 Its tuff ring, created by many explosive eruptions, is made of a mixture of volcanic ash, lapilli and fragmented sandstone country rock.Its eruption followed soon (in geological terms) after the neighbouring Grafton Volcano was created, destroying that volcano's eastern parts and burying the rest.Originally, the crater floor was filled with a lava lake, the western half collapsed slightly and became a freshwater lake which later turned into a swamp and slowly filled up with alluvium and sediment, before being drained by Europeans for use as playing fields and parkland.