EFFECTS OF GLACIATION. The general configu ration of Continental lands has not changed ap preciably since the opening of the Glacial period. The mountain systems had acquired their eleva tion before that time, and, in a general way, the drainage was directed toward the same chan nels that now carry the surface waters seaward. The ice exerted a powerful influence, however, upon minor land forms or types of scenery. In its progress from north to south and from high land to lowland it pushed along the soil and dis integrated rock, the accumulations of long pe riods of subaErial decay, and deposited them in great moraines, which still give a peculiar aspect to the scenery of glaciated regions. At the same time the sand and stones incorporated in the mass of moving ice were efficient agents of ero sion; rock surfaces, wherever exposed, were smoothed and striated, prominences assumed a more rounded form, and the valleys were widened and deepened. The rock striations show that the direction of flow was influenced by mountain ranges of considerable elevation, although small inequalities in the surface caused no deviation. Most of the lakes in Northern Europe and Amer ica had their origin in this period. Lake basins were scooped out by erosion, and temporary lakes were formed by obstruction of valleys during the retreat of the ice-sheet. One of the largest temporary basins, which has been surveyed by means of the old beach lines, extended from the northern parts of Minnesota and North Dakota far into Canada, and covered an area exceeding that of all the Great Lakes combined. All lakes which are of pre-glacial origin show evidence of having been expanded during this period. The Great Lakes are bordered by a succession of ter races, the highest of which stand nearly 500 feet above the present water-level. The ice-sheet for a time stretched across the Saint Lawrence val ley, turning the drainage of Lake Ontario into the Mohawk and Hudson; and Lake Michigan, obstructed at its outlet, overflowed toward the southwest into the Illinois River.
A further important result wrought during the Glacial period was the removal of the soil that had been derived in situ by weathering, and the substitution of a covering of 'drift' (q.v.). This glacial material was spread very unevenly over the land. In the Laurentian highlands of Canada, where the ice-sheet formed, the surface is bare rock or at most thinly covered with soil. Fur ther south the drift accumulated along lines marking the advance and retreat of the ice in great heaps of boulders, gravel, sand, and clay. Such terminal moraines are strongly developed in New England, New York, Ohio, and the Northern States as far west as Kansas. A second portion of the transported material was distributed beneath the ice-sheet as 'boulder clay' (q.v.) or 'till,' in the form of a ground mo raine. The boulder-clay is a compact, tenacious clay containing boulders of varying size and gen erally unstratified, although traces of bedding are sometimes present. It rests directly upon the rock, which is usually smooth and striated. The boulder-clay is distributed unevenly, gathering into smoothly arched ridges and mounds called `eskers' and 'drumlins' (qq.v.), and at times thin ning out so as to leave the rock formations ex posed.
The extraordinary changes of climate indicated by the Glacial period led to migrations of the fauna and flora inhabiting the arctic and temperate zones. As the temperature fell, such animals as were unable to endure extreme cold worked southward, while some species found their way from the far north into regions from which they have long since disappeared; remains of the polar bear, reindeer, and Arctic fox occur in the glacial deposits of Southern Europe. With the retreat of the ice, the Arctic fauna and flora were able to adjust themselves to the changing conditions by withdrawing from southern lati tudes or by ascending the slopes of mountains. The oscillation of the climate was thus accom panied by a variation of the life forms in each particular region.