Magnetic survey of the Holy Island Dyke on Holy Island, Northumberland

The Holy Island Dyke on Holy Island, Northumberland strikes E-W along the south coast of the island. It intrudes Lower Carboniferous rocks and crops out as five discrete segments of dolerite. Flat tops on the two largest segments, benches on the southern flanks of three segments, and flat surfaces with amygdales at several locations on the outcrop have all been cited as evidence that the exposures represent the original top surface of the dyke, where it terminated within the country rocks. The results of a magnetic survey of the dyke show that the principal outcrops are connected to the northern edge of a sill whose top surface lies within 10 m of the present ground surface. The sill is exposed on the foreshore adjacent to St Cuthbert’s Isle in the west, where it forms an outcrop near the low water mark that has not previously been reported. The sill is about 100 m wide in the N-S direction and, at its southern edge, is connected to a subsurface dyke that extends downwards. Thus the dolerite outcrops appear to be part of a step-and-stair transgression of bedding. This interpretation accounts for all the features previously cited as evidence of the upward termination of the Holy Island Dyke within the sedimentary succession