46 DR T. J. JEHU ON 



The seaward terminations of these valleys or depressions are drowned, and their lower 

 reaches must have been eroded during pre-Glacial times, when the land stood higher and 

 extended further out to the sea than it does at present. The rook rubble is the result 

 of subaerial work under severe climatic conditions. It is generally admitted that ice 

 in some form or other has aided in the formation of such deposits. The rock fragments 

 have all the appearance of being frost-riven, but apart from this there is not much trace 

 of weathering. This " Head " may therefore be regarded as angular debris of local deriva- 

 tion moved forward or downward by the action of frost, melting snows, and thawing sub- 

 soils. On the steeper slopes the action of gravity alone would account for the slipping 

 down of the material. Sometimes, on the other hand, the rubble is a confused mass of 

 broken rock showing no arrangement. 



As a rule this rock rubble is overlain by the Lower Boulder Clay, but there are places 

 where the Lower Boulder Clay seems to be absent and where the rubble is immediately 

 followed by the LTpper Drift deposits. The rubble always lies directly on the bed-rock, 

 and the fact that it is generally seen to have the Lower Boulder Clay superposed proves 

 that it is the lowest of the superficial deposits which occur in the area. In the eastern 

 part of the region this rock rubble is not seen unless it be represented by the shattered 

 rock which underlies the Lower Boulder Clay at some places. 



The Lower Boulder Clay sometimes rests directly on the bed-rock, and where this is 

 the case the rock surface as it passes under the clay has a markedly glaciated aspect ; some- 

 times it is separated from the rock by a foot or so of broken and confused rock material, 

 and sometimes, as on the rocky slopes at the seaward termination of valleys or inlets, it 

 rests on an accumulation of rock rubble or " Head " of a considerable thickness. It is 

 the most widespread of all the Drift deposits. This clay is very tough, compact, and 

 homogeneous, having a massive appearance as displayed on the cliff face. It 

 is often seen to be traversed by joints, and to break off in huge slabs along the joint 

 faces as at Porth Nigel. The colour is dark bluish-grey, but after drying it becomes 

 more of a light bluish-grey. Boulders are scattered sparingly through it, and are sub- 

 angular or more or less rounded and often intensely glaciated. Distant erratics are 

 common, and include Carboniferous Limestone, schists, etc., from Anglesey, various 

 granites and porphyrites from the south of Scotland, and possibly some granites and 

 lavas from the Lake District. It is interesting to note the occurrence of the Ailsa 

 Craig riebeckite micro-granite at many places in this Boulder Clay and the frequent 

 presence of chalk-flints which must have come from Cretaceous deposits on the floor 

 of the Irish Sea or in the north-east of Ireland. The Boulder Clay is shelly, 

 especially in the western portion of the peninsula ; and though the shell-remains are 

 generally very fragmentary, some complete shells were obtained and a considerable 

 number of forms have been identified. These include some species which are Arctic 

 and Scandinavian and do not live now in British Seas, such as Trophon (Boreotrophon) 

 clathratus, Trophon (Boreotrophon) scalariformis, and Astarte (Tridonta) arctica; 

 some of a northern type but not confined to high latitudes, such as Cyprina islandica 



