FOSSILIFEROUS MARINE CLAYS 535 



upper surface overlain by a horizontal deposit of peat and soil, can on\y 

 be explained by erosion and subsidence since the clay was deposited. The 

 chief question in this connection is the age of the clay. It overlies till 

 and is therefore Quaternary. When its topography is compared with the 

 fiat and little eroded clays of the Maine coast, it must be considered some- 

 what older than they; and if they are of post-Wisconsin age, this clay 

 probably corresponds with the greatly eroded "high-level" clays of Maine 

 which are under discussion here. The borings indicate an elevation of the 

 coast (post-Montauk) to at least 50 feet above the present sealevel at 

 Portland, followed by erosion, which is indicated by the soil and oxida- 

 tion zones to have been interglacial ; then came a subsidence to at least 

 80 feet below sealevel, when the later clays were deposited, followed by the 

 recent elevation, which raised the clays at least 80 feet above tide at this 

 place. 



Extensive boring at Boston has revealed similar erosion of the clays, 

 and indication of an interglacial stage is furnished by occasional examples 

 of oxidized deposits overlain by more recent deposits of clay, gravel, and 

 till. A detailed study of samples from these borings has been made by 

 Crosby,* who reports a superficial yellowing on the clay underlying the 

 harbor silts. The clay at Boston is characterized by an unconformity 

 similar to that at Portland. Borings along the line of the subways, the 

 East Boston tunnel, and extensive bridge and well boring in the, city and 

 harbor show a widespread deposit of clay extending from 5 feet above tide 

 to 200 feet below tide. The yellow, or oxidized clay, is sometimes found 

 as much as 30 feet below low water. Eegarding this feature, Crosby 

 writes as follows : 



"Oxidation makes the clay, and also the boulder-clay, harder and firmer, by 

 cementing the clay particles by iron oxide ; and this may explain the fact that 

 the boring records often indicate 'hard clay' above, passing downward tlirough 

 'stiff clay' to 'soft clay.' " 



The following is selected by the present Avriter as a typical boring sec- 

 tion. It is situated at the corner of Beach street and Atlantic avenue : 



Section, of Boring at Boston 



Material Feet 



4. Filling 19 



3. Hard yellow clay, sand 7 



2. Hard yellow clay 5 



1. Soft blue clay 28 



Total 59 



* A study of the geology of the Charles River estuary and the formation of Boston 

 harbor, in "Report of the committee on the Chai-les River dam," Boston, 1903, pp. 

 845 .SOS). 



