The Whin sill, a dolerite intrusive into the Carboniferous of northern England, shows a fairly simple pattern over most of its length of some 20 0 miles, but in the region of the North Tyne it becomes more complex and consists of a series intruded at various levels, with offshoots from the uppermost surface along low-angle faults in the overlying strata. Repetition of the Oxford limestone beds at one area suggests that the main sill was there intruded along a fracture plane.

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