366 POPULAR SCIENCE MONTHLY. 



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 be- 

 tween the Scioto River, in Ohio, and the Atlantic coast, the maxi- 

 mum retreat of that portion being twenty-five miles or more in 

 New Jersey. A cool temperate climate and coniferous forests ex- 

 tended up to the retreating ice border in the upper Mississippi 

 region. This great glacial recession was attended with much ero- 

 sion of the early drift. A corresponding interruption of the 

 severity of the Ice age in Europe is named by Geikie the Helve- 

 tian 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 Iowan 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 Iowan stage was terminated by a depression of the ice-bur- 

 dened 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 Iowan boundaries was rapid under the tem- 

 perate (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 



