288 
ME. PEESTWICH ON THE GEOLOGY OF THE DEPOSITS 
Sir Charles Lyell to the grounding of icebergs on the soft sea-bed, and I am disposed 
to attribute to a somewhat like action, on a small scale, of the river-ice, the analogous 
structure exhibited in the St. Acheul and other high-level gravels (figs. 13, 14, 15, p. 269). 
Mr. A. Murray, in exploring the shores of the Mississagui river, noticed instances of 
similar recent effects of ice, “ where the coarse shingle was loosely piled up in great 
conical heaps. The accumulations were usually at a turn in the river where there was 
a strong current above. The ice, brought down with violence and impinging on the 
side at the turn, appeared to have ploughed up the shingle and pushed it forward on 
to the bank. One of the heaps was estimated to be 10 feet high at the apex, with a 
diameter at the base of 40 to 50 feet ; it rested on closer packed materials of the same 
kind, which also formed the bed and the margin of the stream in the neighbourhood ” *. 
These results agree with and confirm the indications furnished by the organic remains, 
viz. that at the period of the high-level gravels the winter cold, which so froze large rivers 
as to furnish ice-rafts capable of transporting innumerable boulders, many of 5 to 10 
tons weight or more, for great distances, was not less than that of Moscow or Quebec at 
the present day, and that it may have been even lower. It is generally admitted that 
previous to this time, in the pliocene or early post-pliocene period, the cold was still 
more severe. Then the greater part of England was under the sea, whereas Switzerland 
and the greater part of France had emerged from the sea at an earlier, or Miocene period, 
and there is no proof of their having been subsequently submerged. 
It was during this previous period of intense cold that the wonderful extension of 
the Alpine glaciers took place, and that many minor chains, such as the Jura and the 
Vosges, had also their glaciers. On the north of the Alps these old glaciers descended 
to within 1200 to 1000 feet of the present sea-level, whilst those now existing in Swit- 
zerland do not come lower than within 3400 feet of that level. M. Leblanc f has 
calculated that such a difference of level might be accounted for by a reduction in 
the mean annual temperature of 12^° Fahr. But although that might give the limits 
to which glaciers could descend in these latitudes under ordinary circumstances and 
like conditions, it by no means proves that a greatly lower temperature may not have 
accompanied and hastened their enormous growth ; nor, when we look at the length and 
extent of the valleys, of which the fall is but small, over which the old glaciers passed, can 
their progress along surfaces so slightly inclined be compared with that where the inclina- 
tion, as usually in their present beds, is steeper and the channel narrower J. I do not 
believe, therefore, that this estimate of M. Leblanc furnishes us with even an approxi- 
mation to the extreme cold of that glacial period. If, however, we were to assume that, 
* Geological Survey of Canada, for 1858, by Sir W. Logan, p. 103. 
f Bull, de la Soc. Geol. de France, vol. xii. p. 132 (1841). 
J Tbus in the valley of the Aar, where the inclination of the surface is about 2 \°, the glacier of the Aar does 
not come down lower than within 6000 feet of the sea-line ; whereas the glaciers Du Bois and des Bossons, with 
beds inclined at 8° to 10°, descend to within 3500 feet of the sea. On the Italian side of the Alps the old 
glaciers descended lower and nearer to the sea-level than those on the Swiss side. 
