352 Proceedings of the Royal Physical Society. 



into the basin of the Moray Firth, as well as the local ice 

 which streamed outwards in the direction of the Caithness 

 plain, was deflected towards the north-west by reason of the 

 greater force of the Scandinavian mer de glace. The pebbly 

 mud and marine shells would be borne inland from the bed 

 of the North Sea across the low-lying part of Caithness, 

 where they would be commingled with the debris of the 

 flagstones, and any marine deposits which might have been 

 deposited in pre-glacial or inter-glacial times. Blocks of the 

 various secondary formations derived from the areas crossed 

 by the Scotch ice would also be mingled with the ground 

 moraine. 



It is no doubt true, as Mr Jamieson pointed out, that the 

 mollusca are of a less Arctic type than those obtained from 

 the stratified shelly clays of Elie, Errol, and other localities. 

 But this may quite well be explained by supposing that they 

 belong to a pre-glacial or mild inter-glacial period. The 

 evidence in favour of alternations of climate in glacial times 

 is steadily accumulating, during which there were constant 

 migrations of northern and southern fauna. It does not 

 follow, therefore, that because the fauna of the Caithness 

 boulder clay is of a less Arctic type that the deposit does 

 not belong to the boulder clay period. There can be little 

 doubt, from the evidence we have adduced, that the reddish- 

 brown boulder clay of local origin is of the same age as the 

 grey shelly drift. 



The widespread traces of moraines and gravels prove that 

 long after the Scandinavian mer de glace had retreated, and 

 the climatic conditions had become less severe, local glaciers 

 moved outwards from the hilly ground to the west, deposit- 

 ing their materials alike on the red and the shelly boulder 

 clay. 



