CHAPTER VI 



ABNORMAL VARVES, AND DISTURBANCES IN THE 



CLAY 



Abnormally Thick Varves 



Now and then single varves or groups of them are abnormally 

 thick and consist of silt or sand covered by the usual thin clay 

 layer. Such varves, as a rule, do not reflect the climatic condi- 

 tions — the amount of melting — and in other lakes are usually 

 matched by varves of normal thickness. Thick varves can be 

 due to different causes. Some of them were caused by a strong 

 current which for a number of years swept over the locality, 

 depositing silt and sand, and then shifted its course (cf. Fig. 19 

 and pp. 51, 54, 57, 82). The cause of other abnormally thick 

 varves lies in an increase of the drainage area by the uncovering 

 of a wider part of the main valley or of extensive tributary val- 

 leys (cf. p. 82). In still other cases, deposition of thick varves 

 was due to shallowing of the water as the lakes filled up with 

 sediments or drained out. The depth of water at which deposi- 

 tion of abnormal varves begins varied, of course, according to 

 the situation of the locality, the size of the lake, and the amount 

 of water passing through it. In the lake at Concord, N. H., as 

 explained on page 82, the critical depth at some points was 

 reached as much as 100 feet beneath the level of the lake, while 

 in quiet bays clay sedimentation went on until the bottom was 

 built up to within 15 or 20 feet of the lake level (cf. localities 

 57 and 46). 



Drainage Varves 



Most interesting are those abnormal silty and sandy varves 

 which represent drainages of lakes that were ponded between 

 the ice edge and higher land, or by dams of glacial deposits. 



