Glacial History of Western Europe. 515 
them flowed out on to the beds of the North and Irish Seas. The 
boulder-clays represent its moraines. The stratified sands and gravels 
were deposited in lakes formed by the rivers which were dammed up 
by ice-sheets.1 A second interpretation recognizes the presence of 
glaciers in the mountain 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 1400 feet, from which depression 
it slowly recovered. By any such submergence Great Britain and 
Ireland would be broken up into a cluster of hilly isiands, between 
which the tide from an extended Atlantic would sweep eastwards 
twice a day, its currents running strong through the narrower sounds, 
while movements in the reverse direction at the ebb would be much 
less vigorous. The third interpretation, in some respects intermediate, 
was first advanced by the late Professor 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 
freshwater lake, held up by the ice-sheets already mentioned and by 
an isthmus, which at that time occupied the place of the Strait of 
Dover. Thus, these deposits, though directly due to land-ice, were 
actually fluviatile or lacustrine. But this interpretation need not 
detain us. 
Each of the other two hypotheses involves grave difficulties. That 
of great confluent ice-sheets creeping over the British lowlands 
demands, as has been intimated, climatal conditions which are scarcely 
possible, and makes it hard to explain the sands and gravels, sometimes 
with regular alternate bedding, but more generally indicative of strong 
current action, which occur at various elevations to over 1300 feet 
above sea-level, and seem too widespread 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. 
Some authorities, however, attribute such magnitude to the ice- 
sheets radiating from Scandinavia that they depict them, at the time 
of maximum extension, as not only traversing the North Sea bed and 
trespassing upon the coast of England, but also radiating southward 
to overwhelm Denmark and Holland, to invade Northern Germany 
and Poland, to obliterate Hanover, Berlin, and Warsaw, and to stop 
but little short of Dresden and Cracow, while burying Russia on the 
east to within no great distance of the Volga and on the south to the 
neighbourhood of Kief. Their presence, however, so far as I can 
ascertain, is inferred from evidence* very similar to that which we 
1 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 Glenroy, but it is premature to speak of it as ‘‘ conclusively shown” 
(Quart. Journ. Geol. Soc., vol. lviii, p. 473, 1902) until a fundamental difficulty which 
it presents has been discussed and removed. 
2 A valuable summary of it is given in The Great Ice Age, J. Geikie, chs. xxix, 
xxx, 1894. 
