GEOLOGICAL IMPORTANCE OF SEDIMENTATION 543 



other portions of the delta may be dry and barren during a greater 

 portion of the year. The decaying material of such fresh-water 

 swamps, being preserved by the water covering, will serve to deoxi- 

 dize the iron to the ferrous state, and even if the carbon is not suffi- 

 cient in amount to color by its balance the argillaceous strata to 

 brown or black, its former presence will still be indicated by gray 

 or green bands of shales. Thus delta regions of subarid climates 

 are peculiarly liable to be forming deposits which will ultimately 

 become variegated shales, in which maroon, deep red, or vermilion 

 bands will pass, sometimes almost without change of texture, into 

 bands of grayish-white or green. An example of such variegated 

 strata recently made is described by Huntingdon as having formed 

 in the basin of eastern Persia and Sistan.^ 



The seaward portion of the delta surface is. also frequently covered 

 between the distributaries by brackish or salt-water lagoons and 

 bays, as in the Nile and Mississippi deltas, protected from the waves 

 and possibly containing considerable life of estuarine types, whose 

 decay will lead in the same manner to variegated shales. 



In truly arid climates, however, such river or sea lagoons are the 

 seats of progressive evaporation giving rise to such salt pools as 

 front the northern portion of the Caspian Sea or the recent gypsum 

 deposits of the Isthmus of Suez. The degree of aridity and of the 

 severance of the lagoons from the sea will determine the kind and 

 amount of the chemical precipitation. It would seem, therefore, 

 that the mud-cracked red beds originating on the delta surface of an 

 arid climate should frequently be interstratified with mud-cracked 

 beds holding salt or gypsum, a less arid condition leading more 

 usually to the production of variegated shales. 



MUD-CRACKS OF THE LITTORAL ZONE 



Discussion as to present origin. — The littoral zone is one of the 

 most sharply delimited of the natural physiographic divisions, 

 forming a narrow belt between the sea and the land and defined 

 here as comprising the zone between the average highest and lowest 

 tides of the month. To form mud-cracks the deposit must be exposed 

 to the sun or air sufficiently long to be dried out to such a depth 



I Carnegie Institution Publications (1905), pp. 285 ff. 



