POSSILIFEROUS MARINE CLAYS 531 



deposits in New England can be made with certainty on this basis alone. 

 Fossils are of some value, however, in checking the conclusions formed 

 on the basis of stratigraphic evidence. 



Columns 1 to 6 in the table (pages 520-523) are localities in which 

 the clays (Gardiner) are believed to antedate the principal till deposit 

 (page 524). The remaining columns are those containing names of fos- 

 sils found in the more recent clays. It is probable that most of these 

 latter occurrences belong to the main clay deposit, but, owing to the fact 

 that the clays in general can not be differentiated on paleontologic or 

 lithologic evidence, it is possible that some of them may be of post- 

 Wisconsin age (see page 553). 



Distinction between high-level and low-level clays. — A study of the 

 marine clay deposits in the field shows at least two distinct topographic 

 types : 



First. Over wide areas the clay is characterized by a very "old-looking" 

 topography. This type of clay is found in localities more or less pro- 

 tected from erosion in nearly every large valley between Boston and the 

 New Brunswick line. It rises from 20 feet above sealevel near Boston 

 to 70 feet in the Merrimac valley, 100 feet in New Hampshire, and about 

 300 feet in parts of the Kennebec valley in Maine. 



Second. In many regions along the coast, in Massachusetts, New 

 Hampshire, and Maine, there are broad, fiat deposits of clay which do 

 not rise more than 20 to 60 feet above tide. These are not remnants of 

 elevated clays having old topograpliy, but are generally foimd in local- 

 ities exposed to erosion and are commonly characterized by flat surfaces. 

 The valleys cut in them are mostly deep V-shaped valleys, which appear 

 very recent. 



For convenience of description the first type of clay will be spoken 

 of as the "high-level" clay and the second type as the "low-level" clay. 

 The high-level clay comprises the deposits described on pages 531-544, 

 and the low-level clay is described on page 553. 



Relations of the high-level clay. — In Massachusetts and New Hamp- 

 shire even the high-level clay is at such a low elevation that its base is 

 seldom seen, and wells frequently penetrate it to depths of over 150 feet. 

 Numerous well records in Cambridge, Revere, Everett, Saugus, Lynn, 

 Salem, and Newburyport, Massachusetts, and in Exeter, New Hampshire, 

 pass out of clay at depths of 30 to 100 feet into underlying water-bearing 

 gravels. In Maine, and especially in the valleys of the Kennebec and 

 other rivers, the clay can be seen to rest unconformably on the gravels. 

 In places it lies in the bottom of the valle}^, while in others its base rises 

 CO a height of over 150 feet. This indicates that the land may have 



