The erosion exposed the inlet channel of volcano which was active 30 million years ago. We do not know the appearance of volcano on the surface. During the passage of Cretaceous sediments, their contact metamorphosis occurred. The sandstone was solidified by heat and quartz solutions, whilst it was still porous and light to be used for the construction of weirs. The clay rocks were burnt to porcelanite. This suggests it did not include just filling the cavity with magma, but longer volcanic activity which passed heat to the surroundings.