GEOLOGIC HISTORY 763 



the sands and muds deposited along the courses of old-age streams or 

 spread widely by currents in shallow lakes or seas. Some appear to have 

 been largely wind-blown sands. 



The close of Cretaceous time was probably marked by crustal warping 

 in the region just to the west of that under consideration and by suffi- 

 cient uplift in the locality of the Front Eanges to terminate sedimenta- 

 tion there. 



TERTIARY PERIOD 



The greater part of Tertiary time was probably involved in the accu- 

 mulation of the Tatarenda formation, the result of "the many varied 

 agencies responsible for terrestrial sedimentation. In all probability the 

 uppermost of the Tatarenda beds are to be correlated with the strata 

 reported by Berry as carrying marine Pliocene fossils, now at elevations 

 of thirteen and fourteen thousand feet, on the higher slopes of the east- 

 ern Andes. In any event, the accumulation of the Tatarenda beds was 

 stopped by pronounced mountain-making movements. These resulted in 

 the breaking and folding of the strata and gave rise to the long, narrow 

 anticlines and tilted fault-blocks of this portion of the Andes. The 

 thrust seems to have come from the west. Its effects were felt farthest 

 toward the east, in the latitude occupied by the Sierra de Charagua. 

 Both to the north and to the south the orogenic movements seem to have 

 been limited to narrower segments of the earth's crust. 



QUATERNARY PERIOD 



The Pliocene cliastrophism started a long interval of erosion which re- 

 sulted in the formation of a peneplain which may safely be referred to 

 the Pleistocene epoch. This old-age surface beveled the crests of the 

 anticlinal folds and the eastern edges of the tilted fault-blocks. The 

 erosion cycle persisted until only a few low, rounded elevations were left, 

 rising as monadnocks above the lowland plain. 



In comparatively recent time, perhaps at the close of the Pleistocene, 

 this old-age surface was uplifted bodily, so that along the front of the 

 Andes its altitude is now 1,500 feet higher than before. The uplift was 

 differential, with greater elevation toward the west than toward the east. 

 Thus the successive remnants of that quondam graded surface are now 

 found at successively higher altitudes when it is traced westward from 

 the front of the Cordillera Oriental toward its center. 



