DEPARTMENT OF BOTANICxlL RESEARCH. 69 



Superposed Bajadas in the Great Basin Region, by E. E. Free. 



It is customary to regard the bajadas or bordering detrital aprons of 

 the arid-region mountains as smoothly graded slopes produced by the 

 slow and uniform accumulation of mountain debris. Closer study 

 upsets this conception. In nearly every case deeply cut washes and 

 isolated erosion remnants indicate that the slopes have been regraded 

 more than once and that bajadas of different age have been superposed. 

 It appears that these superposed bajadas correspond to the alluvial 

 terraces and hanging deltas of the mountain valleys described by 

 Huntington^ as the result of climatic fluctuations. The superposed 

 bajadas are at least four in number and appear to possess certain 

 individual characteristics which are quite persistent over the whole 

 Great Basin region. For instance, the third of the series (the sub- 

 Recent one) forms the main body of the detrital slopes. It is furrowed 

 by wide, flat-bottomed washes, almost always with an intermediate 

 erosion terrace not far below the top. These washes debouch onto the 

 lower, flatter, and more recent bajada, which is also cut by washes, but 

 of a different type, being deeper, narrower, and without the intermediate 

 terrace. These two younger bajadas are entirely slopes of aggradation. 

 The two older bajadas are represented by remnants only and these are 

 frequently rock in place, and have resulted from erosional planation 

 rather than aggradation. The chief interest of these and other indi- 

 vidual characteristics of the bajadas lies in their persistence in many 

 parts of arid North America under widely varied local conditions, and 

 it seems not impossible that closer study of these characteristics will 

 yield important evidence as to the post-glacial climate of the region. 



ANALYSIS OF THE EFFECT OF CLIMATIC COMPLEXES. AND SEPARATE 



EXTERNAL FACTORS. 



The number of species of plants which have been found to endure 

 the widely varying climates of the Montane plantation (8,000 feet), 

 Xero-montane plantation (5,400 feet). Desert Laboratory (2,300 to 

 2,700 feet), and of the Coastal Laboratory station was noted last year 

 as under a score. The total census of these plantations, however, 

 now runs nearly to a hundred, and twice as many have been dealt with 

 in securing this number of survivals. The most notable fact that has 

 come out of this work so far is that although a majority of the species 

 have matured seeds or perfected propagative bodies, not one has yet 

 been disseminated beyond the limits of small plantations in which they 

 have been placed. A general record receives all of the climatological 

 and ecological facts of importance in the history of the plantations, and 

 the history of all material is made available to the collaborators who 

 undertake problems and use this material for experimental purposes. 



^Especially, Bull. Geol. Soc. Amer., 18, 351-388 (1907); Carnegie Inst. Pub. 192, 23-36 (1914). 



