Hornsby Quarry was a bluestone quarry in Old Man's Valley, Hornsby, that ran under private operators from the early 1900s until 2002. Decommissioned in 2003, it was acquired by Hornsby Shire Council later that year for $26 million following a determination by the NSW Land and Environment Court. What the quarrying exposed is geologically extraordinary: more than 100 metres of cross-section through a Jurassic diatreme volcanic neck, a world-class array of volcanic features described in 2022 as a geoheritage treasure. The Higgins family cemetery, with burials between 1875 and 1925, sits within the site and is separately heritage-listed. Rehabilitation works were approved on 4 November 2020 and the site is being transformed into Hornsby Park, with the quarry void retained as a geological feature.