Joints and other structures in the silurian rocks of the southern Shap Fells,Westmorland

The Coniston Grits and Bannisdale Slates south of Shap are folded into the major Bannisdale Syncline, an asymmetrical fold with a longer northwest limb, a half wave length exceeding 5 miles, and numerous minor parasitic folds with half wave lengths ranging from 200 yds to 2 or 3 yds. These minor folds generally plunge from 5° to 10° to the northeast and occur in belts of tight folds which alternate with belts of steeply, uniformly dipping strata. It is notable that the axial planes of the folds are not parallel to the cleavage and in fact the trend of the former (005°) is about 5° oblique to the latter (060°). There are also complementary dextral and sinistral wrench faults and numerous sets of joints related both to the folds and the faults. The joints present distinctly different patterns on opposing limbs of the minor folds and it is suggested that they formed at different stages of the orogeny. The earliest phase, shortly after initiation of folding, is thought to have been “wrench type” joints, subsequently folded and succeeded by post folding dextral and sinistral “wrench” joints accompanied by Reidel shear and kink bands, and then finally by high and low angle tension joints at a late stage in the orogeny.