Thirty or so pits at the Aket Pits, on the north-western flank of Grimwith Fell, were destroyed when Grimwith Reservoir was extended in the late 1970s. Where seen in California Level, the seam, in the shale under the Grassington Grit, dipped N 30° W at 1 in 4.5 and was 18 inches (0.45m) thick.

Evidence from boreholes put in around the dam site suggested the existence of a second, thinner seam which is not present over the whole area. This would be the 0.15 metre thick coal, around eight metres above the main seam. Other boreholes met with voids at the horizon of the main seam, suggesting that an extensive area of coal had been removed. In order to prevent leaks, the shafts and voids were pressure-grouted using a mix of cement and P.F.A.

Another small, detached area of coal was worked from pits in the Grassington Grit on Hartlington Moor, about 1.7 km west-south-west of the Aket Pits.

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