Purbeck Mineral and Mining Museum
After the closure of Eldon sidings all clay exports were via lorry. For several years the clay was carried from the mines across the A351 by trains to the "Lorry Drop". It was realised that the lorries could take the clay directly from the mines and the cost of maintaining a railway system avoided. The transhipment buildings were raised to accommodate the lorries beneath them and in the early 1970s the railway system sold partly into preservation and the rest scrapped. Use of narrow gauge railways continued underground at Norden No 6 & 7 mines until all mining operations ceased in 1999.
Clay
Although many naturally occurring deposits include both silts and clay, clays are distinguished from other fine-grained soils by differences in size and mineralogy. Silts, which are fine-grained soils that do not include clay minerals, tend to have larger particle sizes than clays. There is, however, some overlap in particle size and other physical properties. The distinction between silt and clay varies by discipline. Geologists and soil scientists usually consider the separation to occur at a particle size of 2 µm (clays being finer than silts), sedimentologists often use 4–5 μm, and colloid chemists use 1 μm. Geotechnical engineers distinguish between silts and clays based on the plasticity properties of the soil, as measured by the soils' Atterberg limits. ISO 14688 grades clay particles as being smaller than 2 μm and silt particles as being larger.