Reasons for Designation
For several millennia the western part of the South West Peninsula, namely Cornwall and West Devon, has been one of the major areas of non-ferrous metal mining in England and its more important and prolific products include copper, tin and arsenic, along with a range of other materials which occur in the same ore bodies. Throughout much of the medieval period most of the tin was extracted from streamworks, whilst the other minerals were derived from relatively shallow openworks or shafts. Streamworking formed one of the chief methods of mining throughout the history of the tin industry, and was the predominant method before the 17th century. The method used the high specific gravity of tin ore, cassiterite, to separate it from surrounding 'waste' such as other minerals, earth and grits. It involved diverting and channelling a water supply across the ground to be worked, carefully controlling its speed of flow to remove waste material while retaining the tin ore. The ore-bearing ground was put into the channel for separating, usually from the upstream or upslope side. The tin ore and larger grits left behind were dug from the channel floor for secondary ore-dressing nearby, while larger rubble was dug out and heaped along the channel's downslope side, creating a linear spoil mound. Once a channel became too wide to control the speed of water flow, the rubble soil was used to define a new one, and so the streamworking advanced upstream or upslope until there was no further ore-bearing ground that was economical to work. The end result of streamworking is a broad steep-sided gully from which an often considerable volume of waste material has been removed; in the floor of the gully, various patterns of linear rubble dumps reveal the method and sequence of working, with the earlier dumps sometimes masked beneath silts washed down from the later working. Outside the streamwork gully may be leats and reservoirs which served its water supply. Streamworking was applied to two types of ore-bearing deposit, both separated from the parent lodes by weathering. In alluvial deposits, tin ore accumulated with other eroded materials in valley floors, becoming partly concentrated by natural sorting in the water-rich environment. Here the valley floor stream provided the water supply necessary for streamworking. In elluvial deposits, tin ore occurs in more poorly sorted subsoils weathered directly over the parent lodes, or on slopes and in hollows down which they have drifted under gravity and subsoil slumping. Exploitation of these by streamworking often required a considerable catchment area, and used leats and small reservoirs. Streamworks provide our main source of evidence for the methods employed in tin mining and its scale in the landscape during the early and post-medieval periods, aspects for which historical documentation is scanty and inadequate. The tin streamworks 245m south east of Higher Numphra is of an early probably medieval date and the finest example of its type in west Penwith. It will contain archaeological, environmental and chemical evidence relating to tin production, the longevity and development of the streamworks and its overall landscape context.
Details
The monument includes a tin streamworks, situated on the lower north west facing slopes of Numphra Common. The streamworks survives as a series of linear earthworks with traces of leats and reservoirs and covers an area of approximately 6ha. The workings reach a depth of some 6m and the associated waste dumps vary in height form 0.2m up to 4m. These lie in parallel lines and represent a highly systematic extraction method using water to extract tin from the surrounding waste materials. At the upper end of the complex there are three reservoirs defined by their dam banks which were used both to collect and control the water used in the streamworking operation. The channels or leats used to convey the water are also visible as earthwork remains. To the east are the pits and associated spoil heaps produced by mineral prospecting techniques. The earthworks were identified by S Gerrard as being of types C and D.
Legacy
The contents of this record have been generated from a legacy data system.
Legacy System number:
CO 1096
Legacy System:
RSM - OCN
End of official list entry
