150 C. F. TOLMAN 
must be laboriously sandpapered to nothingness, before the second 
layer can be touched. MacDougal’ reports that wheel tracks were 
seen on his trip into Lower California, that had been made sixteen 
years ago, and that the gun ruts of the Walker Filibustering Expedi- 
tion are reported to be still visible in places after fifty years. In fact, 
a wheel track on such a pavement as is described above would last 
indefinitely according to ordinary popular measurements. The 
polishing of a horizontal surface is striking, but the erosion accom- 
plished by this action can easily be exaggerated. The wind motion 
is nearly tangent to the ground, and the sand is danced lightly off a 
flat surface. Let a projecting face withstand the wind, however, 
and it immediately concentrates its energy to the work as vigorously 
as a youthful river to an opposing hard stratum. The maximum 
work is done where a crumbly horizontal sandstone attempts to 
withstand the wind. Here no talus protects the face of the cliff, 
and the opposing strata are completely chiseled off before the blast.? 
The wind will undercut by working in the least resistant layer, 
the amount of this carving being limited by the talus from the harder 
layer above. Even the talus, however, is exposed to the windblast, 
and its removal may be assisted by torrential water, and yielding 
rapidly or slowly allows fresh attack on the cliffs. 
A study of almost any erosion forms in an arid country shows 
undercut wind action, and further description is unnecessary. The 
action being most effective against vertical faces of horizontal strata, 
the wind terraces are driven.back along the weak layers. There can 
be no question but that much of the amphitheater work of the Grand 
Canyon of the Colorado, and the level steps of the high plateaus of 
Arizona and Utah are developed by wind erosion. 
Underground water.—The depth at which underground water is 
encountered in bolsons is very variable. Considered in relation to 
surface geology, however, interest is not so much attached to the 
underground water table as to the surface moisture table. Recent 
« MacDougal, ‘‘Botanical Features of North American Deserts,” Publication 99, 
Carnegie Institution of Washington, p. 96. 
2 Whitman Cross, ‘‘ Wind Erosion in the Plateau Country,” Bull. of the Geol. Soc., 
Vol. XIX, pp. 53-62, a striking example of the above. 
3 Keyes, op. cit., pp. 81-83, 85-91, an extreme estimate of the importance of 
wind erosion. 
