<P> Technically a drowned valley, like the other waterways around New York City, the strait was formed approximately 11,000 years ago at the end of the Wisconsin glaciation . The distinct change in the shape of the strait between the lower and upper portions is evidence of this glacial activity . The upper portion (from Long Island Sound to Hell Gate), running largely perpendicular to the glacial motion, is wide, meandering, and has deep narrow bays on both banks, scoured out by the glacier's movement . The lower portion (from Hell Gate to New York Bay) runs north - south, parallel to the glacial motion . It is much narrower, with straight banks . The bays that exist, as well as those that used to exist before being filled in by human activity, are largely wide and shallow . </P> <P> The section known as "Hell Gate"--from the Dutch name Hellegat or "passage to hell" given to the entire river in 1614 by explorer Adriaen Block when he passed through it in his ship Tyger--is a narrow, turbulent, and particularly treacherous stretch of the river . Tides from the Long Island Sound, New York Harbor and the Harlem River meet there, making it difficult to navigate, especially because of the number of rocky islets which once dotted it, with names such as "Frying Pan", "Pot, Bread and Cheese", "Hen and Chicken", "Nigger Head", "Heel Top"; "Flood"; and "Gridiron", roughly 12 islets and reefs in all, all of which led to a number of shipwrecks, including the British frigate Hussar which sank in 1780 while carrying gold and silver intended to pay British troops . The stretch has since been cleared of rocks and widened . Washington Irving wrote of Hell Gate that the current sounded "like a bull bellowing for more drink" at half tide, whilte at full tide it slept "as soundly as an alderman after dinner ." He said it was like "a peaceable fellow enough when he has no liquor at all, or when he has a skinful, but who, when half - seas over, plays the very devil ." The tidal regime is complex, with the two major tides--from the Long Island Sound and from the Atlantic Ocean--separated by about two hours; and this is without consideration of the tidal influence of the Harlem River, all of which creates a "dangerous cataract", as one ship's captain put it . </P> <P> The river is navigable for its entire length of 16 miles (26 km). In 1939 it was reported that the stretch from The Battery to the former Brooklyn Navy Yard near Wallabout Bay, a run of about 1,000 yards (910 m), was 40 feet (12 m) deep, the long section from there, running to the west of Roosevelt Island, through Hell Gate and to Throg's Neck was at least 35 feet (11 m) deep, and then eastward from there the river was, at mean low tide, 168 feet (51 m) deep . </P> <P> The broadness of the river's channel south of Roosevelt Island is caused by the dipping of the hardy Fordham gneiss which underlies the island under the less strong Inwood marble which lies under the river bed . Why the river turns to the east as it approaches the three lower Manhattan bridges is currently geologically unknown . </P>

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