194 The Scottish Naturalist. 



must be considered as affording an additional demonstration of 

 its truth. 



Some years ago I called attention to the fact that in the 

 middle districts of Scotland the boulder -clay not infrequently 

 contains a curious commingling of northern and southern 

 erratics. 1 I showed that this was the case throughout a belt of 

 country extending from the sea-coast near Ayr, north-east to the 

 valley of the Irvine, and thence across the watershed into the 

 Avon, and east to Lesmahagow, then down the valley of the 

 Clyde to Carluke, stretching away to the east by Wilsontown, 

 and thereafter continuing along the crest of the Pentlands and 

 the northern slopes of the Lammermuir Hills, by Reston and 

 Ayton to the sea. "All along this line," I remarked, "we have 

 a ' debatable ground ' of variable breadth, throughout which we 

 find a commingling in the till of stones which have come from 

 the north and south. South of it, characteristic Highland stones 

 do not occur, and north of it stones derived from the south are 

 similarly absent." The explanation of these facts is obvious. The 

 belt of ground referred to was evidently the meeting-place of the 

 Highland and Southern mei's de glace. Here the two opposing 

 ice-flows coalesced and became deflected by their mutual pres- 

 sure to right and left — one great current going east and another 

 west. It is evident that the line of junction between the two 

 mers de glace would not be rigorously maintained in one and the 

 same position during a period of glaciation, but would tend to 

 oscillate backwards and forwards, according as one or the other 

 ice-sheet prevailed. Sometimes the Southern ice- sheet would 

 be enabled to push back the Northern mer de glace, while at 

 other times the converse would take place. Nor is it necessary 

 to suppose that the advance of one ice-sheet was general along 

 the whole line. On the contrary, it is most likely that the move- 

 ment was quite irregular — an ice-sheet advancing in some places, 

 while at other points its line of junction with the opposing ice- 

 sheet remained stationary, or even retrograded. Such move- 

 ments would obviously give rise to oscillations in the sub-glacial 

 debris of clay and stones ; and thus we have a simple and natural 

 explanation of those intercrossings of erratics which are so 

 characteristic of that region which I have termed the " debat- 

 able ground." And this conclusion is borne out by the fact that 

 the glacial striae of the same "debatable ground" afford like 

 evidence of oscillation in the trend of the ice-flow. 



1 ' (J rent Ice Age,' p. 609. ___ 



