THE WORK OF STREAMS 



187 



fluences the rate of its forward growth. Furthermore, great 

 deltas are as a rule sinking slowly, and the relation of up- 

 building to subsidence varies greatly. In some cases, for 

 example the Mississippi and Ganges, rivers have built up 

 their deltas faster than the region has subsided. In other 

 cases, subsidence is so rapid as to prevent the building of deltas 

 above the sea. In the Chesapeake Bay region recent subsi- 

 dence has formed great estuaries, and the rivers are now 

 building marshy bay-head deltas. The delta of the Mississippi 

 has an area of over 12,000 square miles, and the compound 

 delta of the Ganges and Brahmaputra rivers is between 

 50,000 and 60,000 square miles in extent (about as large as the 

 state of Illinois) . As a result of long-continued subsidence and 

 up-building, delta deposits may attain great thickness. 



Ancient delta beds of great thickness, their origin revealed 

 by their structure, occur in certain localities, for example, in 

 the vicinity of Puget Sound. They afford a record of the 

 physical geography of the region at the time when the sedi- 

 ments, later changed into firm rocks, were deposited. 



Questions 



1. How could one distinguish in the field between an ancient 

 alluvial fan and an ancient delta ? 



2. (1) What occasioned the building of the fan shown in Figure 

 197 ? (2) Is the front of the fan the same as when built? (3) If 

 not, how has it been changed, and by what ? (4) Account for the 

 trench which crosses the fan. (5) How may the miniature terraces 

 within the trench be ex- 

 plained ? 



3. What are the gen- 

 eral conditions which oc- 

 casion the development 

 of distributaries ? 



4. What are all the 

 ways in which Plate IX 

 shows that the Missouri 



River is there a deposit- FlG . 197 . _ A smaU fan on tht , hp;t( . h of Lakc 

 ing stream? Michigan. 



