Volcanic Features

Tyrendarra and Harman Valley lava flows and related features

Volcanic materials ae extensive and of considerable geological and geomorphological interest in the Portland Shire. Volcanic features include cones, craters and plugs at eruption points and confined or valley lava flows and borad basalt sheets. At least three phases of volcanic activity have been recognised. The earliest evidence of volcanic activity are basalts dated at 3.91 m.y. to 4.35 m.y. (early Pleistocene) and known as the Hamilton Basalt. There are no obvious eruption points of this volcanic phase in the Portland Shire and the rocks are now deeply weathered and underly a flat to slightly undulating land surface.

The second phase of the volcanic activity (Greenwald - Cobboboonee Basalt) occurred later in the Pleiocene and has been dated from 2.15 m.y. to 3.16 m.y. The earliest eruption points are located along the coast from Cape Nelson to Blacknose Point. Subsequent eruptions occurred from these and other volcanoes in the southern and central sections of the shire.

The peninsulas of the Portland-Bridgewater area are composed principally of basalt from this eruption phase, and Bridgewater Bay and Nelson Bay may be the eroded and faulted remnants of large eruption calderas now flodded by the sea. Other eruption points of this phase are smoothly rounded hills of bedded tuffs with basalt plugs and flows. The extruded material often flowed from fissure eruptions and formed the flat to undulating relief north-east of Portland and near Heywood. A thin veneer of Quaternary siliceous sand now covers much of the basalt material and because of this cover and the weathering of the basalt, outcrops of volcanic material are limited.

The third pase and the most evident sign of volcanic activity in the Shire are the Tyrendarra dn Harman Valley lava flows. Only the lower portion of the Harman Valley flow occurs in Portland Shire but there is along and important sector of the Tyrendarra Flow from Mount Eccles. Both these flows followed existing river valleys - Harman Valley, Fitzroy River and Darlot Creek. The Tyrendarra flow blocked the Fitzroy River and the Darlot Creek leading to the formation of the extensive swamps - the Homerton, Whittlebury and Condah swamps. These swamps have been artificially drained. Darlot Creek and the Fitzroy River have developed new courses and are now twin lateral streams at the lower margins of the Tyrendarra flow.

The Tyrendarra flow has been little modified by weathering and erosion and retains many characteristics of a young flow. Radiocarbon dating of wood covered by the Tyrendarra flow and peats in the Condah Swamp indicate that the flow occurred between approximately 6,200 and 19,300 years ago. A more precise date cannot be determined without further deailed mapping and dating.

The Harman Valley flow is also a young flow and displays several different and distinctive surface features with the Tyrendarra flow. It appears to be slightly older than the Tyrendarra flow.