COLD AND ICE ON THE EARTH 



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 west- 

 ward 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 thick- 

 ness. There were periods of retreat and advance, of pro- 

 gress 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. 



At last, after many of these swallow-like retreats and 

 advances, the warmer climate at length came to stay, 

 and the ice retreated farther and farther to the north. 

 It still remained among the mountains, so that we might 



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