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

The Holy Island Dyke on Holy Island, Northumberland strikes E-W along the s outh coast of the island. It intrudes Lower Carboniferous rocks and crops o ut as five discrete segments of dolerite. Flat tops on the two largest segm ents, benches on the southern flanks of three segments, and flat surfaces w ith amygdales at several locations on the outcrop have all been cited as ev idence that the exposures represent the original top surface of the dyke, w here it terminated within the country rocks. The results of a magnetic surv ey of the dyke show that the principal outcrops are connected to the northe rn 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 Isl e in the west, where it forms an outcrop near the low water mark that has n ot previously been reported. The sill is about 100 m wide in the N-S direct ion and, at its southern edge, is connected to a subsurface dyke that exten ds downwards. Thus the dolerite outcrops appear to be part of a step-and-st air transgression of bedding. This interpretation accounts for all the feat ures previously cited as evidence of the upward termination of the Holy Isl and Dyke within the sedimentary succession.