping stretch of country a score 

 and a half miles in width. This valley is now occupied by a 

 diminutive stream, shrunken from a once much larger size, 

 by tiltings and warpings of the earth's crust which involved 

 the slopes of its head drainage and diverted it westward. 

 Numerous water gaps and remnants of former valleys, to 

 the west of the present Puerco Valley, outline in part the 

 work of the Puerco and some of its tributaries before fortune 

 worked adversely to the river. There are many strong evi- 

 dences that the Puerco may have been crowded esatward by 

 the stupendous out-pouring of lava from Mt. Taylor toward 

 the north and east. However, running water, and plenty of 

 it, must be accountable for the broad Puerco valley of today. 

 Between the river and the divide to the east, much and in- 

 tense block faulting has occured. Many of the blocks are 

 inclined at angles of 45 degrees or more and are conspicuous 

 features in this locality. On the west side of the river there 

 is comparatively little faulting, but erosion, caused by the 

 run-off of the higher plains to the west, has made the country 

 considerably rougher. Still further west erosion has cut 

 the, now high plateau, into numerous Mesas and denuded 

 many volcanic necks, which characterize this section of the 

 country. Along the upper portion of the Puerco, the valley 

 widens considerably as the river bends westward to skirt 

 the Nacimiento Mountains which limit the western exten- 

 sion of the Jemez Plateau, but continues northward again until 

 it finds its source in the southern portion of Rio Arriba 

 County. 



THE ESTANCIA VALLEY 



Beyond the clinoplains which, as has been stated, lie east 

 of the Rio Grande River, the Sandias and Manzanos true 

 granite cored, Rocky Mountain types of mountains rise six 

 thousand feet or more above the river, capped with Carboni- 

 ferous limestone, which dips at comparatively low angles to 

 the east beneath Estancia Valley; but, from the west, due to 

 a huge fault scarp these mountains present an almost per- 



