TOPOGEAPHIC FEATURES OF THE DESERT BASHsTS. 23 



THE BASINS OF THE LAVA PLATEAU. 



The eastern two-thirds of Oregon and the southeastern quarter of 

 Idaho, with contiguous portions of Nevada and California, are covered 

 by great sheets of Tertiary lavas. In its northerly portions the 

 plateau thus formad, though considerably dissected, is substantially 

 level, but its southern portion has been invaded by the area of uplift 

 and faultmg which created the valleys of the Great Basin, and has 

 been split into a number of valleys and ranges of purely structural 

 origin. In the main there has been little flexure and the faulting is 

 usually of a simple monoclinal type. As before, the main lines of 

 displacement run north and south, but there has been a significant 

 degree of irregular movement along lines otherwise directed, and the 

 valleys of the region possess only in lesser degree the simplicity and 

 regularity of structure characteristic of the trough valleys of central 

 Nevada. The topography being dependent on the monoclinal struc- 

 ture, is everywhere much the same. The valleys are long and rela- 

 tively narrow, with a gentle, somewhat dissected slope on one side 

 and a steep fault scarp on the other. 



Many of the valleys of this region possessed from the beginning an 

 open drainage to the sea or soon attamed it through the breaching of 

 the surrounding divides. (PL II, fig. 1.) Many of these still retain 

 this open drainage or have lost it only recently. However, the por- 

 tion of the area contiguous to the Great Basin proper has been, like it, 

 a region of low and topographically insufficient rainfall and many of 

 its valleys have never had a seaward drainage. All of the valleys 

 which are now areas of inclosed or restricted drainage are briefly de- 

 scribed below. The Honey Lake Basin, described among the Lahontan 

 group, is not essentially dissimilar to the basins of the lava plateau, 

 and owes its connection v/ith the larger group to the chance occur- 

 rence of a low pass leading thereto. 



THE CHRISTMAS LAKE VALLEY. 



The Christmas Lake Valley is the extreme northwest basin of the group and is per- 

 haps the least typical of all. It lies about at the extremity of the region of profound 

 monoclinal faulting and is characterized more by gentle folding and by minor and 

 irregular displacement than by the well-defined fault lines so prominent to the south. 

 The basin is bordered on all sides by rolling plateaus formed by gentle folding of the 

 lava and modified by a comparatively slight subsequent erosion. Undoubtedly these 

 rolling plateaus once possessed more or less well-defined drainage systems, but 

 increased desiccation has entirely destroyed them or reduced them to mere vestiges. 

 The whole region is now one of no determined drainage. This makes it nearly impos- 

 sible to fix accurately the boundaries of the basin. On all sides the plateau is dotted 

 with innumerable small pans or playas each of which receives and retains the drainage 

 of a greater or lesser surrounding area. (PI. II, fig. 2.) Most of these small basins rep- 

 resent irregularities in the folding of the plateau and are therefore structural and 

 original, but there can be little doubt that nearly if not quite all of them once over- 

 flowed either inward toward the Christmas Lake Valley or outward into the surround- 



