714 



PRINCIPLES OF STRATIGRAPHY 



gently rounded or convex ridges often with a groove on top, and 

 separated by sharp depressions. This mold is distinguished from 

 the normal current ripple mainly by its symmetry. In the desert, 

 wind ripples of all dimensions and of great variety of form occur. 

 They vary in width from 2 cm. or less to a meter or more. Some- 

 times they are sharp angled, at others rovmd. They generally 

 occur in long parallel series, often branching repeatedly, the 

 branch running parallel to the main stem. (Walther-22 : 5^5 [^7P] •) 

 CFigs. 140, 142.) 



The width and height of the ripple-mark are dependent on the 

 size of the grain of sand and the strength of the wind. Uniform 

 currents will produce an elongation of the ripple which always 

 extends at right angles to the direction of the wind. (Hunt-14.) 

 In many respects the arrangement of wind ripples and of sand 



Fig. 142. Plan of Eolian ripple-marks in coarse desert sand. (After Wal- 

 ther.) (Compare with Fig. 140.) 



dunes in deserts is very similar, the former being reduced replicas 

 of the latter. 



In the distribution of the material of the wind ripple, there 

 appears also to be a distinctive character, in that the coarser mate- 

 rial is found on the crest of the ripple, instead of in the trough, as 

 in subaqueous ripples. 



II. Impressions of Animals and Plants in Transit. Ani- 

 mals walking or crawling over a surface of mud or sand commonly 

 leave characteristic impressions. Plants rafted along by the wind 

 or waves, and medusae floating and dragging their tentacles, also 

 make characteristic markings, though these are not always readily 

 determinable. All such markings are depressions, and they will 

 be reproduced in relief on the under side of the next overlying 

 stratum. Occasionally the burrows of certain animals will be 

 marked by elevations or knobs. These structures will be dis- 

 cussed in their biological relations in the chapter on fossils, and 

 they are mentioned here only to complete the survey of minor 

 original features of sediments. 



