164 ELEMENTARY GEOLOGY 
almost any stream bed one may see the variations and 
understand their cause. 
Again, the work of valley deepening is almost con- 
fined to the narrow line occupied by the stream, for 
the erosive action of the river itself is mainly confined 
to its bed. It does a little more than this, for every 
Fie. 79. 
A stream, by meandering, cutting into its bank, so that it is necessary to 
protect it by posts. 
stream meanders about, and in the course of time, 
may abandon one channel and pursue another. A raft 
of logs or a small avalanche may be the means of send- 
ing it over against the opposite bank. So it changes 
about, now eating at the base of a cliff on one side, 
and again, having abandoned this course, it cuts at the 
