306 HANDBOOK OF NATURE STUDY 



difficult to prove where the detached stones or bowlders 

 came from. 



Everywhere in our Northern States, but especially along 

 rivers, streams, and lake shores, we find large and small 

 stones which do not consist of the same material as the 

 nearest stratified rock ; they are nearly always much harder 

 than the nearest sandstone and limestone, and show a great 

 variety of color and texture. Occasionally a bowlder is 

 found which is so large that twenty horses could not move 

 it an inch. Shall we believe that once water rushed over all 

 our Northern States with such terrific velocity that it could 

 move and scatter these monstrous rocks ? No, we must 

 look to another agency for an explanation of the presence 

 of these bowlders. 



Geologists have proved beyond a reasonable doubt that there 

 tvas a time when this country was covered by a sheet of ice 

 thousands of feet thick. This ice began to form in Canada 

 and in the Great Lake regions and was thickest there. On 

 account of the slope of the land, and by its own weight, it 

 crept slowly southward until it covered all our Northern 

 States from Long Island to near the mouth of the Ohio 

 Biver ; from there its southern margin extended in a north- 

 westerly direction to the mouth of the Missouri, and ex- 

 tended up that river into the Yellowstone Park region. 

 This slowly moving ice field broke many pieces of rock 

 from mountain and hill sides, and others it carried along on 

 the bottom. When the climate grew warmer again, the ice 

 melted and dropped stones and soil where we find them 

 to-day. The bowlders are of the same structure as the 

 rocks farther north over which the glacier flowed. The 

 farther south one goes, the smaller the bowlders grow, and 

 finally none are found any more. Some of you may know 



They should write up, in brief form, the results of the trips and walks 

 they take from time to time. 



