Geology - South-East Brisbane (Rock Types and Landforms)

Villanova is situated on a hill of meta-sedimentary rock (i.e. metamorphosed rock with sedimentary features such as bedding planes still visible) - see map (pink) for location. These meta-sediments form a band running through much of Brisbane, underlie other rocks of this area and are found along a wide zone for some distance both north and south of our city. The key rocks of this group in SE Brisbane are meta-greywacke, meta-argillite, and meta-quartzite - mostly forming gently hilly country. The highest hills in our area (eg Whites Hill and Mt Gravatt) are composed of the most resistant of these rocks, meta-quartzite. In places much of the bedrock has been weathered and bleached.

On the northside of Brisbane, a more highly metamorphosed rock, phyllite, has formed more hilly topography. These metamorphic rocks formed on a regional scale. Brisbane's near west has exposures of granite (igneous intrusive) and hornfels (contact metamorphism, local in scale).

The meta-sedimentary rocks of the Brisbane area formed from sediments deposited offshore, at an oceanic plate subduction boundary, about 370 to 290 million years ago. As the oceanic plate was dragged below the continental plate, sediments were scraped off, compacted, folded and uplifted. This created the present coastal zone along southern QLD and northern NSW. Erosion, subsequent deposition and volcanic activity have created a complex regional geology.

Within several kilometres of our college (see map - dark blue) is an igneous rock, ignimbrite (Brisbane tuff). This is a tough, welded rock with attractive (mainly) pink or grey surfaces popular for building and walls when it was readily available. Its age is approximately 220 million years. It resulted from an ash flow; a dense cloud blasted out of an explosive volcano at an extremely high temperature. When it settled the ash was still hot enough to weld into a tough rock filling valleys and low lying areas some distance from the original cone. Further from the flow source, the rock formed is less firmly welded (as the original flow cooled).

This rock is quite thick in places, and forms the well known cliffs overlooking the Brisbane River near the Storey Bridge and the City Gardens.

Another rock near the college (see map - light blue) is the group of sedimentary rocks known as the Tingalpa formation, of age approximately 220 million years. This formed from sediments in shallow swamps and includes shale, coal and sandstone. Fossils (especially ferns) typical of the time are found in shale and claystone. These rocks are quite soft and hence readily weathered. Plant fossils can be readily viewed and collected in cuttings such as that shown below. Gentle folding has taken place since formation. In general, the topography above these soft sedimentaries is gently undulating.

Sedimentary rocks of considerable thickness are found to the west (between Brisbane and Ipswich), including coal measures which are commercially utilised.

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[ Last updated: 11 Feb 2006 by Mr C.Grant ]

go to Field Geology Days of Excellence		go to ROCK LANDFORMS of Australia & NZ Web Site
go to Metamorphic Aureole of Brisbane		go to Fossils of Brisbane Area

go to Field Geology Days of Excellence		go to ROCK LANDFORMS of Australia & NZ Web Site
go to Metamorphic Aureole of Brisbane		go to Fossils of Brisbane Area