Page:Popular Science Monthly Volume 9.djvu/570

544 glacier in what is known as a terminal moraine, which in many cases stretched completely across the valley and marks the place where the terminal face of the glacier was stationary for a considerable period of time before it melted away, and allowed the water to accumulate in the space once filled by the ice. These glacier-built dams are to be met with in all countries which have been subjected to glacial action, and are especially well marked amid the Alps and in Scotland, where they have been most thoroughly studied, on the Scandinavian peninsula, in the Northern States of the Union, and amid the southern Alps of New Zealand. As the bottom of the valley in which such a lake is formed is usually worn deeper by the action of the glacier during the formation of the terminal moraine, this second form of lake-basin is quite often combined with the first.

To this second class also belong the thousands of little lakelets scattered over the Northern States, which are confined on all sides by banks of drift-material, and fill nearly every depression and hollow in the huge banks of glacier-worn débris—known as till, kaims, eskers, etc., scattered so plentifully throughout our Northern country. We have seen many of these pretty little lakelets through New York, Ohio, and westward. Near Plainfield, New Jersey, scores may be passed in a morning's walk. At the latter place they occupy the hollows and dells in the drift, which is there of great thickness, and formed not only from the Triassic sandstone which underlies it, but also to a large extent from the limestone and gneiss found in place only in the northern portion of the State. Intermingled with these are many blocks of the peculiar reddish conglomerate found in situ in Morris County, which show unmistakably the direction from which the drift has traveled. Many of these stones are glacier-worn, and have without doubt been transported from their northern homes by the agency of ice; not in one or two isolated instances, but in sufficient quantity to cover the country for miles in extent. These little lakelets, becoming filled with vegetable matter, form peat-bogs, which promise to become of considerable agricultural value in the future; these peat-bogs not only contain many wonderful things for the eyes of those who are fortunate enough to possess a microscope, but also in them are sometimes found the bones of the huge mastodon, which at no very distant time inhabited this continent.

3. The formation of lakes by a sinking of their bottoms, although at first sight seeming to be the simplest and most common mode of their formation, is really the most unusual. Lake Superior is described as filling one of these depressions, as the rocks on its shores are found to dip toward the centre of the lake, and the basin seems to have been formed by a subsidence at that point, although greatly modified in after-time by the erosion of the ice during the glacial period. The valley into which the Jordan empties is another such region of subsidence.