186 ELEMENTARY GEOLOGY 
delta, is the fact that the ocean bottom near the river 
mouth is sinking, so that the rate of sediment deposit 
is not able to raise the bottom as fast as it settles, even 
though the river is large and muddy. 
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Fig. 97. 
A map of the Mississippi delta. Lands subjected to overflow marked by dashes. 
In some cases, as for instance along the coast north 
of Florida, the rivers, instead of entering the sea over 
deltas, have their mouths in bays or estuaries. This is 
due to the fact that the land has been so recently low- 
ered that the ocean enters the river valley (Plate 13). 
