HEWETTJ RIO GRANDE VALLEY, NEAV MEXICO 15 



The leading geologic features of the Rio Grande valley are con- 

 spicuously expressed. One finds few areas in which the geologic 

 topography seems particularly obscure. The general structure of 

 the valley varies but Uttle throughout the course herein described. 

 The great sjmcHnal trough, about 40 niiles wide, Hes between two 

 imperfectly parallel mountain ranges. On the east is the Santa Fe 

 range, which is a continuous chain, a prolongation of the Sangre de 

 Cristo range of Colorado, embracing the highest peaks in New Mexico, 

 some of which reach an altitude of 13,000 feet. On the west is the 

 Jemez range, made up of a number of subranges and spurs locally 

 known as the Tierra Amarilla, the VaUes, the Gallinas, and the 

 Nacimiento ranges. These mountains are characterized by broadly 

 rounded contours with elevated valleys between the ranges. The 

 highest peaks are in the neighborhood of 11,000 feet in altitude. 



The river keeps toward the western side of the trough. On each 

 side is a narrow strip of highly productive irrigable bottom land, well 

 adapted to raising grain and fruit and to gardening. On the west 

 side this strip is only a few hundred yards wide, being closely hmited 

 by the abrupt escarpments of tlie Pajarito plateau, which hes between 

 the river and the foothills of the Jemez range. The main flood 

 plain, only a mile or two wide, is on the east side. This is limited 

 by an expanse of broken country about 20 miles in width extending 

 back to the Santa Fe range. This strip is characterized in part by 

 naked arenaceous bluffs, which in places are eroded into weird 

 castellated bastions, and in part by rounded gravel-covered hills. 

 The naked Tertiary sands of this valley are nonproductive, and large 

 areas are so situated that they will always be in a state of denudation. 

 There are occasional threads of fertihty along the meager water- 

 courses, and in places the bench lands have undergone a degree of 

 aggradation by reason of the distribution of material from adjacent 

 mountain sides ; this has resulted in a covering of moderately fertile 

 gravel and silt, which supports a fair amount of grass and other 

 vegetation. Extensive fluviatile deposits nearer the river indicate 

 the existence at some former time of a much greater river than the 

 jiresent Rio Grande. . Evidence pointing in this direction is repeated 

 and emphasized in the course of the Rio Grande after its emergence 

 from White Rock canyon opposite Santa Fe. 



At the lower end of the Espanola valle}^, just below San Ildefonso, 

 the river enters the canyon, cutting its channel through another 

 wide-sweeping lava flow. For a distance of two or three miles the 

 railway follows the bank of the river down this canyon, which for 

 the next 20 miles is passable only by an Indian trail, even this having 

 to leave the canyon toward the lower end and pass out over the 

 mesas. The canyon ends just above the Indian village of Cochiti; 

 thence the flood plain is larger and stiU on the east side of the river. 



