GABDINBE CLAY 517 



5. By their occurrence on the immediate surfaces of the pre-Glacial 

 bed-rock and usually occupying depressions in it. 



UNCONFORMITY SUCCEEDINO DEPOSITION OF OLD TILLS 



The old tills are found only locally, indicating that they were exten- 

 sively eroded before the deposition of the overlying drifts. The stage of 

 erosion must have been a long one, and created a great gap in the Pleisto- 

 cene record, aboi;t which nothing is loiown, as no contact has been found 

 between the tills and the next succeeding deposits. If the old till is of 

 pre-Ivausan age, the erosion may have extended from the Aftonian to the 

 Yarmouth stage. If the till corresponds to the Kansan, the erosion 

 came in Tarmouth time. It is possible that this stage of unconformity 

 was the time during which some of the deep river gorges of the New 

 England coast were eroded. As stated on page 513, the principal erosion 

 of the Hudson valley presumably took place at this time. 



GARDINER CLAY 



General statement. — With the exception of the ancient tills, the oldest 

 deposit recognized north of Boston consists of stratified clay exposed 

 underlying the drumlins at Winthrop, Massachusetts, and described by 

 Stimpson in 1854.* In the blue clay at that locality 14 species of fossils 

 were found and are listed in column 4 of the table (page 520). 



Dodge, in 1888, found strata of sand and clay outcropping below Great 

 head in the same town, and collected the species of fossils noted in column 

 5. Upham, in the same 3'ear, found fossils in a stratum of blue clay 

 under Grovers cliff, in the same town, and this exposure was seen and 

 verified by the present writer in 1905. 



Relations and correlations. — In none of tlie three cases noted above 

 can the base of the clay be seen. In all three cases the stratified deposits 

 are overlain by the hard boulder-clay of the drumlins, rising to heights 

 of over 50 feet above tide. The clay found below the drumlins is be- 

 lieved to be of the same age as the Gardiner clay described by Fuller, f 

 and to date from the "Sankaty" sub-epoch of WoodM^orth.J 



Distinction from other clays. — This clay is distinguished from Tertiary 

 clay by its color, by its general appearance, and by its fossils. It is dis- 

 tinguished from more recent clays by being overlain by drum! in till 

 (]\Iontauk). The fact that the till of the drumlins in Boston harbor 

 contains large numbers of fossil shells§ of tlie same and similar species 



♦ Proc. Boston Soc. Nat. Hist., vol. Iv, pp. 9-10. 



t Bull. Geol. Soc. Am., vol. 16, 1005, pp. 375-378 ; Bull. no. 285, U. S. Geological Sur- 

 vey, 1906, p. 433. 



t Seventeenth Ann. Kept. U. S. Geological Survey, 1806, p. 966. 



I Upham: Proc. Boston Soc. Nat. Hist., vol. xxiv, 1888, pp. 127-141. 



