30 GENESIS AND DEVELOPMENT 



dune strata, cut in all the four directions of the compass, can be seen. 

 Successive layers dip in various directions, and are abruptly cut short, 

 showing that the growing dune hill was partly cut down by storms and 

 was again and again built up by such disasters. The consolidation 

 of sand is best observed in ripples and rarely well shown in dunes, 

 because the latter are the result of changing winds, and the time 

 involved in their formation is too great for observation. 



The fundamental forms of sand dunes include the transverse and the 

 longitudinal. The former, which is the most common on sea coasts, 

 especially where the wind is of moderate strength or the sand strip 

 comparatively narrow, is that placed at right angles to the prevailing 

 wind; the latter, formed where the wind is so strong as to prevent free 

 lateral growth, is that following the direction of the wind; between 

 these two there is an intermediate form; when varying winds act upon 

 this latter form, conical dunes are produced. They are, as a rule, 

 stationary, while the longitudinal form represents the most rapidly 

 traveling dune. 



The dunes which are placed parallel to the direction of the prevailing 

 winds have originated quite differently from those which are placed at 

 right angles to the wind. As we have seen, the usual mode of develop- 

 ment of a dune is that the sand forms a ridge transverse to the direction 

 of the wind. The sand is blown up on the lower slope on the front, and 

 when it reaches the top it falls down along the lee slope; the ridge 

 growing until it has reached considerable height. The parallel longi- 

 tudinal dunes are, however, formed through the central part of the dune, 

 being blown further and further forward, while the ends are kept back 

 by various forces. The rule is that such a horseshoe-shaped or parabolic 

 dune on the seashore moves with the convex side in the direction of the 

 prevailing wind. 



Apparently diverse, even opposing effect is produced in sandy deserts, 

 if the observations of Rolland and many other travellers regarding the 

 dunes called Barchanes are correct. In Traite de Geologic of Lapparent, 

 3 Ed., 1893, p. 140, we find the following opinion expressed about these 

 wandering dunes: "Enfin la forme de dunes en marche doit etre 

 generalement celle d'un croissant tournant sa convexite vers le vent; 

 par les particules sableuses, ayant moins de hauteur a franchir sur les 

 bords de la dune qu'en son centre, eheminent plus vite a droite et a 

 gauche. La crete doit done se courber en protectant deux pointes vers 

 l'interieur. Cette forme en croissant a ete bien constatee par tous les 

 voyageurs qui ont parcouru le Sahara et les deserts americains." 



