RECORDS LEFT BY RIVERS 



smaller, and become rounded and smoothed as it goes 

 farther and farther across the plain. At many places it 

 deposits gravel or sand, more especially at the inner side 

 of the curves which the stream makes as it winds down 

 the valley. When the stream runs low in summer, strips 

 of bare sand and shingle are seen at each of these bends ; 

 and the stones are always well smoothed and lie on the 

 whole regularly. Those that are oblong are so placed 

 that the greater length of the stone points across the 

 stream ; those that are flat usually slope upstream. 

 These facts, though apparently insignificant, are really of 

 importance, because they point to us a method by which 

 geology can determine, after a river has disappeared, the 

 slope of the bed and the direction of the curves which 

 once it had. If we examine the steep banks or cliffs by 

 the side of a river the layers of gravel or shingle in the 

 strata may be found to lie not flat on one another but in 

 sloping planes. That at once will furnish a clue to the 

 direction of the river. Another thing of great import- 

 ance are the terraces which a river forms by the side of 

 itself. When it overflows in floods it deposits mud on 

 either side, and when after the flood it subsides the mud 

 is left. If the reader will imagine the river in the course 

 of ages sinking lower into its bed he will see that succes- 

 sive eras of flood-levels will leave their mark in a series of 

 steps, or river terraces as they are called, on either side of 

 the channel. 



But besides the stones and gravel and mud carried 

 down by a river, we must also consider the fate of the 

 remains of plants and animals that are swept along by it, 



55 



