558 HEALD AND MATHER RECONNAISSANCE OF EASTERN ANDES 



At many localities, especially within a few miles of the mountain 

 front, there are numerous low hillocks which were undoubtedly once 

 sand-dunes, but whose migration has been stopped by the growth of 

 grass and trees. Between these irregular mounds there is. little drain- 

 age and in rainy weather there are many shallow ponds without outlets. 



Stratigraphy 



character of the bocks 



The rocks in this region are dominantly sandstones and shales. They 

 range in age from Devonian to Pleistocene and have an aggregate thick- 

 ness of at least 30,000 feet, and perhaps double that. Xo intrusive 

 rocks were seen, nor was there evidence in the stream gravels that any 

 considerable body of such rocks exists in the general region passed 

 through. The oldest rocks in the stratigraphic section examined are 

 metamorphosed to a certain extent, but the metamorphism is com- 

 paratively slight. Some of the sandstones are quartzitic, but interbedded 

 shales are not greatly indurated. Shales predominate throughout the 

 section, although there are also great thicknesses of sandstone, with 

 minor amounts of conglomerate, tillite, and limestone, and also surficial 

 deposits, such as stream gravels, sands, and clays. 



ARAN I FORMATION 



The oldest rocks in the section studied were called the Arani forma- 

 tion by the writers because of their excellent exposures in the abrupt 

 slopes that rim the Arani Basin. A small collection of brachiopods 

 obtained from a quartzitic sandstone some two miles north of the town 

 of Arani indicates that these beds are Devonian in age. The entire 

 thickness of the formation could not be estimated. It is known to exceed 

 3,000 feet and probably totals three or four times that thickness. It is 

 composed of quartzites, quartzitic sandstone, and indurated shales. The 

 quartzites commonly weather to a rusty reddish color, although freshly 

 broken surfaces are dark gray or bluish gray, with occasional pinkish 

 tints. They are extremely hard, dense, and break with a conchoidal 

 fracture. Individual beds range in thickness from a few inches to 7 or 

 8 feet, with an average less than a foot. 



The quartzitic sandstones are similar in appearance to the quartzites, 

 but are less dense. Many of the sandstone beds show spheroidal con- 

 cretionary structures, which weather into concentric circles that may 

 be as much as 6 feet in diameter. 



The shales are commonlv brown or reddish brown on weathered sur- 



