PRE-WISCONSIN EVENTS 385 



100 and 40 foot levels. At points on the coasts of Long island, Marthas 

 Vineyard, Buzzards bay, and cape Cod there are indications of terraces 

 at both these levels ; but if ever present on Fishers island they were so 

 altered by the Wisconsin ice invasion as to be practically unrecognizable. 

 The only suggestion of such a terrace was near the south end of West 

 harbor, where an imperfect 40-foot level was observed. 



During this period not only was marine erosion at work, but streams 

 were actively deepening the valleys. On Long island quite a complete 

 drainage system with channels 40 to 80 or more feet deep was developed, 

 being In marked contrast to the post-Wisconsin erosion, which has only 

 cut a few insignificant gullies. In fact, the length of the erosion interval 

 known as the vineyard interval, seems to have been very many, per- 

 haps fifty, times as long as the post- Wisconsin period. ' 



In the central portion of the country the erosion between the Wiscon- 

 sin and the Iowan stages of glaciation was very slight, indicating a period 

 of rather limited length and one not in harmony with the great length 

 of the Vineyard interval. It seems likely, therefore, that the latter in- 

 terval covers more than the brief Peorian stage, representing rather the 

 combined Sangamon, Iowan, and Peorian stages. The faot that the Iowan 

 ice did not reach very far south in the Mississippi valley suggests that 

 it may likewise have fallen short of reaching the Long Island region. If 

 the Montauk stage is to be regarded as the equivalent of the Illinoian, 

 there is certainly no evidence of the Iowan on the island. 



Wisconsin deposits. — The Wisconsin drift, as already indicated in con- 

 nection with the descriptions of several of the older deposits, is commonly 

 very thin, being usually represented by a sheet only a few feet in thick- 

 ness. The ice during its maximum advance completely covered the 

 Fishers Island region, reaching as far as Block island, 10 to 15 miles 

 farther to the south. Allowing a gradient of 40 feet to the mile of the 

 ice surface, which is one frequently assumed, there would have been 

 from 400 to 600 feet of ice over Fishers island. Notwithstanding this 

 considerable thickness, the ice seems to have accomplished very little in 

 the way of erosion. It is doubtless true that there was a further round- 

 ing of the pre- Wisconsin hills, although the main sculpturing, if due at 

 all to ice-action, was accomplished by the more powerful Montauk sheet. 

 In fact, beyond a moderate rounding of some of the hills, the scooping 

 out of a few basins in the soft sands, the shoving up of low barriers of 

 drift across old valleys, and the deposition of a thin drift mantle, the 

 Wisconsin ice appears to have accomplished but little. The absence of 

 evidence of pronounced erosion and the slight thickness of the deposits 

 both point to a relatively slow and weak movement of the ice, and it is 

 probable that the period of occupation was not a long one. 



