OF SAND FORMATIONS ON MARINE COASTS. 31 



Now the question arises: How is this phenomenon to be explained? 

 Cornish 1) refers to the development of barchanes in the following 

 words: "They form here and there upon the desert floor where the 

 wind will let them. It appears that they neither occur in localities 

 where the sheet of wind has everywhere a complete mastery over the 

 sand, nor where the burden of all the flying sand is everywhere too great 

 for the carrying power of the wind ; they dot the desert plain in localities 

 where the sheet of wind has, for the most part, the mastery of the sand, 

 but drops its burden here and there at certain points or more probably 

 along certain strips." 



"The horns or cusps of the barchanes, pointing to leeward, are readily 

 explained, for the lowest parts of the dune travel quickest. A form as of 

 the moon in her first quarter (i. e. that is to say with the cusps pointing 

 in the direction of motion) is the form of front proper to a traveling 

 sand-wave as viewed in plan. In this case gravity does not operate, so 

 that the incoherence of sand does not hinder the formation of the cusp 

 as it does in the profile of dunes." 



This explanation seems negatived by the fact that the cusps generally 

 are very insignificant as compared with the body of the dune, and in 

 most cases a difference in size of the cusps can be recognized. Thus 

 Lessar 2 ) observed in the Kara-Kum desert in Central Asia that the 

 southwesterly cusp always was longer. This seems to indicate that the 

 cusps are formed in a way similar to that of the low ridges which have 

 often been noticed on the lee side of both stationary and wandering 

 dunes on sea coasts. These lee ridges are usually placed at right angles 

 to the length of the dune and are formed by the combined action of the 

 eddy in the lee and the current sweeping around the side of the dune. 

 If we accept this explanation for the formation of cusps in lee of the 

 barchanes, the original form of which tends to the oval, according to 

 SokoloffZ), we will find a satisfactory solution of the action of wind in 

 the development of the barchane without having to theorize about the 

 lower cusps of the dune moving more rapidly than the higher, which 

 cannot be correct, as we know that the force of the wind is considerably 

 greater on the higher parts of the dune hill than on the lower, and 

 that consequently the central and highest part travels quicker. Steen- 

 strup 4) has also shown that a parabolic dune never can move with the 



1) On the formation of sand dunes, p. 290. 



2) P. M. Lessdr: Die Wtiste Kara-Kum. "Izwestija" Russ. Geogr. Soc. 

 20: p. 115. 1884. 



3) Die Diinen. 1894. p. 164. 



4) Om Klitternes vandring. 1894. p. 14. 



