RIVER EROSION 163 
Valley Deepening. The kind of rock over which the 
river flows, is another cause for variation in the rate of 
valley deepening. Other things being equal, a stream 
wears a soft stratum more easily than a hard one. The 
stream does not cut in all parts of its bed with equal 
ease, but may be engaged in building a bar in one place, 
Fig. 78. 
Pot-hole being scoured out by stream action. 
while just above or below this it is eroding a deep 
gorge. In the beds of rivers we often find saucer or 
kettle shaped holes, commonly called pot-holes (Fig. 78). 
These are formed where the current favors rapid deep- 
ening by constantly whirling pebbles around, and thus 
grinding out saucer-like depressions. The reasons for 
these differences in cutting, are many and local, and in 
