THE GLACIAL DEPOSITS OF WESTERN CARNARVONSHIRE. 51 



These shelly sands and gravels evidently represent the materials of a sea-bottom 

 carried onwards and upwards by an ice-sheet and rearranged by fluvio-glacial action. 

 That is to say, they are the remanies of the great ice-sheet which came from the north 

 and passed over the floor of the Irish Sea. The materials were laid down during the 

 retreat of the ice-front, and at that time the lower parts of Lleyn must have been under 

 water. The boulder gravel may be partly due to the action of torrential waters issuing 

 from the melting ice-sheet and glaciers. No deposits indicating a genial inter-glacial 

 epoch have been discovered in Western Carnarvonshire ; but this is not a matter for 

 surprise, for such deposits, if they once existed, would stand very little chance of being 

 preserved in a region so exposed as this. Whether the sands and gravels indicate a 

 mere temporary retreat of the ice-sheet or the oncoming of a true inter-glacial epoch 

 it is impossible to decide from the evidence afforded by a study of this region alone. 



The Upper Boulder Clay again is very sporadic in its occurrence and varies much 

 in thickness and character. Sometimes it is a typical tumultuous unstratified till with 

 boulders of all shapes and sizes scattered pell-mell throughout the clayey matrix ; at 

 other places it has some of the character of rubble drift. As compared with the Lower 

 Boulder Clay the Upper Till is less compact and homogeneous, more stony in character, 

 and more weathered and friable. It often passes into a sandy clay. Where it becomes 

 a rubble drift it consists of an agglomeration of subangular and more or less rounded 

 boulders crowded thickly together in earthy debris. The included boulders are 

 derived in the main from Welsh rocks, but many far-travelled stones are also found. 

 Ice-scratched boulders are fairly common. The distant erratics are of similar origin to 

 those found in the lower deposits, and include Scottish granites, the Ailsa Craig rock, 

 and chalk-flints. 



Shell -fragments are again present, but these are scarcer and even less well preserved 

 than those found in the Lower Boulder Clay. This Upper Boulder Clay is well 

 displayed in the coast cliffs of Dinas Dinlle, and again between Point Maen Dulan and 

 Clynnog, where it often forms the entire cliff. West of Clynnog it is not so conspicuous 

 in the coast sections, but in many of the bays it can be seen to cap the cliffs which 

 are formed of the Drift deposits. As a rule the Upper Clay rests on the Gravel 

 and Sand series, but occasionally the two boulder clays occur together, the Upper 

 Clay resting directly on the Lower, and in such cases it is sometimes difficult to 

 separate them. 



In some of the smaller bays on the south-west coast of Lleyn the Lower Boulder 

 Clay is not exposed ; in the cliff sections and at the sides of these bays the Upper 

 Boulder Clay rests immediately on the rocky slope or is separated from it by some rock 

 rubble. East of Afonwen the Upper Boulder Clay becomes again conspicuous. There 

 it forms the upper part of the cliff, becoming more prominent and well-defined as we 

 approach Criccieth. To the east of the town it forms the entire cliff. In the coast 

 sections east of Afonwen no shell-remains were noted in this clay, and the included 

 boulders seem to be entirely of native origin. 



