T. Mellard Reach — Qlaclal Geology. 315 



So far as the Drift in the districts described in these pages is 

 concerned, I long ago arrived at the opinion that no good could he 

 got out of it — that is, no intelligible story — until we recognized it 

 as one series of glacial deposits. On this principle, with the facts 

 already stated before us, I propose to discuss the two opposing 

 theories of the Glacial Drift, namely, the land-ice and the glacio- 

 marine or submergence theory, 



T/ie Land-Ice Theory. 



The present form of this theory postulates that during the period 

 when the Drift, as we see it now, was formed, the relative levels of 

 the sea and land were the same as now, and that a great ice-sheet 

 advancing from the North over the Irish-Sea bottom ploughed out 

 the deposits, re-arranged them, and pushing them before it, or 

 conveying the materials frozen in the bottom of the ice, spread 

 them over the lowland plains, and even pushed the sands and 

 gravels with shells up to the high levels in the instances already 

 named. To the obvious objection that the Drift bears indisputable 

 signs of aqueous deposition, it is answered that the current-bedded 

 sands and gravels are due to the washing from the melting of the ice 

 at the termination of and underneath the Mer-de-glace. 



Physical conditions involved. 



Before describing what the deposits themselves have to say to 

 this theory, let us picture to ourselves what it as a matter of physics 

 involves. The gathering ground of such a glacier could at first 

 only be the land area which, by the terms of the postulate, was the 

 same as now. A glance at the Map of the British Isles is sufficient 

 to show that the snow-field could not have been above double the 

 area of the sea it had to displace. The ice-front advancing over the 

 Irish Sea would have had an average length of one hundred miles. 

 The waste by melting would have been enormous, so that an 

 intensity of glacial conditions far in excess of those of Greenland of 

 the present day would have to be granted. 



Frohable Effect on the Sea Bottom. 



If, however, for the sake of discussing the question, we grant 

 both the necessary conditions and their result, the ice-sheet, what 

 would be the probable efi'ect on the sea bottom ? Unfortunately we 

 have very little to guide us, for it is one of the weakest places in 

 this theory that there are no analogous modern examples that we 

 can appeal to. Even with those glaciers that displace the sea from 

 an inlet like the Malaspina Glacier of Mount St. Elias in Alaska/ it 

 is impossible to say what efi'ect the glacier has had on any deposits 

 that may previously have occupied the bottom. We know, however, 

 that glaciers frequently over-ride loose deposits on land without 

 displacing them, while in others they erode their beds. By the 

 terms of the postulate Pre-glacial deposits must have occupied the 

 Irish Sea before the over-riding of the ice-sheet bringing its load of 

 ^ Israel Eussell, — National Geographic Magazine, May, 1891. 



