CHAP. viir. ICE-ACTION IN THE BEDS OF RIVERS. 139 



When '' ice-jams" occur on the St. Lawrence and other 

 Canadian rivers (lat. 46° N.), the sheets of ice, which become 

 packed or forced under or over one another, assume in most 

 cases a highly inclined and sometimes even a vertical position. 

 They are often observed to be coated on one side with mud, 

 sand, or gravel frozen on to them, derived from shallows in 

 the river on which they rested when congelation first reached 

 the bottom. 



As often as portions of these packs melt near the margin 

 of the river, the layers of mud, sand, and gravel, which result 

 from their liquefiiction, cannot fail to assume a very abnormal 

 arrangement, — very perplexing to a geologist who should 

 undertake to interpret them without having the ice-clue in 

 his mind. 



Mr. Prestwich has suggested that ground-ice may have 

 had its influence in modifying the ancient alluvium of the 

 Somme.* It is certain that ice in this form plays an active 

 part every winter in giving motion to stones and gravel in 

 the beds of rivers in European Russia and Siberia. It appears 

 that when in those countries the streams are reduced nearly 

 to the freezing-point, congelation begins frequently at the 

 bottom: the reason being, according to Arago, that the current 

 is slowest there, and the gravel and large stones, having parted 

 with much of their heat by radiation, acquire a temperature 

 below the average of the main body of the river. It is, 

 therefore, when the water is clear, and the sky free from 

 clouds, that ground-ice forms most readily, and oftener on 

 pebbly than on muddy bottoms. Fragments of such ice, 

 rising occasionally to the surface, bring up with them gravel, 

 and even large stones. 



Without dwelling longer on the various ways in which ice 

 may affect the forms of stratification in drift, so as to 

 cause bendings and foldings in which the underlying or over- 



* Prestwich, Memoir read to Royal Society, April, 1862. 



