The glacial geology of Northumberland ioi 



its effect being recognisable to a distance of 14 miles from the 

 coast. As a result of this the Tweed and local ice was thrust 

 up the valley of the Till, but was barred near Hedgeley by the 

 Cheviot ice. This, restrained to the south by the great 

 western sheet, was driven down the Aln and diverted near 

 Alnmouth along the coast, which it traversed in a southerly 

 direction. The position of equilibrium of these sheets at the 

 latest stage of retreat will be considered after the next line of 

 evidence has been treated of, viz., the evidence derived from 

 the glacial lakes. 



IV. — The Forsaken Water Courses. 



The occurrence of these in the district lying between the 

 Tyne and the Wansbeck has already been recorded [18] ; 

 further investigation extending over the greater part of North- 

 umberland has disclosed many others, and about seventy are 

 marked on the accompanying map (No. 1). They occur in two 

 positions ; firstly, and most frequently, cutting the watersheds 

 between the pre-glacial valleys or the spurs and subsidiary 

 water-partings connected therewith ; secondly, as trenches 

 running along a hill-side, roughly parallel to the water-parting. 

 Occasionally, as on Harden Hill, the two types are combined, 

 the upper part of the watercourse cutting along one side of the 

 ridge, then swerving sharply across the divide and dropping 

 abruptly in cascade-fashion down the other side. A great 

 number of these valleys are quite dry; when of considerable 

 length they usually carry small streams in their lower parts, 

 the upper parts being dry or at most flooded in wet weather 

 with stagnant water. As the gradient at the intake is often 

 very slight, a small artificial dam suffices to convert them into 

 lakes ; such is the origin of the Rothley Lakes near Ewesley. 

 Natural dams are formed in them either by the meeting of 

 screes from opposite sides, when a series of basins is produced 

 (as at Middledene and The Kettles, D2 and C16 in map 

 No. 1), or by detrital matter thrown down by an entering 

 stream. The latter type (the " corroms " of Kendall) is not 

 uncommon on a small scale, and frequently results in the 

 diversion of local drainage near the intake. 



