Alluvium, the soil formed by the sediment brought down by rivers and spread by their action, especially when in flood, over level tracts. Such tracts occur mostly in the lower parts of the course of a river, and in traversing them its course will be comparatively slow, whilst in the approximately stagnant and shallow water of floods deposition will be specially facilitated. Alluvium consists largely of fine-grained loam or brick-earth, with river sands and gravels mainly in former channels, and even occasionally extensive stretches of shingle. It may often contain beds of freshwater or estuarine shells, layers of peat or lignite, formed from swamp vegetation bordering the river, or local accumulations of drift wood from such natural rafts as those produced by trees blown by wind into the waters of the Mississippi. More violent floods, such as those produced by the blocking by ice of the mouths of such rivers as those that flow northward into the Arctic Ocean, may carry coarse gravel and deposit it in considerable thicknesses. The deltas of rivers are entirely alluvial in origin, the rivers cutting their way in numerous channels through the matter which they themselves previously deposited. The whole of Lower Egypt and of Holland is thus comparatively modern alluvium.