The bearing capacity of soil is an important consideration in construction projects. The bearing capacity of soil is the pressure that a sample of soil can withstand without collapsing. The bearing capacity varies with the different cohesion of the soil. Cohesion is the force that holds molecules and particles together within the soil. Rock is a continuous mass of solid mineral material, such as granite or limestone, that can only be removed by drilling or blasting. The rock is never completely formed from a single large segment, but is crossed by a system of joints that divide it into irregular blocks. Despite these joints, rock is generally considered the strongest and most stable material on which a building can be founded. Soil is a general term that refers to earthy material that is particulate. If a single piece of ground is too large to lift with one hand, it is considered a boulder. It takes your whole hand to lift a particle, yet it is a pebble. If a particle can be lifted without difficulty with the thumb and forefinger, the soil is classified as gravel. If individual particles can be seen but are too small to pick up individually, the soil is known as sand. Both sand and gravel are coarse-grained soils, meaning that at least half of the material is retained on a number 200 sieve. Peat, loam and other organic soils are not reliable for supporting building foundations. Due to the high concentration of organic matter, they are spongy and compress easily. Their properties can change over time due to changing water content or biological activity in the soil. Clay soils are generally defined as very cohesive, as they maintain measurable shear strength (the ability of materials to resist forces that can cause the internal structure of a material to slide against itself), in the absence of confining forces. The ability of a cohesionless or frictional soil such as sand or silt to support a building depends on the friction between its particles. Sand confined from the surrounding soil within the earth can support a heavy building, while a conical pile of sand on the surface of the earth cannot support anything, because there is little or no shear resistance between the particles. Under each construction site there is a certain number of overlapping layers, also called strata, of different soils. In ancient times the action of natural forces allowed the deposition of the layers one on top of the other.