212 



PHYSICAL GEOLOGY 



As already indicated, glacier deposits are unstratified, 

 and consist commonly of materials of many kinds and sizes. 

 Ice-ground clays usually retain the chemical character of 



FIG. 223. Drumlin near McFarland, Wis. (Alden, U.S. Geol. Surv.) 



the parent rocks. Stream-borne silts, in contrast, are gen- 

 erally the product of weathering, and therefore differ 

 chemically from the rocks from which they were derived. 

 Melting ice has sometimes left great bowlders in seemingly 

 insecure positions, forming " perched bowlders," " rocking 

 stones," etc. 



In some places till has lodged beneath ice sheets to form 

 oval hills, called drumlins (Figs. 223 and 224). Drumlins 

 vary in length from less than 100 feet to more than a mile, 

 and in height from 15 or 20 to 150 or 200 feet. They are 

 common in eastern Massachusetts, in parts of New York and 

 Wisconsin, and in some other localities. In contrast with 



FIG. 224. Drumlin one mile northeast of Gleasondale, Mass. 

 (Alden, U.S. Geol. Surv.) 



ice-worn rock hills (p. 219), the shorter and steeper slopes of 

 drumlins generally face the direction from which the ice sheets 

 came. 



The surfaces of glacier deposits are characteristic (p. 205). 



