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attention yesterday. As we look down into the sunlit pool we see 

 that the eddy is gone for the volume of water is not great enough 

 to cause it to revolve, but there in the rock on the bottom is a deep 

 basin-like hole. In the bottom of this liole we shall see a number 

 of well-rounded stones, with perhaps some sand and gravel. These 

 stones are the tools which, whirled about by the eddying water, have 

 cut the basin-like holes. Holes of this sort are common in rocky- 

 stream beds, especially in the neighborhood of falls or in places 

 where falls have once been ; they are called pot-holes and represent 

 another form of stream cutting. (Fig. 87.) 



86. — A brook building a delta into a lake. Formerly the brook flowed straight 

 ahead, but its own delta lias caused it to change its direction. 



Next let us visit the flood plains which we saw forming when the 

 water was high. Now we shall find the brook flowinir in its channel 

 with the flood plain deposits left high and dry. If we dig down 

 into the flood plain we shall see that it is made up of successive lay- 

 ers varying in thickness and in the size of the fragments. Each of 

 these layers represents a period of high water, and the size of the 

 fragments in the layer tells us something of the strength of the cur- 

 rent, and, therefore, of the intensity of the flood. Some layers are 



525 



