98 THE GLACIAL PHENOMENA OF THE COUNTRY 



escaped in part over the surface of this. The ascertainable 

 facts and legitimate assumptions bearing on the question 

 are : — (i) that the lowest portion of the watershed between 

 the Tyne and upper Ouseburn valleys probably did not 

 exceed 280 feet in height; (2) that the sudden stoppage of the 

 lower ends of the swires points to the streams which flowed 

 through them having entered a lake ; (3) that the upper 

 Ouseburn was flooded during part of the time that the 

 Pont Lake was in being, and that successive levels of this 

 Ouseburn Lake were 280, 266, and 250 feet, which are 

 the respective heights of the mouths of the Callerton, 

 Luddick, and the Woolsington and Newbiggin swires. In the 

 absence then of trustworthy evidence I would suggest that the 

 waters of the Pont Lake reached the Tyne valley by way of 

 Dewley, flowing over the ice in the Ouseburn valley during 

 the period of existence of the overflow stream at Heddon, 

 and breaching the watershed (when this had become free from 

 ice) during the time when the direct overflow was through the 

 Callerton, Luddick, and Woolsington Slacks. 



The assumption we have made of temporary ice-dammed 

 lakes in the valleys of the Wansbeck, the Pont and Blyth, and 

 the Ouseburn, whilst it gives a clear and simple explanation 

 of the phenomena of the swires, renders it likely that other 

 evidence of their existence will remain in the shape of strand- 

 lines, beach-deposits, etc. Such evidence is, however, not with 

 certainty forthcoming, a fact which may be explained by the 

 almost complete absence of sections in the likely places, the ex- 

 tensive cultivation of the land, and the short term of life of these 

 glacial lakes. Exception may perhaps be made of the pebbly 

 deposits near Middle Duddo, described in the chapter on the 

 " Drift," though even here the evidence is far from satis- 

 factory. 



It seems probable, however, that the smoothing out of the 

 drift in the lower lands, and the formation of surface sands, 

 which occur on all the sites of these hypothetical lakes, date 

 from this period, and that many smaller lakes or loughs were 



