Soil is the most common of all materials encountered in construction operations. Probably because it is so common a material, it usually does not receive the attention it deserves. Soil provides the foundation bed upon which most structures are built; it has to be excavated in large quantities; and it supports not only the pavements of all roads and highways. Soil is therefore not only the most common material encountered in building but probably the most important. It must be regarded not merely as "mud" but as a vitally important building material. The word soil is used by engineers and builders to describe all the loose material in the crust of the earth. Sand and gravel consist of coarse particles which may range in size from about 3 inches in diameter down to small sand grains which can just be distinguished with the unaided eye as separate grains. The most important property to watch for is whether the mixture of particles is well graded or not. The word silt is used to describe a third main group of soils in between the two main groups, sand and gravel. This group, which is of great importance in Canada, consists of solids that are made up of particles which are just finer than the finest sand particles, and yet are not as small as the microscopic particles that constitute clays. Since the soil particles in silts cannot be seen, we need not trouble about their actual size as silts can be recognized by other characteristics. Essentially silts consist of particles of fresh ground rock which have not had time to change their character into the minerals which make up clay. Clays are the soils which are made up of the finest possible particles, particles which cannot possibly be seen by the eye or even with ordinary magnifying glasses. In addition to the effects of particle size the special properties of clays are caused by the presence of "clay minerals." These minerals result from changes in the bed-rock material, changes which took place through different kinds of natural weathering. It is the combination of these clay minerals with water which gives to clays their very special properties- the stickiness of prairie gumbo, for example. Glacial till is a very common type of soil in some areas. The name is given to the mixture of all sizes of soil particles which results from the movement of glaciers over solid rock without any subsequent sorting of the material. Glacial till is usually a very hard and compact soil containing boulders, gravel, sand, silt, and clay-sized particles. It is often so hard that it is sometimes called hardpan but if the material is dry, it is easily to disintegrate. Special care must be taken in all studies to guard against mistaking fill for natural soil. In many city areas, depressions have been filled up with rubbish and soil excavated from other areas. Unless such dumped material has been very carefully compacted, it may be in a very loose state and therefore not a suitable soil for building upon. It can usually be identified by its loose condition and by the presence in it of other material such as twigs, grass, rubbish, etc.