56 DR T. J. JEHU ON THE GLACIAL DEPOSITS OF WESTERN CARNARVONSHIRE. 



imagine any recession of ice from the land area which was not practically contempor- 

 aneous on all sides of the Irish Sea basin. 



When the ice-sheet which deposited the Lower Boulder Clay shrank away from the 

 hills and the area now known as Lleyn emerged from its icy covering, the ice probably 

 still lingered for a time in the basin of the Irish Sea. It was probably during this time 

 of shrinkage and recession that the sands and gravels lying between the two boulder 

 clays were accumulated. Owing to melting at the edge of the ice-sheet and to the 

 ponding up of the land drainage, much of Western Carnarvonshire must for a time have 

 been submerged under fresh water, and there is no need to assume any sinking of the 

 land relatively to the sea of which there is no clear evidence in this region. The facts 

 which militate against the supposition that these shelly sands and gravels are ordinary 

 marine deposits laid down when the land stood lower relatively to the sea are summarised 

 by Professor Kendall in Caevill Lewis' Glacial Geology of Great Britain and 

 Ireland (1894), Appendix viii. 



Stratified Drift deposits — shell-bearing sands and gravels — are met with lying 

 between two boulder clays at various elevations on both sides of the Irish Sea — in 

 Ireland as well as in West Lancashire, Cheshire and Wales. If these deposits were laid 

 down in their present position during a retreat of the ice-sheet from the hills, it means 

 that a shrinkage took place from the land area on both sides of the Irish Sea as well as 

 about the Isle of Man. The recession of the ice would be accelerated by the action of 

 land streams impinging on to ice, and wasting at the surface would become more rapid 

 as the surface was brought lower. The shrinking mass would thus be attacked on all 

 sides. Granting, therefore, as Mr Lamplugh does, a widespread change of climate to 

 account for the shrinking of the ice-sheet from the Isle of Man, and recognising that a 

 similar shrinkage must have taken place on all sides of the Irish Sea basin, the proba- 

 bility is that the ice- plateau vanished entirely and that an inter-glacial epoch intervened 

 between the times of the deposition of the two boulder clays. 



The Upper Boulder Clay would thus denote a recurrence of severe glacial conditions 

 and the advance of another ice-sheet. It has already been stated that, as far as Lleyn 

 is concerned, the two boulder clays might be regarded as the product of one mer de 

 glace subject to considerable oscillation, but a review of the whole of the Irish Sea area 

 renders it more probable that they are the products of the ice-sheets of two glacial 

 epochs separated by an inter-glacial epoch. 



(The author is indebted to the Carnegie Trust for a Research Grant towards the 

 expenses connected with this work.) 



