GLACIATION ON THE HIGH PLATEAU OF BOLIVIA 585 



At about midway in this series of glacial deposits as exposed in the valley near 

 La Paz is a bed of volcanic tuff some 20 to 60 feet in thickness. As there are no 

 volcanic rocks known in the Cordilleras Real in this region, it is evident this tuff 

 must have come from some of the volcanic cones on the western side of the plateau 

 basin, some 60 miles distant. At a short distance below the tuff there is a wel 

 marked soil zone, very rich in organic matter. Out of these evident glacial deposits 

 is cut the great valley of the upper La Paz river. A few miles above the city of 

 La Paz, in the La Paz River valley, there is developed a massive series of moraines, 

 both lateral and terminal, filling the valley to an elevation of between 300 to 400 

 feet. These moraines now stand at an elevation of about 12,500 feet above the 

 sea and are of the characteristic valley type. The floor of this old valley glacier 

 is well marked, and where it reveals the rocks near its upper portion the rocks are 

 perfectly striated and polished. Following up this old glacial floor into the higher 

 regions the old cirques are very clearly shown, but do not carry perpetual snows 

 at the present time. The present glaciers are confined to the minor high moun- 

 tain gorges and valleys which feed into the old cirques. The lower limit of the 

 present glaciers is now about 17,030 feet above the sea. The upper portion of the 

 valley of the La Paz is cut partly in the glacial series of the Titicaca basin and 

 partly in the shales and slates forming the mountain axis, but the valley is quite 

 uniform in its characteristics and bears unmistakable evidence of being entirely a 

 valley of* river erosion, and the ancient glacier which occupied it produced very 

 little effect on the general form of the valley. In the valley of the Sorata there 

 was a great valley glacier, similar to the one in the La Paz valley, which extended 

 down to at least 7,000 feet above sealevel. Evidence of these valley glaciers ap- 

 pears on most of the mountains of the plateau, on both the east and west sides of 

 the Titicaca basin. Many of these mountains, whose summits do not now rise 

 into the region of perpetual snows, have a system of very marked moraines on 

 their flanks extending down to a level of the floor of the Titicaca basin. 



The glacial succession in this region can easily be divided with three major 

 divisions with well marked interglacial intervals. The first period was evidently 

 of greatest duration and extent. The interglacial interval between the second 

 and third periods is marked by the erosion of the great valley of the La Paz and 

 Sorata rivers. .Since the last period the glaciers have receded from an elevation 

 of abont 8,000 feet to 17,000 feet above the sealevel. 



Extending southward from lake Titicaca for a distance of about 600 miles is an 

 extensive plain bordered on the east by the Cordilleras Real and on the west by a 

 long chain of volcanic mountains, several of which show still some activity. The 

 plain is very level, with many mountain cones rising abruptly from its surface, 

 showing that the plain has been built up around the mountains, burying the an- 

 cient topography. The material of the plain is the finest silt, such as would be 

 carried in suspension in water. Scattered over the surface are numerous shallow 

 salt lakes and the large fresh water lake Poopo, which receives the discharge from 

 lake Titicaca, through the Desaquadero river. Lake Poopo has an underground 

 outlet to the Pacific. 



Over large areas in the southern portion of this plain there is a layer of salt 

 varying from slight incrustation to 2 feet in thickness. In certain more isolated 

 embayments of the plain are extensive borax deposits. 



On all the mountain slopes bordering the p^ain and also encircling the moun- 

 tains which rise out of the plain there is a series of three well marked beachlines, 



