138 PROCEEDINGS OF THE AMHERST MEETING 



TITLES AXD ABSTRACTS OF PAPERS PRESENTED AT THE MORNING SESSION 



AND DISCUSSIONS THEREON 



FRONT RANGES OF THE ANDES BETWEEN SANTA CRUZ, BOLIVIA, AXD 



EMBARCACION, A RGENTINA 



BY KIRTLEY F. MATHER 



(Abstract) 



The following notes were obtained during a reconnaissance of the eastern- 

 most ranges of the Andes between 18 and 21 degrees of south latitude, made 

 in January and February. 1920. They are published through the courtesy of 

 Richmond Levering and Company, for whom the geologic examinations were 

 made. Only the southern half of the region traversed has been previously 

 described by geologists. 



The mountain ranges along the east front of the Andean Cordillera are 

 long, narrow, sharp-crested ridges, with altitudes of 1,500 to 2.500 feet above 

 their surroundings, which trend in a general north-south direction and are 

 separated by parallel strips of lowland. There is a marked accordance in 

 summit level along their crests from end to end and a regular increase in 

 summit elevation of successive ridges from east to west. Most of the ridges 

 are bordered on either side by steeply inclined hogbacks; others have bold 

 fault-escarpment cliffs along their eastern faces. The Sierra de Charagua, 

 F» to 5 miles wide and 80 miles long, and the Sierra de Aguarague. of similar 

 width and 170 miles in length, are the most striking of these unique mountain 

 ranges. Others which will be described in this paper, and the locations of 

 which are indicated on the accompanying map. are the Sierra de Florida, the 

 Sierra de Limon. the Sierra de Guarui. the Sierra de Mandiyuti. and the 

 Sierra de Yitiacua. 



Certain of the lowland areas between the mountain ranges are occupied by 

 groups of hills or cuestas, which ordinarily consist of several westward-slop- 

 ing hogbacks. The easternmost hogback in each series is abruptly terminated 

 by a prominent fault escarpment; the successive hogbacks generally decrease 

 in elevation from east to west. Individual hogbacks are separated from each 

 other by narrow steep-walled valleys, the eastern wall of which is almost 

 everywhere a dip slope. The most important of these groups of hills are the 

 Cuestas de Oquita and the Cuestas de Pipi. 



The larger streams of this area all flow eastward, with little apparent re- 

 gard for the present topographic features. The Rio Grande and Rio Pilco- 

 maya. for example, have their sources far to the westward, in the heart of 

 the Cordillera Oriental. Flowing eastward, they cut through the successive 

 sierras and cuestas in exceedingly rugged canyons or narrow gorges, which 

 are in sharp contrast to the broad, late-mature valleys which they occupy in 

 crossing the intermontane lowlands. 



Obviously, the present drainage system owes its pattern to inheritance from 

 a previous cycle of erosion. Either the modern stream courses were deter- 

 mined before the deformation of the sierras and cuestas or the whole region 

 has been peneplained subsequent to that folding and faulting. Several hits of 

 data lend strength to the second of the two alternatives. 



