54 



sassafras, the willow and the maple, ihe oak and the beeeh. while over its 

 surface spread many of the coarser grasses, sedges and mosses of the 

 present day. , 



During these long periods of erosion and decay, mild climatic condi- 

 tions had prevailed. But near the close of the Tertiary a change in these 

 conditions came gradually to pass — a change which was most sweeping 

 and far-reaching in its final results. For some, as yet unknown, reason, 

 the mean annual temperature of the northern hemisphere became much 

 lower. The climate of the regions to the east and south of Hudson's Bay 

 became similar to that of Greenland of today, or even colder. The snow, 

 ever falling, never melting, accumulated during hundreds of centuries in 

 one vast field of enormous thickness. Near the bottom of this mass a 

 plastic, porous sort of ice was gradually formed from the snow by the 

 pressure from above. This ice mass or glacier took upon itself a slow, 

 almost imperceptible motion to the south or southwestward, until it 

 covered three-fourths or more of what is now Indiana. As it moved 

 slowly southward great masses of partly-decayed rock and clay from 

 hillsides and jutting cliffs rolled down upon it and were carried on and 

 on until, by the melting of their icy steed, they were dropped hundreds of 

 miles from the parent ledge. Large irregular masses of rock from the 

 region in which the glacier Avas formed were either frozen into its nether 

 portion or rolled along beneath it, and as the ice sheet moved they served 

 as great stone drags, grinding down and smoothing off the hills and 

 ridges and tilling up the valleys, until the irregular, uneven surface of 

 the old preglacial rocks was planed and polished. 



From the stria^ formed by these imprisoned boulders and from other 

 evidence which it is diflicult to otherwise explain, it is now believed that 

 there were several distinct epochs in the glacial period. The great ice 

 sheet, which was at first formed, several times advanced and as often— 

 by an increase of the temperature of the region which it entered— melted 

 and receded; its retreat or recession being each time as gradual as its 

 advance had been. Like a great army which has attempted the invasion 

 of a country and has been compelled to withdraw, it would again assem- 

 ble its forces and start in a slightly different direction. But, perchance, 

 before it had reached the limit of its former invasion a force of circum- 

 stances would render a retreat necessary. Its advancing margin was thus 

 not in a straight line, but in lobes, or long, gradual curves. 



