Geological Features in NorthumberlandGeological Features in NorthumberlandGull Crag and the Rumble ChurnA massive outcrop of the Whin Sill igneous intrustion on which stands Dunstanburgh Castle. Greymare RockAn easy to explore outcrop of the Whin Sill showing obvious geological folding. Sycamore GapA landmark gap in the Whin Sill caused by glacial meltwater.