File:The continent we live on (1961) (20684744025).jpg

Title: The continent we live on
Identifier: continentweliveo00sandrich (find matches)
Year: 1961 (1960s)
Authors: Sanderson, Ivan Terence, 1911-1973
Subjects: Physical geography; Natural history
Publisher: New York : Random House
Contributing Library: New College of California
Digitizing Sponsor: Internet Archive

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second-largest zvading bird. It nests fron! Quebec to Florida hut most of its numbers winter in the South. Center below: Typical of the vast number of wading birds that live in the delta or pass through it on migration are this gallinule, coot, Louisiana Heron, and Snowy Egret. upper hand and burst out with a new tongue of land (Arc II on map). This happened four times, but then the river encountered some assistance, as it were, from the land in its persistent battle with the sea, for it had pushed so far out that it had begun to form a "hook" and thereby created a slight whirligig or counter- current to its left side. This enabled it to make such headway that it did virtually dam itself up (Arc V on the map) and, having thus covered its left flank, it burst out at its old mouth (Arc IV), where it is today, penetrating into "enemy territory." A Louisiana survey has calculated that the Mississippi carries one million tons of sediment to the sea every day of the year. In a year, this is equivalent to a block of land one square mile in area and three hundred feet high. During the eleven thousand years since the last "retreat" of the ice up north, it has there- fore dumped sediment to a depth of two hundred feet over the entire eighteen thousand square miles of its delta: yet that delta is still only just above sea level! Where has it gone? Down below, to form a great inverted dome in the earth's crust, creating thereby complex pressures and tensions that affect a wide area around; for you cannot push a solid into another solid without something giving way. It is a strange thing that deltas, which appear to be the lowest and softest places on earth, really form some of the hardest "nuts" in its crust. When a whole subcontinent sinks, as northern Australia has done, among the last things to go down are the deltas, as is seen in the Aru Islands in the Arafura Sea. which are only the old delta of a vast river that once ran north off that land. DOMES OF SALT AND SULPHUR The amount of sediment actually accumulated in the Mississippi delta is not known, but geologists state that it is more than thirty thousand feet in depth. At the present rate of deposit, this represents six and a half million years, which takes us back to about the beginning of the so-called Pliocene era, the stage
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