SUMMARY 555 



of land to a height of 60 feet or more, and a subsequent subsidence before 

 the deposition at Boston, Portland, and elsewhere of the overlying plains 

 of sand and gravel having every characteristic of retreatal deposits, and 

 which themselves have been elevated and considerably eroded since their 

 formation. 



6. Glacial periods have existed in many parts of the world, and have 

 extended in geological time from the Cambrian to the Recent ; Pleistocene 

 ice-ages, where they occur outside New England, are nearly always recog- 

 nized as several in number ; in New England the land surface is much 

 more diversified and mountainous than elsewhere in the glaciated por- 

 tions of this country, and consequently more favorable to the recurrence 

 of glacial conditions. Therefore, is it not to be expected, even on a theo- 

 retical basis, that a similar sequence of events has transpired here ? 



Summary 



To summarize, there seems to be good evidence, derived from the classes 

 of phenomena noted on page 512 and explained on subsequent pages, 

 that New England, like the rest of the glaciated regions of North Amer- 

 ica and Europe, has been subjected to several glacial invasions. The 

 sequence of deposits is as outlined opposite page 512, and it is believed 

 that they represent three glacial and two interglacial stages. The latest 

 glacial stage that covered the whole region probably corresponds very 

 closely with the Wisconsin glaciation of the middle West. 'Wliether or 

 not the Montauk glaciation occurred simultaneously with the Illinoisan 

 and whether the pre-Montauk tills here are Ivansan or pre-Ivansan in 

 age or still older is only speculation. Ji;dging from the similarity of 

 succession, however, there would seem to be a fair degree of probability 

 that the Montauk glaciation occurred some time near the Illinoisan. 

 The great extent to which weathering has gone in some of the older de- 

 posits would seem to place their date of origin more nearly with the pre- 

 Ivansan than with the Kansan. 



The great diflficulty in any attempted correlation between the East and 

 the West arises through the absence of anything definite in the nature of 

 lowan deposits. It is true that the main pre-Wisconsin deposit of 

 marine clay probal)ly took place at about lowan time, and that its topo- 

 graphic and structural relations are rather similar to those of the water- 

 laid types of lowan loess in the Mississippi valley ; but in composition 

 and origin the two deposits show no similarity. The possible presence 

 of lowan till in northern New England is suggested by the section at 

 Toronto, Canada, 300 miles west of New England, where several distinct 



