264 THE ANDES OF SOUTHERN PERU 



beds upon whose truncated outcrop other inclined beds are laid, 

 Fig. 178. The contrast to these conditions in the case of aggrada- 

 tion by water is so clearly and easily inferred that space will not 

 be taken to point them out. It is also true as a corollary to the 

 above that the greater part of a body of wind-drifted material 

 will consist of cross-bedded layers, and not a series of evenly 

 divided and alternating flat-lying and cross-bedded layers which 

 result from deposition in active and variable currents of water. 



The caution must of course be observed that wind action and 

 water action may alternate in a desert region, as already de- 

 scribed in Tarapaca in northern Chile, so that the whole of a de- 

 posit may exhibit an alternation of cross-bedded and flat-lying lay- 

 ers ; but the former only are due to wind action, the latter to water 

 action. 



Finally it may be noted that the sudden, frequent, and diversi- 

 fied dips in the cross-bedding are peculiarly characteristic of wind 

 action. Although one sees in a given cross-section dips apparently 

 directed only toward the left or the right, excavation will supply 

 a third dimension from which the true dips may be either ob- 

 served or calculated. These show an almost infinite variety of 

 directions of dip, even in restricted areas, a condition due to the 

 following causes : 



(1) the curved fronts of sand dunes, which produce dips con- 

 centric with respect to a point and ranging through 180° of arc; 

 (2) the irregular character of sand dunes in many places, a con- 

 dition due in turn to (a) the changeful character of the strong 

 wind (often not the prevailing wind) to which the formation of 

 the dunes is due, and (b) the influence of the local topography 

 upon wind directions within short distances or upon winds of 

 different directions in which a slight change in wind direction 

 is followed by a large change in the local currents; (3) the fact 

 that all combinations are possible between the erosion levels of 

 the wind in successive generations of dunes blown across a given 

 area, hence any condition at a given level in a dune may be com- 

 bined with any other condition of a succeeding dune; (4) varia- 

 tions in the sizes of successive dunes will lead to further contrasts 



