FOSSILIFEROUS MARINE CLAYS 533 



1. Greater weathering and oxidation than deposits of Wisconsin age. 



2. Occurrence of oxidized zones at the top of huried clay surfaces 

 which were formed by snhaerial erosion. 



3. Occurrence of overlying till and nlorainal deposits. 



4. Folding and erosion due to action of overriding ice. 



5. Eeworked upper portion of clays. 



6. Older topography and greater elevation of these clays than of cer- 

 tain clays believed to be of Wisconsin age. 



7. Occurrence of overlying buried soils. 



1. Degree of oxidation. — Something of the relative ages of different 

 deposits can be inferred from the relative depths of superficial oxidation 

 or yellowing due to action of the weather. In connection with the Mon- 

 tauk till (page 525), it was stated that tlie oxidation of that material 

 was much deeper than that of the ordinary superficial till. The oxida- 

 tion of the former is 10 to 20 feet deep, Avhile the latter, notwithstanding 

 its looser and more pervious character, is, as a rule, not yellowed over 

 3 to 5 feet. This can be seen in the ordinary boulder-clay over large 

 areas in southern Maine. The marine clays are like the ]\Iontauk till in 

 being rather deeply oxidized, in most places being yellowed to a depth of 

 5 to 12 feet from the surface. The following exposures will serve as 

 examples : 



At many brick-yards in Medford, Chelsea, and Everett, Massachusetts, 

 the upper 5 to 10 feet of the clay are bufE or mottled and in places some- 

 what blocky, below which it is blue drab in color. In this region the 

 upper 3 to 5 feet are in some places rather stony and appear to be in 

 reality till composed largely of reworked clay. 



At Trask's brick-yard, Danvers, 9 feet of buff gray clay have been seen 

 to overlie the blue clay. 



At a brick-yard in Lewiston, Maine, about 200 feet above tide, 12 feet 

 of buff clay are exposed over 20 feet or more of blue clay. 



At Mechanic Falls, Maine, 20 feet of clay are exposed in a brick-yard ; 

 the upper 5 feet are very yellow, while below that it is blue. 



Near the village of Eliot, Maine, where 10 feet of clay can be seen to 

 be overlain by 2 to 5 feet of sand, the upper 5 to 8 feet of clay are gray- 

 ish, but the lower part is blue. 



In Brewer, Maine, 30 feet of Avell stratified clay are exposed, and the 

 upper 8 to 15 feet are yellowish, while the rest is blue. 



In Chelsea, Maine, the upper 12 feet of the clay are buff brown, the 

 rest being blue drab. 



Some places have been seen where the yellowing extends only 2 to 3 

 feet from the surface. This might be supposed to favor the view, ad- 



