736 K. F. MATHER FRONT RANGES OF THE ANDES 



more prominently cross-bedded than at the type locality. The exposure 

 in that gorge is at the point where the stream cuts across the crest of 

 the Charagua anticline. South of the Pilcomayo in the Quebrada de 

 Los Monos the upper 500 feet of the Oquita formation are much finer 

 grained than the equivalent beds to the northward. Instead of sand- 

 stones, there are very thin-bedded and finely laminated, but not fissile, 

 shales of a prevailing maroon or lavender shade. Xear the upper limit 

 of the formation, there is a dark blue to black member which contains 

 occasional lenses of blue gray, non-crystalline limestone 2 or 3 inches 

 thick. 



The Oquita formation would, therefore, appear to be the result of 

 sedimentation in a large basin to which streams were bringing material 

 chiefly from the north or northeast. Conditions of accumulation within 

 this basin must have been quite variable and may have run the entire 

 gamut from fluviatile to marine. 



Mandiyuti conglomerate. — This is the most striking formation of the 

 Bermejo series; it forms many of the rugged cliffs and much of the 

 picturesque scenery in each of the sierras and cuestas which were sur- 

 veyed. Its name is derived from the Sierra de Mandiyuti, in which 

 range it is well developed and marvelously displayed. The lavish ex- 

 posures in the canyon of Mandiyuti Creek below the falls form the type 

 locality. At this place the Mandiyuti formation comprises a pebbly 

 conglomerate member overlain by somewhat finer grained yellow sand- 

 stones, which are in turn overlain by an upper conglomerate member. 

 Each member is approximately a thousand feet in thickness. 



Elsewhere throughout the Front Kanges this formation is for the most 

 part massively bedded conglomerate and grit, but it includes also a 

 liberal percentage of sandstone and sandy shale. The majority of its 

 beds display brilliant tints of various shades of red and purple; maroon, 

 carmen, lavender, and vermilion are typical. Some of its strata, how- 

 ever, are green, gray, or white. The red tints are not alone due to 

 weathering at the surface; rather, the formation is a series of true "lied 

 Beds." 



The conglomerates and grits contain a varied assortment of pebbles 

 of all sizes and shapes. Quartzite, quartz, chert, and granitic igneous 

 rocks are the most common kinds. Most of them are well rounded and 

 polished by stream or current action, but in every locality there are many 

 angular pebbles with faceted faces. Some of these are unquestionably 

 shaped by glacial action. Most of the pebbles are small, but stones three 

 or four inches in diameter are not rare, and in certain places boulders as 

 great as 3 feet in length have been observed. The formation is evidently 



