Page:Popular Science Monthly Volume 49.djvu/384

366 boundary northward about five hundred miles to Barnesville, Minn., in the Red River Valley, and two hundred and fifty miles or more in Illinois, according to Leverett; but probably little between the Scioto River, in Ohio, and the Atlantic coast, the maximum retreat of that portion being twenty-five miles or more in New Jersey. A cool temperate climate and coniferous forests extended up to the retreating ice border in the upper Mississippi region. This great glacial recession was attended with much erosion of the early drift. A corresponding interruption of the severity of the Ice age in Europe is named by Geikie the Helvetian stage or epoch. The greater part of the drift area in Russia was then permanently relinquished by the much diminished ice sheet, which also retreated considerably on all its sides. During this stage the two continents probably retained mainly a large part of their preglacial altitude. The decrease of the ice sheet may have been caused by the astronomic cycle which brought our winters of the northern hemisphere in perihelion between twenty-five thousand and fifteen thousand years ago.

In the lowan stage renewed ice accumulation covered the Aftonian forest beds, so that the continental glacier extended again into Iowa, to a distance of three hundred and fifty miles or more from its most northern indentation by the Aftonian retreat, and in Illinois it readvanced about one hundred and fifty miles, while its boundary eastward from Ohio probably remained with little change. At the same time, apparently, was the Polandian stage of renewed growth of the European ice sheet, probably advancing its boundaries in some portions hundreds of miles from the Helvetian retreat.

These foregoing stages belong to the early and longer part of the Glacial period, which may be called pre-eminently the Glacial epoch, including the times of mainly very cold and snowy climate which tended to the formation and preservation of the ice sheet. The lowan stage was terminated by a depression of the ice-burdened area mostly somewhat below its present height, as shown by fossiliferous marine beds overlying the glacial drift up to three hundred feet above the sea in Maine, five hundred and sixty feet at Montreal, three hundred to four hundred feet from south to north in the basin of Lake Champlain, three hundred to five hundred feet southwest of Hudson and James Bays, and similar or less altitudes on the coasts of British Columbia, the British Isles, Germany, Scandinavia, and Spitzbergen. Glacial recession from the lowan boundaries was rapid under the temperate (and in summers warm or hot) climate belonging to the more southern parts of the drift-bearing areas when reduced from their great preglacial elevation to their present height or lower. The finer portion of the englacial drift, swept down from the