lOO THE GLACIAL PHENOMENA OF THE COUNTRY 



left when the ice finally passed away. Some of these, as for 

 example near Middleton* in the Wansbeck valley, and between 

 Fenwick and Stamfordham in the Pont valley, probably existed 

 until comparatively recent times, and Prestwick Carrf was 

 only completely drained about 50 years ago. There can be 

 little reasonable doubt that these lakes were the puny repre- 

 sentatives of the large ice-dammed lakes existing at the close 

 of the Glacial Period. 



Though the explanation given above may be admitted in 

 the case of the " dry " slacks, objection may be made to it 

 when the slack contains a burn, even though only a small one, 

 so that it is important to have some criterion to enable one to 

 distinguish the "swires" (in the sense of forsaken channels of 

 streams derived from ice-dammed lakes) from ordinary valleys. 

 Apart then from the obvious disproportion between the size 

 of the burn and its windings and those of the swires, the one 

 unmistakeable feature of the swires is the constant parallelism 

 of their banks ; in other words they have been cut by streams 

 derived from without, which have not been sensibly augmented 

 in volume during their passage through the swires. 



As bearing on the same subject, it may be remarked that 

 the position of overflow of a glacially-dammed lake would be 

 where the containing watershed was lowest, and as in many 

 cases the lowering would have been brought about by the 

 cutting back of some stream, it is not surprising that these 

 temporary glacial streams frequently followed the path of the 

 head waters of a normal stream, and that on the cessation of 

 the glacial stream the normal one should again flow along its 

 wonted course. 



* Leboizr, "Handbook to tlie Geology and Natural History of Northumberland 

 and Durham," p. 14 (1886). 



t In an old (scientifically worthless) account of the Garr, reference is made to 

 the discovery of " seven feet of marl overlain by three feet of a peaty nature. The 

 marl contained shells of helix and turbo [i.e., Planorbis and Paludina] and quan- 

 tities of bog-moss, and stems and leaves of aquatic plants near the upper part of 

 the stratum. At the bottom the shells were incrusted hard together." See "An 

 Historical and Descriptive Account of Prestwick Carr and its Environs," by D. 

 JIaddison. 1830. 



