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SCIENCE 



[N. S. Vol. XXXII. No. 820 



moraines. The stratified sands and gravels 

 were deposited in lakes formed by the 

 rivers which were dammed up by ice- 

 sheets.*^ A second interpretation recog- 

 nizes the presence of glaciers in the moun- 

 tain regions, but maintains that the land, 

 at the outset rather above its present level, 

 gradually sank beneath the sea, till the 

 depth of water over the eastern coast of 

 England was fully 500 feet, and over the 

 western nearly 1,400 feet, from which de- 

 pression it slowly recovered. By any such 

 submergence Great Britain and Ireland 

 woiiLd be broken up into a cluster of hilly 

 islands, between which the tide from an 

 extended Atlantic would sweep eastwards 

 twice a day, its currents running strong 

 through the narrower sounds, while move- 

 ments in the reverse direction at the ebb 

 would be much less vigorous. The third 

 interpretation, in some respects interme- 

 diate, was first advanced by the late Pro- 

 fessor Carvill Lewis, who held that the 

 peculiar boulder clays and associated sands 

 (such as those of East Anglia), which, as 

 was then thought, were not found more 

 than about 450 feet above the present sea- 

 level, had been deposited in a great fresh- 

 water lake, held up by the ice-sheets al- 

 ready mentioned and by an isthmus, which 

 at that time occupied the place of the Strait 

 of Dover. Thus, these deposits, though 

 indirectly due to land-ice, were actually 

 fluviatile or lacustrine. But this interpre- 

 tation need not detain us, though the for- 

 mer existence of such lakes is still main- 

 tained, on a small scale in Britain, on a 

 much larger one in North America, be- 



"See Warren Upham, " Monogr. U. S. Geol. 

 Survey," XXV., 1896. This explanation commends 

 itself to the majority of British geologists as an 

 explanation of the noted parallel roads of Glen- 

 roy, but it is premature to speak of it as " con- 

 clusively shown" {Quart. Journ. Geol. Soc, 

 LVIII.,. 1902, 473) until a fundamental difficulty 

 which it presents has been discussed and removed. 



cause, as was pointed out when it was first 

 advanced, it fails to explain the numerous 

 erratic blocks and shell-bearing sands which 

 occur far above the margin of the hypo- 

 thetical lake. 



Each of the other two hypotheses involves 

 grave difficulties. That of great confluent 

 ice-sheets creeping over the British low- 

 lands demands, as has been intimated, eli- 

 matal conditions which are scarcely pos- 

 sible, and makes it hard to explain the 

 sands and gravels, sometimes with regular 

 alternate bedding, but more generally in- 

 dicative of strong euiTent action, which 

 occur at various elevations to over 1,300 

 feet above sea-level, and seem too wide- 

 spread to have been formed either beneath 

 an ice-sheet or in lakes held up by one ; for 

 the latter, if of any size, would speedily 

 check the velocity of influent streams. Also 

 the mixture and crossing of boulders, which 

 we have described, are inexplicable without 

 the most extraordinary oscillations in the 

 size of the contributing glaciers. To sup- 

 pose that the Scandinavian ice reached to 

 Bedfordshire and Herts and then retired in 

 favor of north British glaciers, or vice 

 versa, assumes an amount of variation 

 which, so far as I am aware, is without a 

 parallel elsewhere. So also the mixture of 

 boulders from south Scotland, the Lake 

 District and north Wales which lie, espe- 

 cially in parts of Staffordshire and Shrop- 

 shire, as if dropped upon the surface, far 

 exceeds what may reasonably be attributed 

 to variations amplified by lateral spreading 

 of mountain glaciers on reaching a lowland, 

 while the frequent presence of shells in the 

 drifts, dozens of miles away from the pres- 

 ent coast, implies a rather improbable 

 scooping up of the sea-bed without much 

 injury to such fragile ob.jects. The ice 

 also must have been curiously inconstant in 

 its operations. It is supposed in one place 

 to have glided gently over its bed, in an- 

 other to have gripped and torn out huge 



