One of the things that scientists know for certain about the waves
which dumped billions of tonnes of sandstone on Sydney was that no-one was
around to catch them.
Dr Patrick Conaghan, a senior lecturer in the School of Earth Sciences at
Macquarie University, believes a succession of catastrophic, massive flood waves
possibly 20 metres high and up to 250 kilometres wide swept down from an
ancient lake that stretched from Murrurundi, north of Sydney, to the Carnarvon
ranges in central Queensland.
His theory about how Sydney got its sandstone can be perfectly illustrated
by recently discovered "fossilised waves" in cliffs above the Wolli Creek
At the time of the flood-waves, in the Triassic period about 237 million
years ago, the South Pole was somewhere west of Walgett, in north-west NSW, and
Australia was still part of the super continent of Gondwanaland.
The Sydney basin was a sandy river-plain vegetated by ferns and seed-ferns
and inhabited by amphibians, reptiles, fish and freshwater sharks.
"Imagine a sudden wall of water up to 20 metres high coming down from
Murrurundi at enormous speed ... on a front up to 250 kilometres wide," Dr
"Imagine that it would tear the whole landscape apart - and that's what it
The water came from what Dr Conaghan calls Lake Napperby, which occupied
what is now the Gunnedah basin to the north of the Sydney basin. Periodically,
glaciers and small ice caps in the New England Highlands east of the lake
suddenly released bursts of glacial lake water.
Dr Conaghan compared this to emptying a full bucket of water into another
full bucket, causing an instantaneous spill.
As far as he was aware, the catastrophic flood deposits of the Hawkesbury
sandstone were about 235 million years older than any other similar deposits in
the world. The rocks have been dated from plant micro-fossils of spores and
The fossil waves at Wolli were formed as the floodwaters began to recede.
The waves, which geologists call recumbent cross-beds, record the
distortion of sediment layers caused by the massive floods and are found around
But Dr Conaghan said the Wolli waves, which were three metres tall, were
twice as large as any other known.
Dr Conaghan was shown the cliff where the waves are preserved by members
of the Wolli Creek Preservation Society. They are in a weed-infested area of
Earlwood that is littered with bricks and rubbish.
He believes the waves should be given heritage listing because of the
insight they give into the palaeogeography of the Sydney region.
The flood-waves, superconcentrated with sand and mud, sporadically
devastated Sydney's Triassic geography and plants and animals for nearly two
The initial force of the floods left kilometre-long layers of almost
structureless sediment which were up to tens of metres thick. Today, these
layers are quarried to produce almost flawless sandstone used for building.
The source of this sandstone had puzzled Dr Conaghan and other geologists
Such sediment is extremely rare if not absent in the deposits of other
rivers around the world. Also, Dr Conaghan said the volumes and velocities
necessary to explain these types of sediment flows must have been huge.
It was the discovery of Lake Napperby a few years ago by researchers from
the University of NSW and the Geological Survey of NSW that allowed him to put
his theory of the origin of Sydney's sandstone.
In the days after the peak of these periodic flood waves, the area from
Mittagong to the Hunter Valley was inundated with fast-flowing water. These
receding floodwaters continued to transport large volumes of sediment for many
It was in these secondary sediment deposits that the recumbent crossbeds
formed at places such as Wolli Creek.