584 PROCEEDINGS OF THE SAINT LOUIS MEETING 



Session of the Cordilleran Section, Saturday, January 2 



The section was called to order at 10 a m, January 2, 1904, Professor 

 Hilgard in the chair. 



The following papers were read and discussed : 



GLACIATION ON THE HIGH PLATEAU OF BOLIVIA, SOUTH AMERICA 



BY W. G. TIGHT 



[Abstract'] 



The region under consideration includes the great elevated basin at the northern 

 end of which is located lake Titicaca and the Cordilleras Real, which form the 

 eastern wall of the basin, which are commonly known as the eastern Andes of 

 Bolivia. The western rim of the basin is marked by the high iine of volcanic 

 mountains which run in a general northwest and southeast direction, roughly 

 parallel to the west coast of South America. The Cordilleras Real were studied 

 somewhat in detail for a distance of about 150 miles in length in the region lying 

 directly east of lake Titicaca and between the high peaks of Illamani near La Paz 

 and mount Sorata. These peaks reach an elevation of over 23,000 feet above the 

 sea. South of mount Illamani the Cordilleras have been cut through by the La 

 Paz river, which rises on the western slopes of the mountains. North of mount 

 Sorata the Sorata river has also cut through the axis of the range, and extended 

 its headwaters many miles to the southward along the western side of the moun- 

 tains. The topographic features are very different on the two slopes of this great 

 mountain system, due to the fact that on the western side the baselevel of erosion 

 is the level of the Titicaca basin which is approximately 12,500 feet above the sea, 

 while on the eastern side the baselevel of erosion extends down to the low basin 

 of the Amazon. 



As a result the eastern slopes are very rough and rugged and the mountain 

 gorges of great magnitude as compared to those of the western side of the axis of 

 the system. 



The rainfall is also very great as compared to that of the western side, so that 

 all the streams on the east carry a much greater volume of water. The eastern 

 streams are encroaching very rapidly onto the drainage areas of the western 

 streams. Among these encroaching streams the La Paz river has been most active. 

 After working its way through the axis of the system its headwaters have dis- 

 covered a vast system of stratified sands, gravels, and clays, which fill the great 

 interior Titicaca basin. Into these easily eroded deposits the streams are work- 

 ing rapidly towa rd lake Titicaca. Near the city of La Paz the walls of the valley 

 stand in almost vertical cliffs, exposing the strata for a depth of almost 2,000 feet. 

 These strata are composed of stratified beds of gravel, sands, and clays. 



The clay layers are typical boulder clays of evident glacial origin. Near the top 

 of the series is a bed of boulder clay, about 250 feet thick, in which almost every 

 stone, from those a fraction of an inch in diameter up to boulders weighing thou- 

 sands of pounds, are most perfectly striated and polished. Striated pebbles and 

 boulders were found very generally distributed throughout the entire series, being 

 wanting only in the perfectly sorted and bedded gravels of the lower beds. 



