436 J. BARRELL RECOGNITION OF ANCIENT DELTA DEPOSITS 



which imply aridity from those other deposits where the wind lias been 

 merely a cooperating factor and has but minor climatic significance. 



COMBINATIONS OF WIND AND WATER ACTION 



Kinds of coinhined structures and textures. — The character and occur- 

 rence of such structures have been discussed elsewhere^^ and they need 

 here be enimierated only. First, dune structures and dune textures pre- 

 vail over the delta of the Indus, the material being derived from the 

 river sands ; second, facetted pebbles in sand deposits are common in the 

 Great Basin of the United States from the fluviatile intermixture of 

 sand and gravel, the latter being then subjected to wind scour \)\ the 

 sand; third, somewhat etched pebbles and millet-seed sand grains show 

 a lesser degree of the same combined action in formations which are 

 dominantly fluvial or pluvial; fourth, subangular pebbles, showing a 

 dominance of disintegrating rather than decomposing activities in 

 weathering, and exhibiting, furthermore, a lack of sorting in transporta- 

 tion characterize the local alluvium of semi-arid to arid climates, such 

 as in the alluvial fans of the mountainous parts of Arizona and Xew 

 Mexico ; fifth, mud cracks made by the drying out of the floodplain clays 

 may be filled by wind-blown sand and have been recorded as existing at 

 the present time in both South Africa and South America. 



Relative association of eolian action with fiuviatile and marine de- 

 posits. — Dune structures bespeak a dominance of wind action, but in 

 ancient formations appear to be relatively rare. Much more usual are 

 the other marks of the wind which are subordinate to wave or river 

 action. The more common of these are seen in the unsorted wash de- 

 posits, marking the local deposits of basins, in which wind may have 

 played a very minor part ; and tlie mud cracks filled l)y wind-blown sand 

 extensively developed over semi-arid or arid floodplaiiis. These struc- 

 tures in their modern exami)les are ty])ically associations of river and 

 wind action rather than shore and wind action, and are not by any 

 means restricted to truly desert climates. 



Those combined structures which are now found to an appreciable 

 extent associated with the margin of the sea are the millet-seed texture 

 and dune structures. The latter, however, are restricted to limited belts 

 of what are chiefly submarine sands. Since the form of the sand grains 

 persists after the dunes are destroyed, it is a much more pervasive plie- 

 nomenon and is the feature which is more likely than the others to be 

 developed in sands associated witli the ancient epicontinental seas. 



^.Joseph Barrel! : Relations between climate and lorreslrial deposits. Journal of 

 Geology, vol. xvi, 1908, pp. 279-284. 



