DIVERSITY OF THE GLACIAL PERIOD 775 



tion uncertain. Separated as this is from the preceding glacial 

 epoch by heavy beds of clay and fossiliferous sands, and from 

 that following by marked erosion, this period seems to deserve 

 the rank of an epoch. 



6. The exact nature of the erosion in the interval between 

 the folding and the deposition of the Tisbury beds is difficult to 

 determine. That suggested by the outcrops is a decapitation of 

 the folds by wave action, the land standing somewhat lower 

 than today. The nonconformity shown is very marked and 

 forms a ready plane of reference. 



7. The Tisbury or Manhasset beds represent glacial outwash 

 when the land stood at least 200 feet lower on Long Island, and 

 140 feet lower on Marthas Vineyard, than it does today, and the 

 bowlders in the Marthas Vineyard and the bowlder beds in the 

 Long Island deposits perhaps represent the deposits of floating 

 ice. 



8. In the Vineyard interval, which separated the Tisbury from 

 the Wisconsin, there was widespread erosion ; the land stood 

 somewhat higher than today, and the surface of the preceding 

 deposits was greatly trenched by stream erosion. 



9. In the last ice advance the change in the shape of the ice 

 lobes, shown by the crossing of the two outer Wisconsin 

 moraines, suggests the twofold division of the Wisconsin found 

 by Leveret in Ohio. 



CORRELATIONS. 



The correlation of the Pleistocene succession here shown with 

 that in the Mississippi valley is necessarily more or less tentative, 

 but cerain points are worthy of note : (i) the number of glacial 

 epochs is in each case the same, and (2) the correlation of the 

 Pensauken and extra-morainic drift of New Jersey with the pre- 

 Kansan, or sub-Aftonian, which will follow if these glacial 

 epochs in each case are to be regarded as synonymous, is sup- 

 ported by a growing feeling among Pleistocene geologists that 

 the old extra-moranic drift of New Jersey is rather older than 

 the Kansan ; hence the growing tendency to say Kansan or pre- 

 Kansan in speaking of these deposits. These results are remark- 

 ably in accord with this idea, and if the oldest and youngest 



