NATURE STUDY IN SCHOOLS. 187 



VACATION STUDIES IN GEOLOGY. 



BY 



M. Eva Warren. 



(Read before the Maynard Chapter of the Newton Natural History Society, 



Sep. 15, 1899.) 



Although the results of glacial action can be seen in every gravel-pit 

 and on every stony hill-side in the form of boulders, rounded pebbles, and 

 other debris, it is not always so easy to find the marks known as glacial 

 scratches made in the solid, or bed rock. These scratches were caused by 

 the passage of the great ice sheet, which, in ages long passed, swept slowly, 

 but with irresistable force over the country. 



This ice-sheet ( which we have reason to believe was about a mile in 

 thickness) broke off fragments of rock from "projecting ledges which it passed, 

 and often carried them along with it embedded in its substance. 



Following the law of gravitation, these rock fragments gradually set- 

 tled down into the ice-mass, the heaviest sinking to the bottom. Thus, in 

 time, the lower surface of the glacier must have been thickly studded with 

 pieces of rock of various sizes and forms, making of it a huge sheet of sand 

 paper covering an area of thousands of square miles. 



A foot of cubic ice weighs one hundred pounds. A pile of these cu- 

 bical blocks a mile in height would weigh nearly three hundred tons. From 

 this we can get some idea of the pressure of this immense mass. 



Think then of this huge sheet of ice moving slowly over the rocks 

 which form the earth's crust. Its grains were often huge rock points, and 

 these, with three hundred tons pressure above them, were rasping over the 

 rocks beneath ; and no matter how hard their surface, it must needs show 

 some evidence of this powerful sand-papering. 



We should expect to find such evidence on the exposed rocky hill- tops ; 

 but if we look for this, we shall, in most cases, be dissapointed. We have 

 not perhaps taken into consideration the lapse of time which has occurred 

 since the ice passed over the hill-tops. Rock exposed to the action of the 

 elements for a thousand years changes greatly, being worn away by the rains 

 of summer and the ice and snow of winter. How great then must have 

 been the effect of the wear and tear of fifty thousand years ! 



For evidence of this glacial marking, we must find some place which 

 has not been exposed to such corrosive action. Such places are to be found 



