83 THE GLACIAL PHENOMENA OF THE COUNTRY 



The gradients here are quite inappreciable, and although 

 the information is not such as to warrant any extensive 

 deductions concerning the pre-glacial land-surface, yet it may 

 safely be affirmed that there was a large tract of flat land here 

 at the meeting-place of numerous smaller burns with the two 

 main arteries, the " Blyth " and the " Pont." 



The Pre-glacial Streams. — The Upper Blyth and the How 

 Burn now flow in drift-filled valleys which are superimposed in 

 great measure upon pre-glacial valleys. The former stream 

 in its course from its source to Blagdon, a distance of about 

 14 miles, only once (near Trewick) forsakes the drift to cut into 

 the rocky southern bank of the pre-glacial valley. The pre- 

 glacial " Blyth " thus flowed, in the main, in the direction of 

 the present river, and some indication of the extent to which 

 the old valley has been filled up by glacial deposits is given by 

 the data supphed by borings and sinkings along its course. 

 Thus near Brandy well Hall the thickness of drift is 72 feet 

 and the rock-surface 508 feet above sea-level; half a mile east 

 of this the drift is 63 feet thick and the rock-surface at 462 

 feet, and at Bradford, a mile further east, the values are 89 

 and 396 feet respectively. Still lower down, at Carter Moor, 

 the drift reaches the great thickness of 117 feet and the rock- 

 surface is only 56 feet above sea-level. 



Of the smaller burns, the Belsay Burn and the Ogle Burn 

 do not appear as the representatives of any pre-glacial 

 streams ; their courses have been determined entirely by the 

 land-surface and the exigencies of drainage existing at the 

 close of the Glacial Period, without any reference to the 

 contour of the pre-glacial surface. The same applies naturally 

 to those small streams, as e.g. the March Burn, Swallow 

 Sike, and the Small Burn, which run entirely over superficial 

 deposits. The Blackheddon Burn in the first four or five 

 miles of its course seems to flow in the drift-filled valley of 

 some pre-existing stream ; it touches rock last near Robsheugh 

 at 400 feet, and from there to its meeting with the Pont flows 

 over an enormous thickness of drift, 1 19 feet being registered at 

 Milburn, 122 at West Coldcoats, and 119 at Middle Coldcoats. 



