6o8 



PRINCIPLES OF STRATIGRAPHY 



maputra and the Indus, on the Indian Ocean ; of the Niger and the 

 Orinoco, on the Atlantic Ocean ; and the Mississippi delta on the 

 Gulf of Mexico. Of the numerous deltas on the protected waters 

 of Europe and Asia may be mentioned that of the Danube on the 

 Black Sea, the Volga on the Caspian, and those of the Oxus (Amu- 

 darja) and the Jaxartes (Syr-darja) on the Aral Sea. 



Form and Rate of Grozvth of Deltas. The form of the deltas 

 varies greatly from the typical triangular outline resembling the 

 Greek letter delta (A) characteristic of the Nile delta (the type 

 of deltas; Fig. 125) to the long, narrow estuarine filling of the 

 Mackenzie mouth, on the one hand, and the very broad, but short, 

 cuspate delta of the Tiber, or the still narrower strip-like or stunted 

 delta formed by the Cavonne on the Gulf of Taranto, southern Italy, 



on the other. The mouth of the delta-building river may advance 

 singly (unilobate) without dividing into distributaries, as is nearly 

 the case with the Ebro on the northeast coast of Spain, or it may 

 be multilobate with the distributaries pushing each its own narrow 

 lobe forward, which may even become a finger-like extension, as 

 in the remarkable Mississippi delta. If the distributaries are nu- 

 merous, they may form a network of streams, as on the Nile delta, 

 which advances by a continuous, more or less scalloped front. 



In size the deltas of the present day vary from an insignificant 

 deposit at the mouth of a small stream to areas covering many 

 thousands of square miles, as in the deltas of the Nile, the Lena, 

 and the Mississippi. Confluent deltas of several streams occur, 

 making irregular deposits with many lagoons, as in the Rhine- 

 Meuse-Ems delta ; while deltas building on a coast with many 



