Photographs by Harry Whaite 1912 - 2011

 

 

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THE PREHISTORIC HISTORY OF THE SYDNEY DISTRICT

By GRACE EDGECOMBE, B.Sc.

The Sydney Bush Walker 1936

"The Earth's crust is the Geology Professor's Daily Bread," or, to be more precise, his daily sandwich, a four-decker effect, with the oldest rocks at the bottom. Needless to say, the lower layers can't be seen unless the "sandwich" is tilted. As regards the Sydney area, only the two central layers, the Palaeozoic or older, and Mesozoic or younger, are to be found.

In case you should have a passion for antiques,: the oldest Palaeozoic rocks we can provide are to be found in the Shoalhaven Gorge, below Badgery's Crossing, where you can see really grandfatherly slates, folded and up-ended in a most exciting manner—because, like an apple, the older a rock the more wrinkled it is. Those wrinkles are on a. pretty large scale, a single fold may be hundreds of feet in depth; so that you can see it must have been quite a time since they were laid down, as mud, in a nice, quiet sea full of—well, things like jelly-fish and blue-bottles. Pretty tough on surfers, if there had been any surfers. How do we know there were any such animals ? The expert will answer, with lofty scorn, that there are fossil remains. At this point the unregenerate friend usually remarks that he can produce a better-looking fossil with a piece of slate and a pencil—and what is worse, he does. But, if you yearn for a gen-u-ine 100% graptolite, there are some to be found in the blue grey slaty rocks around the Jenolan River—Cox's River Junction.

These folded slates, then, were once all the rocks we had. The climate grew warmer, coral reefs flourished, and volcanoes were strewn around the landscape; in fact, it was all too picturesque and tropical for words. It was during this period (which rejoices in the name "Silurian") that the limestone for the Jenolan, Wombeyan, Colong—and any other caves in the area—was formed by the coral reefs, while from the volcanoes came the molten rock which now forms the rugged country along the upper Jenolan River.

In the course of time the coral reefs became somewhat less important and the Volcanoes more so, especially during what is called the Devonian period. These volcanic rocks are usually very fine-grained and tough—some of them are called "tuffs"; but probably not for that reason. Since they are comparatively older, and hard, you will usually find them in the lower, more rugged parts of our mountain valleys— in the Tonalli River at Yerranderie, in Jooriland Creek to the south and in Cox's River above Hartley.

Before finally disposing of these particular rocks, I must mention something else that happened to them. From deep down in the earth came masses of molten rock, which never reached the surface, but eventually solidified, forming an enormous area of crystalline rocks— granite and its friends and relations—easily recognisable by their "spotted dog" appearance. Bathurst, Hartley, Rydal, part of Megalong and the Upper Cox are all situated on or near this granite mass.

At this stage, the whole area had been worn down to a level plain. Then Sydney—pardon, I mean the future site of Sydney—became the centre of a depression. Just a little one at first, but it "growed," just like Topsy, and formed a huge basin-shaped sea, in which hundreds of feet of sand were deposited, coal was formed, and submarine volcanoes were probably active. The coal measures appear on the surface as a huge semi-circle swinging from Newcastle, through Lithgow,

to Bulli. The volcanoes are responsible for the open, fertile country round Kiama and Gerringong, and for the ridges of Saddleback and Cambewarra; incidentally these dark "basalts" and "andesites" provide something extra special in the way of slippery mud in wet weather.

Finally comes the Mesozoic, the third decker of the sandwich, and the upper three layers of this "basin" which were formed from muds and sands deposited in fresh water. The lowest, the Narrabeen shales, appear on the north, and, strange to say, at Narrabeen; and on the south, between Era and Stanwell Park. Their most noticeable feature is the chocolate-coloured shale, on which the cabbage-palm flourishes. Above the Narrabeen beds, filling most of the centre of the "basin," and producing our most characteristic mountain scenery, is our Hawkesbury sandstone—by no means confined to the Hawkesbury River—but everywhere forming those vertical walls of extraordinarily uniform golden-coloured rock which constitute our "mountains," with their level tops. These are only interrupted where a volcano (they seem to have been laid on) left sheets of basalt, the remnants of which form Mounts Hay, King George, Tomah, Wilson, Colong, and the Far Peak.

Eventually, river systems, the Cox, Wollondilly, Nepean. and the Grose, carved valleys for themselves out of the four thousand foot tableland, and produced our present day scenery, which fortunately is beautiful enough to survive even the geological (and other) remarks I have been making about it.