THE WORK OF SNOW AND ICE 141 



to make the streams flow more steadily through the year, by holding 

 back some of the water of wet times, and letting it flow out in times 

 of drought. The flow of streams from lakes is much steadier than 

 the flow of streams which have no such reservoirs to draw upon. 

 The drift is, on the whole, thicker than the mantle rock of other 

 regions, and therefore is able to hold more of the rain which falls 

 in wet weather. This water it yields up slowly, and so makes the 

 supply of ground-water to streams more steady than it would be 

 otherwise. 



Much of the drift clay (rock flour) is used for the manufacture 

 of brick, tile, etc., and the gravel of the drift is used for road making, 

 and in the manufacture of various sorts of cements. 



The results of glaciation were not always helpful to man. Thus, 

 in some places the quality of the soil was injured, for the drift of 

 some places is stony, and great labor is necessary to put it in 

 workable condition. In some places, too, it is too sandy or gravelly 

 to make good soil, and in other places its surface is too rough. 



On the whole it seems probable that the glaciated area of the 

 United States was benefited by the work of the ice. 



REFERENCES 



A. EXISTING GLACIERS 



1. Standard text-books on Geology. 



2. SALISBURY, Physiography, Advanced Course, Chapter V: Henry Holt 

 &Co. 



3. RUSSELL, Glaciers of North America: Ginn & Co., 1897. 



4. RUSSELL, Glaciers of Mount Rainier: 18th Ann. Rept. U. S. Geol. 

 Surv., Ft. II, 1896-1897, pp. 349-415. 



5. TYNDALL, The Glaciers of the Alps: Murray. 1860. 



6. Greenland Glaciers: Jour. Geol., Vol. IV, 1896, pp. 769-810. 



7. Alaskan Glaciers: Nat. Geog. Mag., Vol. Ill, pp. 176-188 (RUSSELL), 

 Vol. IV, pp. 19-55 (REID), Jour. Geol., Vol. I, pp. 219-245 (RUSSELL). 



B. RESULTS OF GLACIATION 



8. CHAMBERLIN AND SALISBURY, Earth History, Vol. Ill, pp. 327-446: 

 Henry Holt & Co., 1906. Also College Geology, Chapter XXX. 



9. SALISBURY, The Drift: Jour, of Geol., Vol. II, pp. 708-724 and 837- 

 851, and Vol. Ill, pp. 70-97. 



