Their advance to the south was checked by the indomitable opposition of the Araucanians, but from the southern Andes the Spaniards overflowed on to the great plains which now form the interior of the Argentine Republic. The first permanent settlement at the mouth of the river Plate at Buenos Aires dates from 1580.
In the valleys of rivers which have overflowed their banks and on level bench lands there is considerable silt and vegetable loam mixed with glacial clay; but on the hills and ridges of western Washington the soil is almost wholly a glacial deposit consisting principally of clay but usually containing some sand and gravel.
Most of the small closed basins, however, contain "playas," or alkali mud flats, that are overflowed when the tributary streams are supplied with storm water.
But between the Lake of Zurich and the Walensee the huge desolate alluvial plain grew ever in size, while great damage was done by the river, which overflowed its bed and the dykes built to protect the region near it.
Its present deserted and malarious state is probably owing to the silting up of the mouth of the Silarus, which has overflowed its bed, and converted the plain into unproductive marshy ground.