I. Bowman — Physiography of the Central Andes. 393 



spite of the dissection of mature slopes everywhere so prom- 

 inent, the streams of many of the basins and valleys are not 

 flowing upon rock but upon alluvium. An episode has occurred 

 which for a time greatly decreased the dissection of the 

 lower slopes. That episode is glaciation and its effects are 

 now expressed by moraines, hanging valleys, striated surfaces, 

 and valley-head cirques in the mountains ; and by alluvium in 

 the basins and valleys.* At present the streams are actively 

 removing the alluvium of the valley floors and deeply trench- 

 ing the alluvial fans of the valley sides. The map of a three- 



FiCx. 25. 



Fig. 25. Cliza river and terraces at Cliza, Bolivia. 



mile stretch of the Cliza river, fig. 25, likewise shows dissection 

 by the succession of terraces bordering the basin streams, and 

 due to the normal sidewise swinging of the river in the down- 

 cutting accomplished since the glacial period. The sketch, 

 fig. 26, shows typical relationships of uppermost peneplain, 

 lower mature slopes, later dissection, partial valley filling, and 

 the renewed dissection of post-glacial time, the dissection now 

 in progress. 



*A paper on "The Glaciation of the Central Andes" will be published 

 elsewhere at an early date. 



