THE STORY OF NINETEENTH-CENTURY SCIENCE 



side of the Rocky range, Tertiary strata were slowly 

 formed to the ultimate depth of two or three miles, en- 

 closing here and there those vertebrate remains which 

 were to be exposed again to view by denudation when 

 the land rose still higher, and then, in our own time, to 

 tell so wonderful a story to the paleontologist. 



Finally the interior seas were filled, and the shore 

 lines of the continent assumed nearly their present out- 

 line. 



Then came the long winter of the glacial epoch per- 

 haps of a succession of glacial epochs. The ice sheet 

 extended southward to about the fortieth parallel, driv- 

 ing some animals before it, and destroying those that 

 were unable to migrate. At its fulness, the great ice 

 mass lay almost a mile in depth over New England, as 

 attested by the scratched and polished rock surfaces and 

 deposited erratics in the White Mountains. Such a mass 

 presses down with a weight of about one hundred and 

 twenty-five tons to the square foot, according to Dr. 

 CrolFs estimate. It crushed and ground everything be- 

 neath it more or less, and in some regions planed off 

 hilly surfaces into prairies. Creeping slowly forward, it 

 carried all manner of debris with it. When it melted 

 away its terminal moraine built up the nucleus of the 

 land masses now known as Long Island and Staten Isl- 

 and ; other of its deposits formed the " drumlins " about 

 Boston famous as Bunker and Breeds hills; and it left a 

 long irregular line of ridges of " till " or bowlder clay 

 and scattered erratics clear across the country at about 

 the latitude of New York City. 



As the ice sheet slowly receded it left minor moraines 

 all along its course. Sometimes its deposits dammed up 

 river courses or inequalities in the surface, to form the 



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