We didn't exactly discover it yesterday - geologists have known for a long time about the Hornsby diatreme, but yours truly knew almost nothing about diatremes generally, let alone this one. So I discovered a lot myself.
Such as that diatremes are volcanic pipes through which hot gases and water vapour erupt explosively. What's left behind is not so much a crater, more a crack, and the surrounding materials, from lots of different geological eras, fall in on it to form a relatively shallow valley. And an exciting hunting ground for geologists.
The Hornsby one dates back to events of 200 million years ago, and it's the biggest such feature in the Sydney region. The area includes the Hornsby Quarry, parts of the Blue Gum Walk, and the Rosemead Reserve.
Which is where I met my guides and fellow expeditioners on Hornsby Council's geology walk. It was one of a regular series of free guided bushwalks they put on, and which serve as an excellent gentle introduction to bushwalking for a lot of people. They tend to have ornithological or botanical experts along too, and in this case a personable and renowned local geologist, to help you get more out of the experience.
It made for a very pleasant and informative morning excursion, and I'm glad I discovered it. Other local councils do similar things. Check it out.
