most resistant minerals suggests a deep weathering of the bedrock in the area supplying the 

 materials deposited. 



Above the lower horizons where coarse material is common, the Magothy formation 

 shows no consistant composition or typical cross section. Locally there are thick beds of clay 

 which can apparently be traced at most for a mile or so, only to lose their individuality and 

 blend into the general succession of sand and clays. Such a pattern is more common in terrestrial 

 than in marine deposits, but in any case reflects changes in the drainage pattern of the streams 

 supplying the sediment and of the flow of water in those streams. In a gravel pit at Port 

 Washington a clay filled stream channel a few hundred feet wide can actually be seen. Changes 

 in composition between clay and sand do not always represent uplift or subsidence of the land, 

 as has been argued by some of the earlier students of Long Island geology. The evidence 

 available suggests a relatively slow but steady sinking of the area that is now Long Island 

 during the entire period that the so-called Magothy formation was being deposited, with a 

 continuous accumulation of sediment, although at very different rates at different places and 

 at different times. 



This brings to a close of what is known of the history of the formation of the Cretaceous 

 beds on Long Island. At that time these beds reached well north of their present limit, possibly 

 fifty miles or more although there is now little evidence on which to base such a figure. This 

 did not bring to a close however the Cretaceous deposition in this area if one may judge by 

 what is known of the geologic history of New Jersey, where several younger formations were 

 laid down before the end of the period. 



At the close of the Cretaceous the sea retreated from the area that is now Long Island, 

 the land was upraised and probably gently tilted once more to the southeast, and some erosion 

 took place. It has been suggested that the sea advanced again over the land several times during 

 the Tertiary periods which followed, possibly even as far as the former limit of the Cretaceous 

 seas but no definite record of such advances has been found. 



TERTIARY HISTORY 



During the latter part of the Tertiary, erosion became more important than deposition 

 arid vast amounts of material were removed from the Long Island area and surrounding areas. 

 Sea level, at least during parts of this period, was somewhat lower than at present and the 

 resulting erosion created for the first time the important characteristic topographic features 

 of present-day Long Island. The depression which is now Long Island Sound was first formed 

 at this time, probably as the valley of a large stream which drained out to the east, although 

 this question is the subject of a controversy that will be discussed below. 



Whatever the details may have been it is certain that the depression that now forms the 

 Sound was located by the relative ease of erosion of the weathered bedrock which formed the 

 surface of the Fall Zone peneplain and of the Lloyd sand. South of this depression, the more 

 resistant massive clays which comprise the upper part of the Raritan formation formed the 

 core of a line of hills the upper part of which was composed of the sands and clays of the 

 Magothy formation. These hills, considering the lowered sea level of that time, must have 

 risen 400 to 600 feet above the sea. The slope south from this line of hills was gentle and while 

 undulating, was relatively smooth. The northern slope, down into the depression which is now 

 the Sound, was steeper and was carved into a series of short steep straight valleys more or 

 less parallel to one another. These valleys, altered to some extent by subsequent glacial scour 

 and partly filled by debris left by the retreating ice sheets, now form the embayments of the 

 north shore of Long Island. 



37 



