THE MUSEUM 



73 



were born and lived and died and were 

 covered up in these ancient times, to 

 be seen by us to-day as stones which 

 have the shape of animals. 



These high mountains have been cut 

 down by the action of the elements; 

 the valleys in part have been filled up, 

 and this ancient sea bottom covered 

 by soil which was once a part of the 

 tops and sides of these mountains. 

 This work has been going on for all 

 time; it is going on to-day, and it will 

 continue to go cm. 



One of the most potent agents of 

 this denudation was ice, although the 

 length of time during which this agent 

 has acted is small by comparison with 

 the age of the earth. Over much of 

 this continent it has now almost ceased 

 to act, yet in the furthest north it is 

 still doing the work which once it did 

 over the whole of the northern North 

 America. 



That division of geological time in 

 which mammals had their greatest de- 

 velopment is called the tertiary period. 

 And it was during the ages which im- 

 mediately followed the close of that 

 period that the whole northern North 

 America was covered with an enor- 

 mous ice sheet thousands of feet in 

 thickness. Beneath this frozen mass 

 were deeply buried the whole of British 

 America, all of New England, a very 

 large portion of the Middle States and 

 much of the Rocky Mountains. In 

 northern New England the upper sur- 

 face of this ice sheet was at least 6,000 

 feet above the sea level, while it is es- 

 timated that still further north it was 

 not less than 13,000 feet in thickness 

 This vast mass was not, as might be 

 thought, at rest. Instead, it had a 

 general slow, but more or less constant 

 motion southward. This was the only 

 direction in which it could move, be- 

 cause to the south the ice was con- 

 stantly melting and disappearing, while 

 to the north it was always firm and un- 

 yielding, and tended constantly to in- 

 crease in thickness, and to push away 

 a portion of the mass in the direction 

 of least resistance. This pressure was 



constant and ever increasing, and urged 

 on by it, the ice sheet moved steadily 

 southward, creeping up high mountain 

 slopes, and then, when their summits 

 were reached, overtopping them and 

 pushing its way down on the other 

 side. 



It is a matter of common knowledge 

 that ice is to a certain extent plastic; 

 in other words, it can be made by 

 pressure to take certain forms, as wax 

 does, though of course it is not soft 

 like wax. Dr. Kane speaks of a table 

 of ice 8 feet thick and 20 wide, sup- 

 ported only at the sides, which in two 

 months' time, while the temperature 

 was constantly below the freezing 

 point, became by its own weight so 

 deeply bent that its center was depres- 

 sed 5 feet. By pressing ice through a 

 round hole it may be made to take the 

 shape of a long cylinder, or ice by 

 pressure may be made to copy a seal 

 or a mould. This being the case, we 

 may understand how this great ice 

 sheet moved forward, adapting itself 

 to the inequalities of the land, filling 

 up valleys, climbing mountain sides, 

 overtopping them and then flowing 

 down beyond them. The motion of 

 the ice sheet was slow, but it was sure. 

 It flowed onward slowly, as molasses 

 would flow on a cold day, but better 

 still, is the illustration employed by 

 Prof. Dana, who says: f 'If stiff pitch 

 be gradually dropped over a horizontal 

 surface it will spread and continue to 

 do so so long as the supply is kept up, 

 and if that surface rises at an angle in 

 one direction and there is no escape in 

 the other, it will first fill the space to 

 the level of the edge and then drop 

 over and continue onward its flow. So 

 glaciers, if the accumulation is ade- 

 quate, may go across valleys and over 

 elevated ridges. " 



It may be asked where did all this 

 ice come from and how did it accumu- 

 late to the enormous thickness already 

 named? The answer is simple: It is 

 the accumulated excess of the precipi- 

 tation of many ages over the annual 

 melting. The ice of the great ice 



