For Those Who Love Learning

Tonight I am going to take you on a journey, a journey through time both ancient and modern.

The cover of the book you see here is safely reproduced, I believe, as it is no longer in print and as one of the authors is my father. It is an excellent publication released circa 1968 and containing a wealth of information on the geology of the Adelaide Region. I bring it to you now as a direct result of the farcical Year 10 Coastal Excursion.

You may (or may not) recall that one of our ‘educational’ stops was at the remarkable

Hallett Cove where “many of the important geological events recorded in South Australia can be studied” (Talbot & Nesbitt 1968 p18). Of course there was a complete lack of interest in this activity by any of my students but I did reflect that although I had a sketchy knowledge of the area gained through the information plaques dotted about the Cove and the question book the students were required to complete, I would love to have the opportunity to gain a greater understanding of the region.

This is ironic. As a child I had been on the Hallett Cove excursion with my father and his Geology 101 students on a number of occasions. They usually took place on a Saturday and afforded my mother some much needed ‘child free time’ as well as opportunities for father-daughter bonding between said father, my sister and I. It hardly need be said that we gained little in terms of education from these trips, a frequently heard mantra being “Daaaaad. I don’t CARE if that’s a nice example of ‘cross-bedding’.” (I might explain here that cross-bedding is a geological term describing a manner in which sediments have been laid down, not a euphemistic reference to adultery).

But I digress. All those childhood opportunities to fully appreciate the geological history of South Australia were lost in a sea of youthful apathy not dissimilar to that of my Year 10 Geography students. Now however, I had the chance to correct this situation. Mum and Dad are here for one more week and I am on holidays so I suggested to Dad that we spend the morning together visiting Hallett Cove.

I will attempt to encapsulate for the lay person, what I learned from my father last Thursday.

Let us begin with geological time. About 600 million years ago, during the Precambrian period, sedimentary rocks were laid down in shallow water, such as a river delta, and about 500 million years ago they were subjected to enormous pressures probably brought about by a collision of continents or at least the plates upon which the continents rode.

These pressures caused an enormous distortion and uplifting resulting in a massive mountain range running north/south along the current coast of South Australia.

This is what’s left of them.

Over two hundred million years passed. The mighty mountain range was eroded away by the elements. Sediments were probably laid down over the top and eroded away again. The Cambrian period gave way to the Ordovician, the Silurian, the Devonian and the Carboniferous. Rocks were formed and laid down, perhaps twisted and changed, we don’t know. Nothing of this two hundred million year's worth of geological history remains. Instead, at the top of this cliff there exists a geological ‘unconformity’. 270 million year old rocks sitting directly on top of 600 million year old rocks, on an irregular surface polished, scratched and shaped by glaciers.

Imagine the force of ice pressing down over time on this rock. Can you see the large undulations in the surface and the smaller, obvious scratches running approximately north/south? Throughout the Permian period sands and clays were deposited onto the exposed Pre Cambrian bed by the retreating glaciers. These appear as a prominent white and red layers across the landscape and contain boulders called ‘erratics’ dropped by floating ice into the lake sediments below. In the photo below look carefully at the red layer in the centre to see an example of one of these ‘dropstones’.

Once you have spotted it you can start to see more of them scattered throughout the Permian layers.

As you can tell from the amazing shapes and colours in this Permian layer, the soft material is easily eroded and indeed in the following 200 million years, this layer and whatever was once laid over the top of it were again stripped by the elements. The next obvious layer in the geological ‘cake’ is a thin, hard band of calcareous sandstone, approximately 5 million years old and containing numerous marine fossils. This is the Pliocene marine bed.

Here you can see Dad examining it and at about the line of the brim of his hat you can see the sediment cutting through the geological sequence.

Above it, relatively recent layers of alluvial sediments attest to the creation of the Adelaide Hills with streams bearing freshwater sediments now running down towards the marine Pliocene layer. These alluvial sediments were laid down in the last million years, known as the Pleistocene period.

One of the features of these more recent rocks is the records they contain pertaining to climate change. In the middle of the sequence there is a spectacular red and white mottled fossil soil known as laterite. These soils are typically seen in tropical climates where heavy rain has leached the nutrients from the soil leaving, in this case, mainly iron oxide and aluminium silicates.

Here’s an example where the laterite is exposed and weathered resulting in this spectacular outcrop. At the time these rocks were soil, our dry old Adelaide coast was a tropical paradise.

Later, near the top, a ‘kunkar’ layer contains the typical nodules of calcium carbonate formed in dry climate soils. It seems ‘climate change’ is nothing new.

This starkly beautiful landscape is described in the line drawing below. The laterite layer can be clearly seen through the middle of the upper red alluvial deposits. Nature has helpfully exposed an entire 270 million years of history for us here.

Finally, look down the coast at the exposed glacial valley.

Can’t see it?

I love that it is so obvious once you know what you’re looking at.

This is an incredibly brief summary of our amazing morning. I learned so much more about the history of geological discovery, beliefs, misconceptions and revelations. I learned about methods for dating rocks and looking critically at ‘geo-morphology’.

I learned about how much has been added to geological knowledge since my Dad’s book was published 40 years ago.

I came away feeling energised, washed clean and intellectually stimulated. I wondered whether I would have felt the same way at the age of 15 and I had to admit that I probably wouldn’t have. Why then am I surprised by the lack of enthusiasm from my pupils?

As a tiny little spooky post script I found this map in Dad’s book.

Can you see Eden Avenue and the words Sturt Gorge? Well, by use of Google Earth I can tell you that situated around about the position of the first letter T in Sturt, is our house. I wonder if you’d told my Father in 1968 that his 7 year old daughter would one day be married and living on this map, whether he would have believed you?

NB: For those of you who subscribe to the Biblical ideas of 7 day creation and a 6000 year old Earth, remember that for God a million years is but the blink of an eye.