The mud, sand, and boulders thus let fall in still water must be exactly like the moraines of terrestrial glaciers, devoid of stratification and organic remains. But occasionally, on the outer side of such packs of stranded bergs, the waves and currents may cause the detached earthy and stony materials to be sorted according to size and weight before they reach the bottom, and to acquire a stratified arrangement. I have already alluded (p. 172) to the large quantity of ice, containing great blocks of stone, which is sometimes seen floating far from land, in the southern or Antarctic seas. After the emergence, therefore, of such a submarine area, the superficial detritus will have no necessary relation to the hills, valleys, and river-plains over which it will be scattered. Many a water-shed may intervene between the starting-point of each erratic or pebble and its final resting-place, and the only means of discovering the country from which it took its departure will consist in a careful comparison of its mineral or fossil contents with those of the parent rocks. [1] As to the former excess of cold, whether brought about by modifications in the height and distribution of the land or by altered astronomical conditions, see Principles, vol. i, (10th ed., 1867), chaps. xii and xiii, “Vicissitudes of Climate.” CHAPTER XII. POST-PLIOCENE PERIOD, continued.—GLACIAL CONDITIONS, concluded. Glaciation of Scandinavia and Russia. — Glaciation of Scotland. — Mammoth in Scotch Till. — Marine Shells in Scotch Glacial Drift. — Their Arctic Character. — Rarity of Organic Remains in Glacial Deposits. — Contorted Strata in Drift. — Glaciation of Wales, England, and Ireland. — Marine Shells of Moel Tryfaen. — Erratics near Chichester. — Glacial Formations of North America. — Many Species of Testacea and Quadrupeds survived the Glacial Cold. — Connection of the Predominance of Lakes with Glacial Action. — Action of Ice in preventing the silting up of Lake-basins. — Absence of Lakes in the Caucasus. — Equatorial Lakes of Africa.