. Nous vivons sur le continent. Géographie physique ; l'histoire naturelle. zvading deuxième oiseau. Il niche fron ! Québec en Floride hut la plupart de ses chiffres l'hiver dans le sud. Ci-dessous centre : typique de la grand nombre d'échassiers qui vivent dans le delta ou passer à travers des migrations sont la gallinule, foulque, Louisiane, Héron et Aigrette neigeuse. dessus et éclatent avec une nouvelle langue de terre (Arc II sur la carte). C'est arrivé quatre fois, mais ensuite la rivière a rencontré un peu d'aide, pour ainsi dire, de la terre dans sa bataille permanente avec la mer, car il avait poussé tellement loin qu'il avait commencé à

• Contributeur: The Book Worm / Alamy Banque D'Images
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. The continent we live on. Physical geography; Natural history. 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. whic

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