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).
Wivenhoe Gravel Pit Wivenhoe Gravel Pit Wivenhoe Gravel Pit is a 2.1 hectare geological Site of Special Scientific Interest north of Wivenhoe in Essex. It is a Geological Conservation Review site. The site is the type locality for the Wivenhoe Gravel, which was laid down by the River Thames before it was diverted south to its present course by the Anglian glaciation around 450,000 years ago. The gravel was laid down during two cold stages with an intervening interglacial, but the fossils in from this warmer period are not distinctive enough to identify which interglacial they come from. The site is important for