RIVER AND LAKE DEPOSITS 185 
and even rapidly growing outward. On the other 
hand, many large rivers, such as the Amazon, have no 
deltas. 
In some cases the absence of deltas is due to the 
fact that ocean waves, tides, and currents, remove the 
sediment about as fast as it is brought to the sea, and 
: al 
e 
a 
Fig. 96. 
A delta at the head of Lake Cayuga upon which the town of Ithaca, N. Y. is built. 
(See also Fig. 94.) 
hence prevent its accumulation near the river mouth. 
In other cases this absence is due to a general lack of 
sediment transported by the streams. If the sea 
reaches a great depth opposite the mouth of the river, 
the development of a delta is also retarded; and in 
many cases, one of the chief causes for the absence of a 
