Mr Hufjh Miller on River- Terracing. 



211 



alluvium ; secondly, a casing of boulder clay ; and thirdly, 

 the outer frame-work of rock. It is clearly impossible to 

 maintain a balance of force and resistance in any curve which 

 in its oblique course shall successively traverse them all.* 

 Hence arises that distortion of open curves into closing loops 

 and horse-shoe bends which renders the courses of some 



Fig- 7. 



Part of the Eiver North Tyne (near Chipchase), illustrating the relation of 



the river to its preglacial course, 



1, 2, Terraces ; A, Gravel banks ; B, Eecent addition to 1. 



streams so tortuous, and which often goes on until the two 

 extremities of the horse-shoe come together, and the water 

 breaks through. Hence, too, mainly arises those sudden 

 shiftings of channel which occur during floods.-|* 

 Some of these facts will be illustrated by Fig. 7. 



* It will be noticed, however, as conducing to this balance, that force and 

 resistance in such cases are both least in the alluvium of the middle, and 

 greatest in the rock at the sides. 



t The heightening of the plane of water above each curve (which in floods 

 has sometimes been observed to be, or to appear, as much as 10 or 12 feet) 

 makes flood-overflow take place first where the water is thus checked. At first 



