BOULDERS 



217 



and then the cHmate became colder 

 and the ice again advanced. For a pic- 

 ture of the northeastern United States and 

 easterii Canada during the Glacial period 

 we may look to Greenland and the An- 

 tarctic continent. 



Before this ice invasion, the northeastern 

 United States and eastern Canada had 

 been above the sea level for tens of mil- 

 lions of years. During all those long 

 ages the rocks had been undergoing 

 disintegration by the chemical and 

 mechanical agencies which have been 

 already referred to. On the lowlands 

 the bed rocks were covered with a thick 

 mantle of disintegrated rock, in which 



FIG. 6. JUDGES' CAVE, WEST ROCK. NEW 

 HAVEN, CONNECTICUT. 



there was a gradual transition from 

 solid rock at the bottom to fine soil at 

 the top. On the hill slopes rested 

 numerous boulders of disintegration, 

 where the more finely disintegrated 

 material had been washed away by the 

 raiub. As the thickening ice sheet 

 began to creep outward from its center, 

 it shoved along vast masses of the dis- 

 integrated material, scouring in most 

 places down to the fresh and unaltered 

 bed rock. Hence it is that in these 

 glaciated regions the mantle of disin- 

 tegrated rock is almost everywhere 

 composed of material which has been 

 transported a greater or less distance 

 from the place where it was formed. 

 Rarely do we find in New England the 

 gradual transition between unaltered 

 rock and soil which is characteristic 

 of the regions beyond the boundaries 

 of the great ice sheets. Such an ice 

 sheet would shove along in its resist- 

 less course boulders of disintegration 



FIG. 



BOULDER NEAR SOUTHINGTON, 

 CONNECTICUT. 



of every size, as well as the finer mate- 

 rial. Herein lies a very striking difit'er- 

 ence between transportation by water 

 and transportation by ice. Water mov- 

 ing slowly can transport only fine pow- 

 der. Water plunging in a fierce torrent 

 can carry boulders a few feet in dia- 

 meter. In general, the weight of the 

 largest block which moving water can 

 carry varies as the sixth power of the 

 velocity. But there is a limit to the 

 size of the stones which can be moved 

 by the fiercest mountain torrents. 

 There is, on the other hand, no limit to 

 the size of the rock masses which can 

 be transported by the slowly creeping 

 glacier. It carries coarse and fine 

 material with equal ease. 



A part of the material which is trans- 

 ported by every glacier is shoved along 

 beneath the ice mass ; and. in the case 

 of a continental ice sheet, much the 

 larger part of the material transported 

 is at the bottom. But some of the mate- 



■ic. 



.!■: ROCK, XE.VR .MIDDLETOWN, 

 CONNECTICUT. 



