224 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Bonneville Bay-deltas. — Almost all the streams entered lake Bouue- 

 ville through momitain gorges, aud the detritus of the Bonneville epoch 

 was deposited iu these narrow estuaries forming bay-deltas, the water 

 at the Bonneville level having entered the previously eroded valleys. 

 When the water fell the detritus was carried away, leaving no deposit 

 to mark the Bonneville shoreline in these larger streams.* 



14. Deltas. 



Ratio between Sea and River Activities. — The shore changes caused 

 by delta growth depend on the ratio between sea and river activity more 

 than upon any other factor ; and deltas would therefore be typically 

 developed either after uplift or depression. The area of a delta is a 

 measure of time since the beginning of a cycle. A large river will soon 

 build a great delta, a precocious infant ; but the delta will attain its 

 maximum area at some period in late adolescence or maturity, after which 

 the delta will diminish in area by the degrading of the river. A smaller 

 stream has a similar maximum area, though its dimensions at any given 

 stage are always less. 



The life of a river is in a sense to be considered apart from the cycles 

 of coastal plain development and also as distinct from the other shore 

 changes, though its life is intimately connected with and a most important 

 part of both sets of processes. The river's aim is to convey the load 

 given it by the laud to the sea. Of itself it would build forward a lobe 

 for each distributary, the shifting of these distributaries on account of the 

 upbuilding causing in time a broad fan-shaped deposit, so well shown in 

 the confluent delta of , the Hoang and Yangtze rivers. 



The sea, on the other hand, desires a straight shoreline. The delta 

 intention is opposed to its attack upon the land, and therefore the sea 

 aims to cut off the front of the lobes and carry the delta waste out to 

 sea, depositing it beyond wave attack or below wave-base. 



The form of a delta front does not indicate sharply the stage of the 

 cycle in which a given re<rion stands. The relative strength of sea and 

 river may cause a given form of delta front at many stages in the life 

 history of a region. The river activity is increasing from birth toward 

 maturity, so that in the case of any given river there will come a time of 

 maximum activity, when the river will be best able to push forward and 

 build a lobate delta into the sea. This time, however, may not be the 

 time when there is the greatest likelihood of the formation of such a 



* Gilbert, Lake Bonneville, 154. 



