A HISTORY OF SCIENCE 



to Dr. Croll's estimate. It crushed and ground every- 

 thing beneath 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 nu- 

 cleus of the land masses now known as Long Island 

 and Staten Island; other of its deposits formed the 

 " dmmlins " about Boston famous as Bunker and 

 Breed's 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 lakes which everywhere abound over Northern ter- 

 ritories. Some glacialists even hold the view first sug- 

 gested by Ramsey, of the British Geological Survey, 

 that the great glacial sheets scooped out the basins of 

 many lakes, including the system that feeds the St. 

 Lawrence. At all events, it left traces of its presence 

 all along the line of its retreat, and its remnants exist 

 to this day as mountain glaciers and the polar ice cap. 

 Indeed, we live on the border of the last glacial epoch, 

 for with the closing of this period the long geologic past 

 merges into the present. 



PAST, PRESENT, AND FUTURE 



And the present, no less than the past, is a time of 

 change. This is the thought which James Hutton con- 

 ceived more than a century ago, but which his contem- 

 poraries and successors were so very slow to appreciate. 



164 



