GEOLOGIC RECONNAISSANCE OF TAOS RANGE 741 



GEOLOGICAL HISTORY 



While the pre-Cambrian history of the Taos Range must neces- 

 sarily remain rather obscure until further investigation and correla- 

 tion with other regions, some of the events may be enumerated 

 with more or less accuracy. Nothing is known about the origin 

 of the ancient gneisses and schists. During the long erosion interval 

 that exposed them and probably reduced the ancient mountains 

 to base level, thick clastic deposits accumulated along the eastern 

 and southern margins of the area now occupied by the granite 

 batholith. 



Upon this time of great erosion a period of intense orogenic 

 movement followed, probably accompanied or closely succeeded 

 by the intrusion of enormous volumes of granitic magma into the 

 overlying schists, gneisses, and sediments. Later a number of 

 basic dikes pushed their way into this batholith. No record of the 

 geologic events that followed is preserved until Pennsylvanian time. 



At the beginning of this period the present site of the range most 

 likely formed the eastern shore of a considerable land mass west 

 and northwest of it. Siebenthal, in his study of the San Luis 

 Valley, has come to the same conclusion.^ The very coarse and 

 angular basal conglomerates of the Pennsylvanian leave no doubt 

 as to the near-shore conditions that existed during their formation. 

 The deposition of the puddingstone conglomerates and breccia 

 and such bowlder beds (some bowlders with a diameter of 25 to 50 

 feet) as S. F. Emmons mentions farther north, on the east side of the 

 Sangre de Cristo Range,^ can have been brought about only by 

 talus and wash from a precipitous coast directly into deep or quiet 

 water. The fact that the pebbles of all conglomerates consist of 

 pre-Cambrian schists, gneisses, quartzites, and granites suggests a 

 land surface composed chiefly of these rocks. The enormous thick- 

 ness of the strata leads also to the conclusion that a gradual sinking 

 of the coast and progressive submergence from the east to the west 

 took place during this period. 



'C. E. Siebenthal, "Geology and Water Resources of the San Luis Valley, 

 Cole," U.S. Geol. Survey Water Supply Paper 240 (1907), pp. 50-51. 



»S. F. Emmons, "Orographic Movements in the Rocky Mountains," Geol. Soc. 

 Amer. Bull., Vol. I, pp. 245-86. 



