Leader, geologist John Martyn, has worked in and published on volcanic environments both ancient and modern, originally having completed his PhD on the Great Rift Valley in Kenya.

Browns Field sits over a diatreme of which there are eight known in the Ku-ring-gai GeoRegion.

Diatremes are massive, cylindrical or dyke-like bodies of fragmental rocks that have been blasted and bored up from depths of kilometres by extreme steam pressure created by igneous intrusion into deep rock aquifers. Most or all in the Sydney Basin are believed to be of Lower to Lower Middle Jurassic age. At the present surface we see their deeply weathered erosional cross sections which lie up to a kilometre below what were their ancient Jurassic eruption surface vents, long eroded away. We won’t clearly see actual volcanic rocks at Browns Field as they are limited to weathered stream bed exposures but we will see the diatreme’s contact zone with Hawkesbury Sandstone.

We’ll certainly see the vegetation at Browns Field, which thrives on deep, fertile, clay-loam soils and is essentially warm temperate rainforest: the common tree species being coachwood, sassafras, lillypilly, cabbage tree palms and sandpaper figs. Understory trees and shrubs include guioa, bolwarra, native hydrangea and brush turpentine Rhodamnia rubescens, the latter now reclassified as critically endangered as the species most sensitive to myrtle rust disease.

The Cooper Crescent arm of the diatreme has been for decades an experimental planting site for subtropical rainforest species found nowhere else in the area. These are diverse and include giant stinging tree, blue quandong, featherwood, white bolly gum and native tamarind. We will do a short road traverse identifying the subtropical species within visual reach but the creek bed is not accessible.

Meet at: Entrance to Browns Field from Campbell Drive, Wahroonga (off Fox Valley Rd)

Bring: Water, snack and wear good walking shoes