650 THE POPULAR SCIENCE MONTHLY. 



polished. The whole surface of New York Island, where the rock is 

 exposed, shows marks of glacial action, the upturned edges of gneiss 

 being ground off to form a nearly plane surface, or, where ridges of 

 more massive rock had existed, these are rounded over to form roches 

 moutonnees. Fine examples of the latter may be seen in Central Park 

 and on the east side of the island near Harlem. 



The material which occupies so much of the troughs of the Hudson 

 and East River is mostly glacial drift, clay, gravel, sand, and bowlders, 

 scraped from the highlands by the great ice-sheet into these preglacial 

 gorges. It is probable they were once filled to the brim, and that they 

 were subsequently reexcavated in part by the floods of water which re- 

 sulted from the melting ice. After these ceased, and they were occu- 

 pied by water standing at its present level, and moved only by tidal 

 action, they were more or less silted up by the deposit of fine mud 

 brought down by the larger and smaller streams, here checked in their 



N.Y. Island Long Island. 



_^L East R 



^/T77TrrT77777TTTTTTmTrpTTr^-^ 



^f'i /''' /;/'/'' |p , ' 1 11 1 1 ' 1 1 1 1 ' 1 \ v . - * 



Waxm/tiiV'li'i i m'm' iuU|[i'V$|l|f 



I I rap 2.Triassic 3.Laurentian 4.Drift 5. Boulder Clay 6.SILT. 



Fig. 3. 



flow and losing their transporting power. The southern and lower 

 portion of New York Island, which was under the lee of the higher, was 

 covered with deposits left by the retreating glacier, and these were 

 never afterward entirely removed. Here are now beds of sand and 

 gravel which have in places been penetrated to the depth of one hun- 

 dred feet or more. On the higher parts of the island and the adjacent 

 country, the rock is generally bare or covered with soil, but even here 

 depressions are filled with bowlders, clay, or gravel, often to the depth of 

 several feet, and large transported bowlders are everywhere scattered 

 over the surface. These latter have sometimes been derived from the 

 rocks of the island, but most of them seem to have come from distant 

 points, and always from the north and west. Rounded masses of trap 

 are very common among the bowlders, and these have been brought 

 across the Hudson, for there is no trap in place on the east side of the 

 river. The trap-ledge which forms the summit of the Palisades is 

 everywhere worn and scratched by glacial action, and the markings 

 which it bears are generally concordant in direction with those of the 

 rocks of New York Island and Westchester County, viz., about north- 

 northwest and south-southeast. Even on the river-face of the hills 

 which form the east bank of the Hudson, the bearing of the glacial 

 scratches is essentially the same, showing that the movement of the 

 great ice-sheet was little affected by any such trifling irregularity of 

 the surface beneath it as the Hudson Valley. 



