184 ELEMENTARY GEOLOGY 
this name is given to all similar deposits at stream 
mouths. Generally, though not mvariably, the delta 
preserves a triangular form. Very often the typical 
outline is destroyed by the action of waves or cur- 
rents, particularly opposite the mouths of small streams 
entering lakes. 
Fic. 95. 
A typical delta form in Switzerland. 
Deltas are present in nearly every lake into which 
good-sized streams flow (Fig. 96); but the largest and 
most famous of the world are found at the mouths of 
rivers which enter the sea. Such mighty streams as 
the Nile, the Mississippi (Fig. 97), the Yellow, the 
Indus, etc., have immense deltas, which are steadily 
