Читать книгу The Romance of Modern Geology. Describing in simple but exact language the making of the earth with some account of prehistoric animal life онлайн

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The sea is not the only carrier which is thus laying down beds of material. The rivers are doing the same thing. Every shower of rain washes some dirt—by which we mean sand or gravel or loam or chalk—from the land into the nearest rivulet. The rivulet hurries with it down to the neighbouring river, and the river carries it down to the sea. If the river is going very fast it carries most of its dirt along with it, and we generally find the river muddy after rain. But when the river slackens its pace, as it usually does when it nears the sea and meets the sea's tides, then it lets the dirt fall; and thus at the river's mouth we find mud-banks or sand-banks. If a river is left long enough to its own devices, these sand-banks will so increase in bulk that the mouth of the river will become shallower and shallower and will spread. It silts up, and when a river is needed for the navigation of ships large sums of money have to be spent, as in the Scheldt or at the mouth of the Thames, in dredging this mud so as to keep the channels clear.