ATLAS NATURALISTS PRESENTATION

5th OCTOBER, 2019

SUMMARY & FOLLOW-UP

Soils are fundamental to ecological systems. They influence the vegetation types which in turn determine the insects and animals that live there. And to really understand soils we need to think about rocks and their origins.

On 5th October, Mick Harewood shared his knowledge of our local geology with an enthusiastic audience of 25 people in the Tura Marrang Library. Mick’s interest in hydrology, soils, and fluvial geomorphology underpins his interest in geology.

Mick has generously shared the notes he prepared for his talk. Thanks Mick!

Sites of interest

Mick has also provided a list of sites around the region where many of the geological formations and rock types he discussed can be viewed. Yesterday I took a tour of some of these, and also looked back on photos I’ve taken on earlier visits. Already I have a better understanding of what I’m seeing. The quotes are from Mick’s paper.

Pambula River mouth and Rotary Park

Red Merimbula metamorphosed mudstone visible on the beach. The red balloon indicates the rocks of interest. You can zoom in or out using the controls in this and other maps.

“Mudstones are a type of sedimentary rock. Sedimentary rocks are laid down in horizontal strata at the bottom of lakes, rivers, estuaries or seas. Metamorphic rocks are sedimentary rocks that have been changed over time by compression, heat, distortion or other processes. They include the spectacular Devonian Merimbula mudstone seen in places along the coast. This metamorphic rock was originally the bed of a Mississippi-sized river that flowed from the McDonald Ranges region to the Gondwana coast.”

Haycock Point

Another excellent viewing site for red mudstone.

View from Boyd Tower

Look to the south as you walk along the track to the tower to view the spectacular folded red mudstone formations. An anticline with a fault line is evident here, illustrating how once-horizontal sedimentary strata can become distorted over millions of years.

Yellow Pinch cuttings along Princes Hwy

Devonian or Silurian metamorphic strata visible.

Another example of metamorphic sedimentary rocks. “Devonian and Silurian sandstones, shales, siltstones and conglomerates do not generally form particularly fertile soils”.

Nethercote

Basalt visible just south of the Nethercote Hall. The rich volcanic soils formed by weathering of the basalt support the dairy farms in this area. Volcanic ash has contributed to a thin layer of richer topsoil in some areas, over less fertile geologies.

“Basalt is a type of igneous rock, extruded as molten magma from under the crust. Its crystals are small in size because the molten rock cools quickly at the surface”.

Basalt forms some of the most fertile soils, with a high mineral content, amazing moisture holding capacity and a generally excellent crumb structure.”

Roadside cuttings on the road back to Eden from Nethercote

Ordovician metasediments can be viewed at road cuttings on the road between Nethercote and the Princes Highway. “Ordovician metasediments are mainly shales, siltstones and mudstones laid down in lakes and seas at a time when there was unmitigated erosion due to the absence of complex life on the land. Near-vertical fissures allow the deep penetration of water so that the surface is rarely properly wetted. These landscapes tend to have a high drainage density (number of gullies per square kilometre) because most of the runoff is stormflow through sub-surface fissures. Gullies tend to be actively incising.” A view of the Nethercote/Nullica area on Google Maps (zoom out on this map) shows the large numbers of gullies in this region.



Bega batholithe

To see the granite of the Bega batholithe, one only has to drive around the Bega Valley. “The Bega Valley is dominated by decomposed granite soils, mainly derived from a type of granite called biotite granodiorite. It forms a rich black loam with high cation exchange capacity and relatively high permeability. It is the basis of the successful local dairy industry in spite of a relatively erratic rainfall distribution.”

Exposed granite boulders can be seen throughout the Bega valley, depending on local topography.

Allan Brook Road (off the Imlay Road) and Yambulla State Forest

These are good sites to view adamellite granite.

