SAND DUNES 555 



ter bodies, passive influence, or receptive waters is not necessarily 

 involved in this concept. Irregular stratification and cross-bedding 

 of a complex type may, however, be regarded as one of the most 

 characteristic structural features of the coarser eolian deposits. 



Eolian cross-bedding is especially characterized by discontinuity. 

 The successive cross-bedded layers or wedges are limited above and 

 below by erosion planes, which are of contemporaneous origin. 

 While deposition goes on in one section of the deposit, erosion of a 

 previously formed one in part supplies the material, or alternate 

 deposition and erosion may characterize any portion of the series. 

 Furthermore, as will be more fully discussed in the summary chap- 

 ter on these structural features, the varying currents will result in 

 the inclination of the cross-bedding in different directions in the 

 successive sections. Horizontal beds will be rare or wanting, a 

 feature readily distinguishing this type from the diagonal bedding 

 due to torrential action. Finally, as Huntington (25) has insisted, 

 the tangency of the inclined beds with reference to the erosion sur- 

 face below, or their gradual bending toward parallelism with this 

 plane as they approach it. is one of the most reliable criteria by 

 which eolian deposits can be recognized. 



Sand Dunes, Their Origin and Form. 



Wherever along the beach the sun has dried out the sands, 

 they have lost the coherency which they possessed when saturated 

 with water, and are then readily blown about by the winds. As a 

 result, sand dunes are formed along most coasts, these dunes grad- 

 ually traveling inland, and burying forests, fertile lands and build- 

 ings unless their landward march is checked by a barrier, either 

 natural or artificial. The distance to which wind-blown material is 

 carried depends chiefly on the fineness of the material, its specific 

 gravity and the strength of the wind. Examples of such transpor- 

 tation have been given in Chapter H. 



The sand particles move under the direct influence of the wind, 

 and their rate of movement depends on their size and the strength 

 of the wind. According to Sokolow (48: /j), when the wind 

 velocity is 4.5 m. per second, quartz sand grains of 0.25 mm. diame- 

 ter will glide only along the surface of tlie ground without rising 

 freely above it. With a wind velocity of 15 meters per second, 

 however, sand grains of i mm. diameter will be raised some dis- 

 tance into the air. With the wind of no great strength and the sand 

 not very fine, movement of the grains is along the surface of the 



