Soil is usually divided into an upper darker-colored layer, termed loam, and into a lower, lighter-colored layer, termed subsoil. The presence of humus, resulting from the decomposition of plant and animal remains is the factor which darkens the color and distinguishes the loam; so that loam is a complex of inorganic rock particles plus more or less humus, colloid compounds, bacteria, living plants and animals. The subsoil is mainly rock particles. The distinctions between these two layers break down in arid soils, and often also in swampy regions. It is this layer of soil on which the water of every rain and flood works, picking part of it up and carrying it along, step by step, to the sea. Though the amount moved on any one day is small, the sum of all the soil transported is enormous, a large river carrying annual incredible amounts. For instance the Mississippi annually deposits in the Gulf of Mexico 476,900,000 metric tons (2204 lb. to the metric ton), of which about a third is in solution. At this rate it takes about 7000 to 9000 years to remove a foot from over the whole drainage basin. This is considerably slower than is the case of some other rivers. While on the one hand soil is being continuously carried away from the surface, on the other hand it is being constantly renewed from below, by the weathering action of water, air and temperature. Gravel Gravel is a mass of loose fragments of rock, which have been rounded by water and deposited with little or no sorting, so that larger and smaller pebbles and sand all occur together. It is the deposit laid down by comparatively fast water in inland lakes or along the storm-beaten shores of the sea. Where a swift stream enters quiet water, as where it empties into a lake, there it quickly drops its coarse material as gravel, usually thus building a delta. Gravel also occurs in stream beds, where for any reason the rate of flow is checked. During the recent glacial period, the ice sheet brought down great masses of unsorted material, which was deposited as till, or in moraines. Much of this was then picked up by the running water and moved longer or shorter distances, so that, all over the glaciated country of the northern and eastern United States, there are unusually large numbers of gravel deposits. Gravels are all water laid, and usually show more or less clearly the bedded or stratified structure. The size of the component pebbles of gravel ranges from great boulders to fine sand, and the finer gravels grade into the coarser sands, the line between gravel and sand being drawn at about the size of a pea, the coarser being gravel, the finer sand. Gravel is widely used as ballast for railroads and in making highways, because of its tendence to pack well, while the hard pebbles resist wear. It is also widely used in concrete work, bonding in well with the cement, and making it go from three to five times as far. Conglomerate Pl. 58