CHAP. XIII. PRODUCED BY GLACIERS AND ICEBERGS. 231 



current varied from time to time in the same place, a strati- 

 fied arrangement. 



In those regions where glaciers reach the sea, and where 

 large masses of ice break off and float away, moraines, such 

 as I have just alluded to, may be transj^orted to indefinite 

 distances, and may be deposited on the bottom of the sea 

 wherever the ice happens to melt. If the liquefaction takes 

 place when the berg has run aground and is stationary, and 

 if there be no current, the heap of angular and rounded stones, 

 mixed with sand and mud, may fall to the bottom in an iin- 

 stratified form, called "till" in Scotland, and which has been 

 shown in the last chapter to abound in the ]Srorfolk cliff's; 

 but should the action of a current intervene at certain points 

 or at certain seasons, then the materials will be sorted as they 

 fall, and arranged in layers according to their relative weight 

 and size. Hence there will be passages from till to stratified 

 clay, gravel, and sand, and intercalations of one in the other. 

 Many of the blocks of stone with which the surfaces of ghiciers 

 ai*e loaded, falling occasionally through fissui*es in the ice, get 

 fixed and frozen into the bottom of the moving mass, and are 

 pushed along under it. In this position, being subjected to 

 great pressure, they scoop out long rectilinear farrows or 

 grooves parallel to each other on the subjacent solid rock. 

 Smaller scratches and strise are made on the polished surface 

 by cr^^stals or projecting edges of the hardest minerals, just 

 as a diamond cuts glass. 



In all countries the fundamental rock on which the boulder 

 formation reposes, if it consists of granite, gneiss, marble, or 

 other hard stone capable of permanently retaining any super- 

 ficial markings which may have been imprinted upon it, is 

 smoothed or polished, and exhibits parallel stria? and furrows 

 having a determinate direction. This pi-evailing direction, 

 both in Europe and North America, is evidently connected 



with the course taken by the erratic blocks in the same dis- 



16 



