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A note about the photos:
Some of the shots seen here were taken from places which are no longer accessible, thanks to the nervous nellies of the NPWS, reacting to The Great North Head Calamity. The visit to Old Man's Hat is still possible (by another route), but not recommended, due to the high tick counts. |
In the early 20th century, one of the major influences in my life, Griffith Taylor, proposed that there was once a river, flowing parallel to the present coast. Some 60 years ago, his Sydneyside Scenery was my bible, and this image comes from that work:
The river entered the present Sydney harbour near Bondi, where there is an old channel, with the bottom far below the present sea level. From there, it would its way up the coast of the northern beaches, before feeding into Broken Bay at Pittwater.
Near North Head, the river passed out into what is now the ocean, running along the line
of Manly's main shopping street, The Corso, and one of my geology lecturers told us that
"in historic times, ocean waves had rolled all the way along the line of the Corso, and into the harbour".
As a writer of history, I have been unable to confirm this, but informal chats with geotech engineers reveals that the whole Manly flat has at least 12 metres of sand, so given the right storm and the right lack of civic responsibility, North Head may one day become again the island that it probably was at some previous time.
Then again, if you look at the pictures on the right, the headland won't be there forever. It is composed of Triassic Hawkesbury sandstone, and sandstone typically has joints, and these make the cliff fall in slabs. For more, see The Great North Head Calamity, cited above. Having written it there, I'm too lazy to add it here.
It is in the nature of sedimentary rock like sandstone that the beds are flat and horizontal, but the Hawkesbury sandstone has a trick up its sleeve: cross bedding. This is explained in a piece I wrote on ant lions. The main point is that you will often find tilted beds as you walk around: these beds are best thought of as fossil sandbanks, but if you want more, look at the links, then search in cross bedding and/or current bedding.
Then again, you could always buy or borrow Australian Backyard Earth Scientist, by the same author. This was written for younger readers, which means adults should be able to understand it :-)
People often ask about the steel tracks, but the scientists tell us that the sand on North
Head is largely aeolian sand, meaning it was blown in. The date for this sand
delivery is set at "during the last Ice Age", and that means there won't be another
delivery until we get another ice age. On current indications, that's a long way off.
Some of the sand comes from sandstone breaking down or weathering, but that's a slow process. The steel tracks keep feet away from the fragile ground. When erosion does happen, like the example near the entry to the Third Cemetery seen above (shot taken 2015), we need to act. The Third Cemetery entry was professionally and beautifully paved, but the right-hand picture shows a simple variant of the bush remedy known as Chuck a log in it. This was on one of our service roads.
In simple terms, a log in a gully slows the water down, so that instead of washing sand and silt away, it drops it. Some of the people in blue shirts could show you where that site was, but it is now hidden by washed-in silt. If you see stones arranged in a gully like then ones you see here, please leave them where they are.
Our other erosion control is on the steeper sandy slopes, where we use dead tree trunks to hold the sand in place.
You can see weathering best along the cliffs, meaning you will
now miss most of this fascinating aspect of geology, but if you are walking in
past Collins Flat, you will see some examples of weathering near the beach.
The cliff on the right is one of the east-facing portions, near the Fairfax loop. In this photograph, you can see cross-bedding, honeycomb weathering and other effects, but this link will bring you up to speed on the science of sandstone.
This file is http://members.ozemail.com.au/~macinnis/nhead/geology.htm.
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http://members.ozemail.com.au/~macinnis/nhead/index.htm
The page was first created on 11 January 2019, last updated 1 August 2020.