Tough, close-grained rocks, such as hard limestone, are sometimes polished to look like marble. But there is a great deal of difference between the smoothing effected by a river or a torrent and that which is produced by a glacier, because the river tosses the rocks and stones in all directions, polishes them on every side, and leaves no distinctive parallel scratches or grooves on them. That can only be done by glaciers which hold the rocks, the rubbers and the rubbed, pressed firmly together and grind them continually in the same way. These scratchings or striations of rocks, the smoothed and grooved surfaces, and the deposited boulder-clay and boulders enable us to trace the march of great ice-sheets over regions of the earth which are now of totally different aspect. From this kind of evidence we have been able to find that the whole of Northern Europe was once buried under a great expanse of snow and ice. The sheet was, as we should expect, thickest in the north and west, and thinned away southward and eastward. Over Scandinavia it was between 6000 and 7000 feet in thickness--as we can tell from the scratches on the sides of the high mountains. Similar marks 3000 feet above the sea-level in the Scottish Highlands lead us to believe that over Scotland the glaciers were 5000 feet thick, and even as far south as the Hartz Mountains in Germany it could not have been far short of 1500 feet in thickness. Imagine this great mass of ice ever slowly moving and ever creeping solemnly down to the sea. By the markings it left we can trace where the greater glaciers slid grandly along. In Scandinavia it swept westwards to the Atlantic and eastwards to the Gulf of Bothnia, then frozen as solid as the Pole. Southward the ice ground its way across Denmark to the Low Countries and North Germany. The Baltic was choked with ice, and so was the North Sea as far south as London. Ice in that day flowed in glaciers from the British Isles, eastwards from Scotland into the hollow of the North Sea, and westwards down all the clefts of the mountains, burying the western isles and breaking off in icebergs that drifted far into the Atlantic. Sir Archibald Geikie says that the western margin of the ice-fields from the south-west of Ireland to the North Cape of Norway must have presented a vast wall of ice 1200 to 1500 miles long and hundreds of feet high--like that great barrier which the Antarctic explorers tell us frown on the waters that lap the boundaries of the south polar land. Northern Europe must have been like North Greenland of to-day. The rock scratches tell us (since even the southern coast of Ireland is intensely ice-worn) that the edge of ice must have extended some distance beyond Cape Clear, rising out of the sea with a precipitous front that faced to the south. Thence the ice-cliff swung eastwards, passing probably along the line of the British Channel and keeping to the north of the valley of the Thames.