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.
Earth structure
Clays typically are phyllosilicate minerals with a sheet-like structure.
The very small clay particles interact with each other physically and chemically. 
Even a small proportion of clay affects the physical properties of the soil much more than might be expected.
Clays such as kaolinite do not expand or contract when wetted or dried, and are useful for brick-making. 
Others, such as smectites, expand or contract considerably when wet or dry, and are not suitable for building.