ABNORMAL VARVES 73 



Disturbances due to grounding of icebergs are usually quite 

 local, even if a great number of varves may have been involved. 

 Sometimes, however, icebergs may have given impulse to 

 extensive slides. 



Dislocations due to a readvancing of the ice sheet were more 

 extensive and thorough. The ice jammed the clay beds together, 

 overturned clay blocks, folded the layers, and kneaded sand and 

 till into the clay. 



Faults in the clay seem generally to have been caused by 

 settling due to melting away of icebergs which, overloaded with 

 material, had sunk to the lake bottom and had become buried 

 in the sediments. 



Disturbances caused by pressure of the clay itself or by the 

 weight of overlying strata are most likely to occur in very fat 

 clay or quicksand. They are treacherous and can sometimes 

 be detected only by comparison of different measurements. 