“This type of granite forms very poor soils.  There is no phosphorus in the parent rock.  Particle size in the surface soil tends to be quite coarse, with clay leached to a subsurface layer (i.e. a Podzol).  Areas of adamellite have generally not been selected for agriculture and support low commercial quality forests.  However, the clay layers below the surface in a podzol can slow the drainage of rainfall and help to form biodiverse heath swamps.”

Surprisingly, the diversity of plant life growing in nutrient poor adamellite granite soils is often much higher than in fertile soils.

Nethercote Falls/Bournda Nature Reserve/Navy Wharf near Edrom

These are all good sites to view rhyolite, a silica-rich igneous rock formed by melting of continental crust. While Kerri and I didn’t include the above rhyolite sites in our geological tour, we have a personal interest in this rock type. A band of rhyolite runs along the Wonboyn River valley, which is where our property is located.

Green Cape

This belt of rhyolite outcrops at Green Cape where it can be seen as columnar blocks on the southern side and tip of the Cape.

Further Exploration

Below is a list of resources that will be helpful if, like me, you’re keen to learn more. I’ve also been poring over geological maps, considering the vegetation in my home forests in light of what I’ve learnt about the underlying rocks. A fascinating exercise!

David Johnson (2009) Geology of Australia. Cambridge University Press. ISBN 978-0-521-767415. This is the book Mick referred to extensively during his talk. A copy is available from the BVSC library. It includes the following powerful analogy of geological time:

“One way to grasp the magnitude of geological time is to compare it to the events in a year, to compress the entire 4.6 billion years into 365 days.  On that scale, the oldest minerals we can date formed in mid-January and we can find living creatures such as algae and bacteria in May. Plants and then animals did not emerge on land until the end of November, and the valuable coal basins of eastern Australia were formed from vast peat lands over a period of 19 hours on 10 December. Dinosaurs roamed central Queensland from shortly after this until 26 December, when Australia separated from Antarctica and started its drift northwards towards Asia. The Great Barrier Reef probably formed mainly after 11.03 pm on 31 December.  Aboriginal people arrived perhaps around 11.54 pm, and the last of the giant kangaroos died out by 11.59.35.  Twenty-five seconds to go. Rome ruled the Western world for 5 seconds from 11.59.50 to 11.59.55. James Cook arrived on the Australian coast at 1 second to midnight. And, as they say, the rest is history.” (page 50, D. Johnson)

Mick also made use of the following local documents.

Beams and Hough. 1977 Geology of the Eden Region Appendix 2a of the Eden Native Forest Management Plan 1982 (State Forests of NSW, May 1983).

Tulau M.J. 1997 Soil Landscapes of the Bega-Goalen Point 1:100000 Sheet. Soil Conservation Service of NSW.  Land and Water Conservation, NSW.

Fryirs K. and Brierley G. 1998 River Styles in Bega/Brogo Catchment: Recovery Potential and target conditions for river rehabilitation. ISBN 0 7313 0397 0 NSW Department of Land and Water Conservation.

I discovered a paper by Ida Brown from 1931, which describes the Devonian rocks of the South Coast.

Brown, I.A. 1931. The Stratigraphical and Structural Geology of the Devonian Rocks of the South Coast of New South Wales. Proceedings Linnean Society of New South Wales 56, 461-496.

I assume that much of her analysis is still considered valid and in any event the paper is of historical interest. It can be viewed and downloaded free at this Biodiversity Heritage Library website.

Finally, here are two links to detailed maps showing the local geology. Both are available for free download as pdf files. Select your desired file type, then click on download. Note that you also have the option of purchasing a print version of the maps for $11.00.

NSW Government Monaro 1:500 000 Geological Map (from 1971)

NSW Government Bega-Mallacoota 1:250 000 Geological Map (from 1995)

Prepared by Paul Whitington, based on Mick’s presentation, notes and some further reading - 8/10/19. This information is based on an interpretation of geological maps of the area. It does not involve direct examination of crystal structures in the rocks.