Its southern margin ran across what is now Holland and skirted the high grounds of Westphalia, Hanover, and the Hartz Mountains--which probably barred its further progress southward. "There is evidence that the ice swept round into the lowlands of Saxony up to the chain of the Erz, Riesen, and Sudenten Mountains, whence its southern limit turned eastward across Silesia, Poland, and Galicia, and then swung round to the north, passing across Russia by way of Kieff and Nijni Novgorod to the Arctic Ocean." In North America there are similar traces of the great ice-sheet, one of the branches of which streamed southward into the basin of the Mississippi, the second moving westward from Hudson Bay to the Rockies, and southward to Iowa, and the third setting out from the great mountain ranges of British Columbia. Right across North America to-day for thousands of miles stretch accumulations and mounds of rock which were pushed forward by the ice, and were dropped by the glaciers when they reached "farthest south." These accumulations are called, from their origin, the great "terminal moraines" of the North American Ice Age. It must not be thought that these great ice-sheets of both hemispheres remained constant in extent and thickness. There were periods of retreat and advance, of progress and shrinking, and the shrinkings of these took place on a large scale, and perhaps lasted for hundreds or thousands of years; so that mixed with the strata of boulder-clay, which are the characteristic strata of the glacial periods, are other strata of sand, ordinary clay, and even peat. Remains of plants and animals are found in these strata, showing that sometimes the glaciers retreated so far and for so long that vegetation sprang up and animals lived on the ground that they had covered--in the intervals when the cold of centuries was replaced by other centuries of mild and equable climate.