72 ICE RECESSION IN NEW ENGLAND 



varves are comparatively thin at locality 91, 3>< miles up the 

 Passumpsic valley. The lakes had probably been dammed on 

 the northwestern slopes of the White Mountains. Varve 6997 

 (normal curve 19, PI. V) offers another interesting case. The 

 varve, at localities 67 and 63, marks a great drainage, measuring 

 16 and 14 inches (40.5 and 35.5 cm.), but is normal from locality 

 68 northward, showing that the drainage originated south of 

 this. Since the ice edge by this time had retired far beyond 

 Woodsville, the lake in question must have been ponded by 

 glacial drift. 



Thick Varves as Indications of Warm Summers 



Some unusually thick varves seem to be a true expression of 

 the amount of melting and probably represent very long and 

 warm summers. They illustrate the necessity- of having measure- 

 ments from different valleys in order to understand the meaning 

 of many varves and also show that, in our caution not to assume 

 too great climatic fluctuations, we may easily go too far (cf. p. 56). 



Disturbances in the Clay 



Such disturbances in the clay as folded and squeezed zones 

 and faults are essentially of negative scientific interest, because 

 they prevent positive observations. They can have been caused 

 by sliding of the clay, grounding of icebergs, readvancing of the 

 ice sheet, melting of buried ice, sliding or washing down of 

 rock or soil from a hillside, pressure due to the weight of accu- 

 mulated strata, etc. 



Disturbances caused by sliding are frequent in regions where 

 the clay is fat and where it rests on an uneven surface. Sometimes 

 the whole clay deposit has slidden. In other cases a few varves, 

 which formed a shearing plane, are crumpled up so as to appear 

 homogeneous, while the zones below and above are undisturbed. 

 Such contorted zones, one-half to one foot thick, can often be 

 traced continuously through long exposures. In greasy clays 

 slides can occur even if the dip of the strata is slight. 



