The Hornsby Quarry Geosite, NSW, Australia—A Geoheritage Treasure

Quarrying for aggregate material at Hornsby in New South Wales (Australia) exposed a >100 m-deep cross-section of the volcanic neck of a Jurassic diatreme, which extruded through the Sydney Basin deposits. The cross-section reveals volcanic features at many scales. Globally, there are very few instances of such excellent 3D exposure. It is arguable that this exposure alone makes it a geosite of international value. However, evaluation of the Hornsby Diatreme using a geoheritage toolkit shows that it meets various geoheritage conservation criteria, being a reference site, and an historically and culturally important site, as well as hosting important archival material. It has significant features at various scales: at the macroscale, where dish beds are visible on all quarry walls, preserving the structure formed during volcanic accretion and later caldera collapse; at the mesoscale, where breccia and surge layers are visible, providing insight into how magma interacted with various host rock types; and at the microscale, where lapilli, chilled margins on host rock blocks, and carbon-rich xenoliths are visible in hand specimens. As such, the Hornsby Quarry hosts a world-class array of volcanic features and preserves an important period of volcanic and post-volcanic history within the Sydney Basin, fostering geoeducation and geotourism.