Silt three grades fine, medium and coarse 0.002 mm to 0.006 mm to 0.020 mm to 0.063 mm). In actuality, silt is chemically distinct from clay, and unlike clay, grains of silt are approximately the same size in all dimensions; furthermore, their size ranges overlap. Clays are formed from thin plate-shaped particles held together by electrostatic forces, so present a cohesion. Pure silts are not cohesive. According to the U.S. Department of Agriculture Soil Texture Classification system, the sand–silt distinction is made at the 0.05 mm particle size. The USDA system has been adopted by the Food and Agriculture Organization (FAO).
Fern House Gravel Pit Fern House Gravel Pit Fern House Gravel Pit is a 1.3 hectare geological Site of Special Scientific Interest in Fern, near Bourne End in Buckinghamshire. It is also a Geological Conservation Review site. This site exposes the Pleistocene Taplow Gravel formation, and may help to elucidate a poorly understood period in the history of the River Thames, between glacial Anglian stage, around 450,000 years ago, when the river was diverted south to its present course, and the warm Ipswichian around 120,000 years ago. Fossils found at the site include straight-tusked elephants and mammoths. The site is on private land with