Teachers Guide
Browns Field – The Fox Valley diatreme

The names? Where do they come from?

Brown? This was from John “Squire” Brown, a local timber getter, the source of several local place names.

Field? The site now encircles a sports oval.

Fox Valley? Not Vulpes vulpes, the pestilential, imported red fox but named after a former grey-headed flying fox colony in a nearby valley.

What the hell is a diatreme?

Well one could easily say that they come from hell if you’re so inclined. The word is Greek derived and means “through a hole or aperture.” Diatremes punch their way through crustal rocks driven by magmatic heat and powered by hot gases. Diatremes actually lie on a spectrum that includes kimberlites! Maybe you’ve heard of kimberlites, some of which contain diamonds, but what’s special about it?

Kimberlite?

Kimberlite diatreme pipes are sourced beneath the crust in the Earth’s upper mantle and powered to the surface by mantle-derived CO2 (that stuff that causes global warming). So what are the Sydney Basin diatremes (at least 150 of them) powered by? The common answer to that is “steam” from hot magma cutting through wet rocks. But Sydney Basin diatremes are basalt associated. Basalt is also derived from the mantle.

What rocks make up Sydney Basin diatremes?

Well one clue has been given away already. Browns Field doesn’t have exposed fresh rocks so we only have a sketchy clue. You can stumble along the little creek downstream from the sandstone trackhead and you will find rocks in its bed and banks, but you’ll probably be scratching your head as we were – they’re extremely weathered and barely recognisable.

We have identified sediments interlayered with what are called pyroclastic rocks – “fire broken” in translation – of course by volcanic processes. They show fragments of basalt. If you can manage to get a peek at the Hornsby diatreme quarry faces, and pick up quarry fragments from around and about, you’ll see they are very much of basalt, and in multiple layers that are dished as they have sagged as the diatreme rocks subsided.

Diatreme vegetation, especially Fox Valley

The main vegetation here is rainforest and you can wander through it on the little track. It’s essentially subtropical though there are a few tropical elements, some from experimental plantings. See if you can identify the main tree species. We can name two – sassafras (Doryphora sassafras) and lillypilly (Acmena smithii), but there are several others, and don’t overlook the shrubs, ferns, climbers and mosses.

In closing

The Fox Valley diatreme is a precious commodity and everything that can be done, must be done to protect and preserve it and its vegetation community.