limestone 
8 Soo 
os 2 $ %00 
ca # %09 pion Ea 
a0 pea 
Fig. 3. 
The process of formation is analogous to that of a mill pond in a 
stream. Instead of there being a damming up of water, there is ponding, 
as it were, of silt and degradational material brought down by the stream, 
and so a local flood plain is formed. In these coves the streams have 
every appearance of old age, with meanders especially well developed. 
The are flood plains, but not flood plains as we usually think of them— 
that is, the ones produced by material deposited during times of overflow. 
They are formed largely as a result of lateral planation. The impor- 
tance to agriculture of this process whereby a stream works back and 
forth over a tract of ground, can not be overestimated. To-day, one of 
the fundamental principles of agriculture is that of rotation of crops. 
When man, in his ignorance or negligence, will not rotate his crops, 
nature rotates the ground and thus guards against the inevitable drain 
on the capacity of the soil. In the case of the Nile, a fresh layer of 
