About
The catchment of Derwent North represents the area to the south of Stoney Middleton down to the base of Longstone Edge at Hassop, and from the settling lagoons of Cavendish Mill in the west to Calver village in the east, including Coombs Dale and Calver Peak/Deep Rake. Virtually all the natural drainage of the high ground to the west drains downwards via the deep vein workings or natural cavities and ultimately mostly exits via springs into the floor of Coombs Dale and from the Sallet Hole mine adit. The more easterly percolation is captured by two soughs from the lead-mining era, Calver Sough and Brightside Sough, which both ultimately drain into the River Derwent near Calver Mill. A watershed proved by dye tracing exists at the westernmost limit, with underground flows instead draining to Cressbrook Dale. Thermal water is known in the area, and an input into Red Rake Mine has shown to be several degrees higher than background temperatures. A fore-reef of Eyam limestone south of Calver village wraps the toe of the anticline along the shale boundary in much the same way as it it does in Castleton.
The area has been extensively mined since at least the late 15th Century, first for lead and in the 20th Century for fluorite in Sallet Hole and Watersaw Mines, this being controversially extended into extensive open-pit work along Deep/High Rake. Additionally, limestone quarrying of questionable legality has removed much of the natural ground on the eastern limb of Longstone Edge down to Backdale Mine.
The geology of the area is relatively simple on the plateau, with faulting (and associated mineralisation) breaking up the fairly flat-lying beds, though clearly some ground has moved again since the time of mineral deposition. Toward the southern side of the anticline the beds are steeply-dipping and parallel to the slope, sometimes rotating to almost vertical. Volcanic horizons of lava and tuff exist on Longstone Edge and caused many serious issues in the mines below.
Natural cave discoveries in the catchment have been relatively limited, although the examples that are present represent several levels of hydrological development, from high-level sediment-filled phreatic caves to deep vadose shafts near the shale boundary, suggesting a larger and more complex system is present, but entering it is proving difficult. Additionally, mining has so disrupted the natural drainage that previously water-bearing caves may now be dry, making them more difficult to discover. It may well be that a lower-level phreatic cave system exists below the shale boundary to the SE, but it would still be completely flooded if so.