DEPARTMENT OF BOTANICAL RESEARCH. 95 



An Ancient Bajada of the Great Basin Region, by E. E. Free. 



Brief announcement of certain generalizations concerning the 

 bajadas or debris-aprons of mountain ranges in the Great Basin and in 

 the arid regions generally was made in the report of this Department 

 for 1914. The essential conclusion was that these bajadas appear to 

 be composed of distinct superposed elements and to indicate a record 

 of alternating erosion and deposition around the mountain base. 

 Further work has been confirmatory of this generalization and has 

 brought out, in a striking way, the great development, wide distribu- 

 tion, and uniform character of the third, or sub-Recent, of these super- 

 posed bajadas. This surface has now been identified in all parts of the 

 Great Basin and has been traced in nearly all of the major river 

 valleys and over many of the mountain passes. It constitutes one of 

 the most important physiographic elements of the Great Basin region, 

 and its remarkable smoothness and regularity of grade suggest its 

 completion during a long period of substantially uniform conditions. 



Stream-cuttings into this ancient surface are everywhere of the 

 flat-floored, steep-sided type previously described. The base to which 

 these newer cuttings is graded seems to differ Uttle, if at all, from that 

 of the ancient surface, the difference being rather one of the curvature 

 of the gradient. This fact and the uniformity of the physiographic 

 elements over an area so wide and so varied make it impossible to con- 

 sider the physiographic changes as the result of structural movement, 

 changes in lake-levels, or other local causes. It is very probable that 

 the cause lies in climatic changes and that significant evidence regard- 

 ing the course of these changes can be obtained by closer study of the 

 physiographic elements described. 



The ancient bajada is well developed on the northern slope of the 

 San Bernardino Mountains at the southern border of the Mohave 

 Desert, southeastern California. Here the Mohave River has cut a 

 typical steep-sided channel through the beds of the ancient bajada and 

 these beds are exposed for a horizontal distance of over 100 miles. 

 An intensive study of this area is in progress, and especial effort is being 

 made to discover paleontological evidence which will enable the deter- 

 mination of the age of the ancient bajada. 



The Stages of Development of Playas, by Ellsworth Huntington. 



The temporary lakes known as playas are peculiarly well developed 

 in the area draining to Death Valley. Some are completely flooded 

 to a depth of a foot or more every year. Others contain large areas 

 which have not been flooded for centuries, and which are being eroded 

 by the wind. Some are floored with clay, and others with deposits 

 of every degree of salinity up to almost pure crystals. Two important 

 problems present themselves for solution. The first is the determina- 

 tion of the physical character of playas in various stages. In the 



