740 K. F. MATHER FROST RANGES OF THE ANDES 



notable parallelism. They are the softest part of the rock and conse- 

 quently on weathering these slabs form pits and cavities such as those on 

 a level with the hammer in the upper part of the exposure shown in 

 figure 1(5. The third type of pebble is illustrated by the large mass of 

 Bermejo sandstone, 5 feet or more in length, in the center of this same 

 photograph. This member of the Tacuru formation is thus composed in' 

 large degree of material derived from the erosion of the Bermejo shales. 

 Presumably it was deposited by torrential streams in alluvial cones or on 

 an alluvial plain. It is, therefore, indicative of an unconformity between 

 the Bermejo and Tacuru formations, although, so far as known, these 

 l>eds are all structurally conformable with each other. 



Xo fossils were found within the Tacuru formation, and it is therefore 

 impossible to state its age with any definiteness. There is little doubt 

 but that the stratigraphic break indicated by the presence of eroded Ber- 

 mejo fragments in the Tacuru formation occurs above the Vitiacua lime- 

 stone, although this limestone is not present between the Bermejo and 

 Tacuru beds in the locality at which the conglomerate was observed. 

 Accepting Bonarellrs reference of the Vitiacua limestone and chert to the 

 late Triassic or early Jurassic period, it would appear likely that the 

 Tacuru formation is of late Mesozoic age. Unfortunately, the fossils 

 Found in the overlying Tatarenda beds to which reference will be made in 

 a subsequent paragraph are by no means diagnostic, so that it is not pos- 

 sible to place an upper time limit on the age of the Tacuru. It is improb- 

 able, however, that this formation could be younger than early Tertiary. 



The Tacuru formation as thus defined is evidently the equivalent of 

 Bonarelli's "Areniscas superiores/' to which a Cretaceous age has been 

 assigned. 12 The only fossils to which Dr. Bonarelli makes reference are 

 undescribed reptilian bones which he considers as in agreement with this 

 assigned age. 



TATARENDA FORMATION 



Overlying the resistant "hogback-maker" sandstone at the top of the 

 Tacuru formation there is a thick series of soft sandstones, shales, uncon- 

 solidated sands, and clays to which the name Tatarenda formation is 

 here applied. These strata have in general been removed from above the 

 older beds in the sierras and cuestas; they are present only beneath the 

 lowland areas and along the outer flanks of the uplands. The type 

 locality is the broad valley between the Sierra de Limon and Sierra de 

 ( 'haragua, in which the little Indian village of Tatarenda is situated. 



The Tatarenda beds are characterized by their lack of resistance tc 



B Bonarelli, 1021. pp. 77-78. 



