The sandstone and concrete monument on the summit of Observation Island (Wudunhu), erected in 1923 by members of HMA Surveying Ship Geranium to commemorate Matthew Flinder's visit in 1802; Flinders was the first to circumnavigate Australia and named Observation Island on 16 December 1802. The obelisk is used as a navigation marker when entering the channel between North and Vanderlin Islands from the north.