BETWEEN THE TYNE AND THE WANSBECK 105 



The assumption we have made of the eastern ice-sheet at 

 the close of the Glacial Period, when the high land to the west 

 was in a large measure free from ice, is possibly connected 

 with the question concerning the origin of the upper drift 

 gravels. Three points bearing upon this matter may be 

 noticed : (i) to account for the swires a southerly component 

 to the motion of this ice-sheet must be assumed ; (2) that the 

 later gravels as mapped by the Geological Survey* are 

 restricted to the country under the 400-foot contour (a fact 

 confirmed by my own observations) ; and (3) that notwith- 

 standing some differences in composition, the kaims are alike 

 in containing, among pebbles of local origin, many porphyritic 

 and felsitic rocks apparently of Cheviot origin, and ashes, 

 lavas, and granites similar to those occurring in the eastern 

 parts of Scotland. Taking these facts into account, it would 

 appear that this eastern ice-sheet moved south along the flat 

 coastal regions of Northumberland from Scotland to the Tyne, 

 bringing with it rocks from these northern parts, which were 

 deposited in the form of kaims at or near the edge of this ice- 

 sheet when it formed the barrier of the Pont and Ouseburn 

 lakes. Such a sheet, sweeping around the flanks of the 

 relatively ice-free Cheviots, would afford a good explanation 

 of many of the swires which, as stated above, form such a 

 prominent feature of that region. 



Some points regarding the formation of a parallel sequence 

 of swires, such as exists on the Dinnington Ridge, may be 

 touched upon here. There is no need to assume anything 

 more than a gradual and continual retreat of the ice to account 

 for such a sequence. The duration of overflow of an ice- 

 dammed lake at any point across its containing watershed 

 will depend upon the ratio between the rate of rock-erosion at 

 that point and the rate of ice-retreat along the watershed, and 

 the cutting of a lower notch by the lake-waters will only take 

 place when the watershed becomes clear of ice at a level yV/i-/ 

 below that of the existing channel at the position where it 



* It may be mentioned here that no distinction is drawn on the Survey Drift 

 Maps between what we have called Later Gravels and Surface Sands. 



