Io6 THE GLACIAL PHENOMENA OF THE COUNTRY 



leaves the lake. A little consideration shows that in the case 

 of a parallel sequence of swires a simple numerical relationship 

 should exist between the depths of the swires and the heights 

 of their intakes ; this relationship is : the height of the in- 

 take of a swire phis the depth of the swire near that point,* 

 equals the height of intake of the next swire higher in the 

 series. 



This is illustrated by the following section which is drawn 

 (only roughly to scale) along the Dinnington Ridge from West 

 Heddon to Prestwick Pit Houses near Woolsington, that is in 

 a line running E.N.E. along the water parting of the ridge. 



Fig. 5. — Section along the Dinnington Ridge from West 

 Heddon to Woolsington (left to right in diagram). 



The ground slopes in the main from west to east, and the ice- 

 sheet, retreating in the same direction, has allowed the Pont 

 Lake to drain across the ridge, cutting channels at con- 

 secutively lower levels. The first channel (on the left in the 

 section) was cut while the ice retreated from A to a position B, 

 at which the ridge was uncovered at a level just below that of 

 the bottom of the first notch. Similarly during the retreat of 

 the ice from B to C, and C to D, the second and third notches 

 respectively were cut, the abandonment of any channel by the 

 outflowing lake-waters only occurring when an outlet at a 

 lower level was afforded. By comparison of the levels as 

 indicated by the dotted lines, it will be readily seen that the 

 numerical rule given above holds good. 



* That is at the original water parting. It may he noted that with the pro- 

 gressive deepening of a swire the intake becomes displaced towards the lake side of 

 the ridge. This point has to be borne in mind when estimating the depth of a swii'e. 



