180 ELEMENTARY GEOLOGY 
the river from overflowing it and submerging the culti- 
vated sections. 
Terraces. — TJerraces are narrow plains parallel to 
the river, ending in an abrupt descent on the side 
toward the stream. Sometimes the river flood, by 
reaching different heights with the varying volume of 
water, builds such narrow plains on either side of the 
river at  dif- 
ferent levels. 
While some 
river terraces 
are made in 
this way, many, 
and perhaps 
most, have been 
caused by exca- 
vation (Fig. 
93). A. Tiver 
in cutting down 
Fig. 92. 
River overflowing its floodplain, New Albany, Indiana. 
through clay or gravel, changes its course by meander- 
ing, and is now on one side of the valley and now 
on the other (Figs. 71 and 79). So in excavating it 
leaves a series of terrace steps. 
Deltas. — Where a river enters a lake, its flow is 
suddenly checked, and the sediment cannot be carried 
far beyond the mouth. This causes a deposit to be 
commenced beneath the lake waters, which im course 
