PHYSIOGRAPHIC AND GEOLOGIC DEVELOPMENT 265 



Fig. 178 — Plan and cross-sections of superimposed sand dunes of conventional 

 outline. In the sections, dune A is supposed to have left only a small 

 basal portion to be covered by dune B. In the same way dune G has advanced to cover 

 both A and B. The basal portions that have remained are exaggerated vertically in 

 order to display the stratification. It is obviously not necessary that the dunes should 

 all be of the same size and shape and advancing in the same direction in order to 

 have the tangential relations here displayed. Nor need the aggrading material be 

 derived from true dunes. The results would be the same in the case of sand drifts with 

 their associated wind eddies. All bedded wind-blown deposits would have the same 

 general relations. No two successive deposits, no matter from what direction the 

 successive drifts or dunes travel, would exactly correspond in direction and amount 

 of dip. 



