"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.
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