The MacDonnell Ranges and Ellery Creek

The MacDonnell Ranges

Approximately 850 million years ago, most of the southern half of the Northern Territory was inundated by a shallow sea - about the same size as today's Mediterranean Sea.
Quartz-rich sediments were the first deposited.

Other sediments were laid on top until the waters retreated 460 million years ago.
At their deepest point these sedimentary layers, known as the Amadeus Basin, were over 10 km thick, causing deposits in the pile to be compacted to solid rock.

Since then, massive earth movements have tilted, buckled and folded these layers.
Approximately 315 million years ago the ancestral MacDonnell Ranges were Himalayan in size.

The same forces that pushed up this mighty mountain chain have fractured the tilted rock layers in this area.
The sediments which had been laid down in horizontal layers were thrust up to their present, near-vertical position.
The resulting zone of broken rock is more easily eroded by running water and creeks forms along these lines of weakness.

Over time wind and rain have carved into the vertical layers to reveal dramatic swirls and curves.

Ellery Creek

An almost complete sequence of sedimentary rocks, laid down over a 400 million year period, can be seen along the banks of Ellery Creek.
This exposure extends for eight kilometres downstream, south from Ellery Gap.

The table above refers to the section of Ellery Creek south from Ellery Gap to Namatjira Drive and covers the period from 850 to 600 million years ago.
During this time different sediments where deposited one on top of the other to produce a layered "rock sandwich" nearly 2 kilometres thick.

Simple algea were the dominant life form on Earth at this time.
This is why few fossils are found in the rock today.

The rock strata exsposed are now tilted to nearly vertical and run at right angles to the creekbanks.

Over the past 315 million years all the softer rocks that lay above the Heavitree Quartzite were removed by erosion.
Running water has gouged out the zone of weakness to create Ellery Gap.

Massive earth movements folded the rock layers and caused great slabs of country to slide up and over one another.
Faults, cracks and other zones of weakness were created as the layers were rearranged.

When the creek is in flood, water rapidly flows through Ellery Gap.
The turbulent, rushing water is loaded with rock fragments and acts like liquid sandpaper, scouring away the walls and creekbed.



No liability for timeliness, integrity and correctness of this document is accepted.
Last updated: Sunday, 13.07.2008 8:42 PM