FOSSILIFEROUS MARINE CLAYS 



541 



like that described on page 539 could be so produced, however. Tn several 

 cases the occurrence of folds in which the inclination of the strata is 

 toward the south seem to indicate a lateral pressure from the north. If 

 lateral pressure was the cause, it can hardh^ have been due to anything 

 but ice. 



The general assumption in the past has been that such instances were 

 due to local readvances of the ice. If, however, the clay at Haverhill, 

 Augusta, Portland, and elscAvhere is of the same age as that in Portland 

 and Boston harbors, as its relations with the underlying clay and overly- 

 ing gravels render probable, the evidence afforded by the subaerial erosion 

 and oxidation of the clay surface must be taken into account as intlicat- 



T\ " ■ -■ ■' . ". — — v—^ii 



j2_ 



-SF*- 



Figure 6. — Section at Munjoy Bill, Portland, Maine. 



Showing contorted clay and its relations to underlying till and overlying gravels. A, 

 hard, blue till, Montauk ; B, stratified sand and clay, unconformable on A ; C, coarse, 

 stratified gravel, resting unconformably on B ; D, heterogeneous, boulder-filled and 

 boulder-strewn gravel, possibly representative of Wisconsin till. 



ing a lapse of thousands of years between the deposition of the c\&y and 

 its folding. On the basis of this kind of evidence it seems probable that 

 ice has covered the greater part of the surface of the New England clays. 

 The absence of folding and of deposits of till over considerable areas is 

 not evidence to the contrary, for in Alaska, where living glaciers are 

 seen, they have Ijeen observed by P. E. Wright and otliers to be at pre,~ent 

 retreating in places over clayey deposits with almost no deposition of 

 till and no perceptible erosion or folding. Absence of erosion may per- 

 haps be due to the presence of water above the deposits during tlie ice- 

 advance. 



5. Eeworked upper part of some clays. — In most of the clay pits which 

 have been examined the upper few feet of clay is different in character 

 from the lower part. The upper 5 to 8 feet is commonly imstratified, 



