RIVER AND LAKE DEPOSITS 181 
of time, grows upward to the very surface, when it be- 
comes a plain. As additional deposits are made, they 
cause this delta plain to grow outward into the lake. 
In some places lakes have been entirely filled by this 
action, while others have been separated by the growth 
of deltas across them (Plate 8). 
This deposit is called a delta, and over it the river 
Fig. 98. 
Terraces of excavation in the Madison valley, Montana. 
flows, often by several mouths. This is due to the fact 
that the delta is a nearly level plain, over which the 
river water cannot all flow through a single channel, 
as it does where the slope is more abrupt in the ordi- 
nary valley above. 
As the floods of the river overspread the delta 
(Fig. 94) through the various mouths, and deposit 
sediment upon it, both the elevation of the delta and 
