accompanied the accumulation on the continents of large volumes of ice. It may therefore be 

 assumed that the Hudson was not then an estuary, as it is today, but a rapidly flowing stream, 

 capable of carrying in flood time the coarse sediments which now form the Jameco gravel. 

 While this hypothetical river of glacial origin seems the most logical source of material of the 

 composition of the Jameco gravel, the main valley of the Hudson, then as now, flowed west of 

 Manhattan and Long Island, and so out to sea. We must imagine that the river was deflected 

 to the east, across Kings and Queens Counties, where the formation is now found, either by 

 a tongue of stagnant ice, or an ice lobe which advanced from the west. Later in the Pleistocene 

 time, probably during the height of the Wisconsin ice advance, part at least of the flow of the 

 Hudson was indeed deflected east through the Harlem River, by the temporary blocking of the 

 main channel by the ice of the glacier itself. 



It is a necessary part of the hypothetical origin of the Jameco outlined above, that the ice 

 in the Hudson River Valley was at that time some distance to the north, so that the melt water 

 would have an opportunity of collecting in the Hudson Valley, along the west bank of which is 

 found the only obvious source of the diabase which forms such an outstandingly large part of 

 the formation. One must assume therefore that the blocking of the lower Hudson Valley which 

 resulted in the deflection of the river eastward over Queens and Kings Counties was the result 

 of the independent advance of a separate lobe of the ice. This lobe might have moved out from 

 the low lands of the Passaic River and blocked the Hudson near what is now the Narrows. 

 Such an independent movement of a lobe of the main ice sheet is entirely in keepin with what 

 we know of the behavior of large ice sheets. 



A second hypothesis of the origin of the Jameco gravel imagines it to be the normal out- 

 wash of a glacier which advanced nearly but not quite to the present north shore of Long Island. 

 A series of streams may be imagined to have flowed from points scattered along the front of the 

 ice, spreading a blanket of gravel and sand for some distance to the south. The topography 

 which must have guided these streams and their deposits is not known to us today. The depression 

 which is now Long Island Sound must have existed then, but how deep or how wide it was, and 

 to what extent it may have been filled in, it is quite impossible to say. The depression may have 

 deflected to the east all of the outwash which reached it, or part of the outwash may have been 

 swept across it into central and eastern Long Island and may underlie some of the embayments 

 on the north shore of Nassau and Suffolk Counties. If this hypothesis is correct and the gravels 

 were deposited by many separate streams, then considerable variation in composition might be 

 expected. Whether some difference between the direction of the movement of the ice sheet of 

 Jameco time and the later glaciations could account for the unusual composition of the Jameco 

 in the type area must remain a matter of speculation. Since we know so little of the earlier 

 Pleistocene history of this area it is possible that the Jameco ice was the first to reach the vicin- 

 ity of northwestern Long Island. If this were the case, then in its passage down the lower valley 

 of the Hudson, where we know that the ice sheets cut diagonally across the edge of the Pali- 

 sades, this glacier might have been able to pry loose and pick up more diabase than the later 

 glaciers. 



On the correct interpretation of the origin of the Jameco gravel depends the recognition of 

 the formation in other areas. If it was deposited during the temporary deflection of the Hudson 

 River then it is probably of very limited extent, and its composition would be much the same 

 throughout. If on the other hand it was the direct outwash of an ice sheet which advanced nearly 

 to the northern edge of Long Island then the area covered by the formation can be expected to 

 be much larger although its composition would presumably vary from place to place. 



The Gardiners clay apparently lies conformably over the Jameco gravel although the con- 

 tact has been observed at only one locality. If one accepts the glacial origin of the Jameco then 

 the Gardiners clay presumably was formed during the following interglacial period. It contains 



42 



