IV THE ICE AGE AND ITS WORK 93 



In consequence of these various difficulties it was sug- 

 gested by the late Mr. Belt that the great Irish Sea ice- 

 sheet had carried up a portion of the sea-bottom em- 

 bedded in its substance, perhaps containing deposits of 

 shells of various periods and thus explaining the inter- 

 mixture of species as well as their fragmentary condition. 

 The fact that boulders and pebbles from Scotland, Ailsa 

 Craig, and Cumberland have been found in the Moel 

 Tryfaen beds almost amounts to a proof that they w^ere so 

 uplifted ; and a recent search has shown that in the other 

 localities where marine shells hav^e been found in drift at 

 great elevations similar foreign rocks occur, rendering it 

 almost certain that the same ice-sheets which have 

 distributed foreign erratics so widely over our country, and 

 which in doing so must have passed over the sea-bottom, 

 have in a few cases carried with them a portion of that 

 sea-bottom, and deposited it with the erratics in the places 

 where both are now found. A full discussion of this point, 

 with replies to various objections, by Mr. P. F. Kendal, 

 will be found in the volume already quoted ; and he has 

 recently adduced a fresh argument against " the great 

 submergence " in the fact that, if it ever occurred, our low- 

 lands must for a long time have formed the bottom of a 

 sea 200 fathoms deep, yet not a single shell characteristic 

 of that depth has yet been discovered in the drift.^ The 

 cumulative evidence against the submergence is now almost, 

 if not quite, conclusive. 



In the brief outline now given of the facts of glacial 

 geology bearing upon the former existence, the thickness, 

 extent, and motion of ice-sheets, it has only been possible 

 to treat the subject very broadly, omitting all those details 

 and minor difficulties which cannot be discussed within the 

 limits of a popular exposition. My object has been to ex- 

 plain the nature and amount of the converging evidence 

 demonstrating the existence of enormous ice-sheets in the 

 northern hemisphere, to serve as a basis for the discussion 

 of the glacial origin of certain classes of lake-basins, which 

 will form the subject of the following chapter. 



^ Wright's Man and the Glacial Period, pp. 167 — 175; Also 

 Geolorjiral Magazine, November, 1892, pp 491 — 500. 



