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200 million years ago
Australia began as part of a great southern super-continent
called Gondwana.
At this time, the climate is luxuriously warm and wet worldwide.
The Earth is covered in rainforest made up of ferns and
gymnosperms, including pines and cycads. |
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45 million years ago
Currents deep within the Earth have caused Gondwana to
break up.
Australia is separating from Antarctica and starting to
move slowly northward, as it still does today.
The break-up of Gondwana is causing dramatic climate changes
and the extinction of many of the old plant and animal groups.
Rainforest still covers much of the Earth, but is made up
more of the new angiosperms, the flowering plants: flowering
trees and palms have started to dominate over pines, cycads
and other gymnosperms. |
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15 million years ago
Major ice sheets have formed in Antarctica and climates
have become cooler and more arid worldwide.
Australia has started to dry out and is now covered mainly
by open woodland, with wattles, Casuarinas and some Eucalypts,
and grasslands.
Rainforest has mostly retreated to the north and east.
Around 10 million years ago
As Australia collides with Asia, mountains are pushed up
in New Guinea and along eastern Australia.
A severe rainshadow forms across inland Australia and central
Australia becomes more arid. |
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2 million years ago
The North Polar ice cap is forming.
With more of the Earth's water tied up in ice, the world
has become much drier.
Much of Australia is now semi-arid.
There are patches of rainforest in central Australia, with
Acacia (wattle) woodland and grasslands dominating. |
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Present
During the ice age peak 18,000 years ago, the driest
period in Australia's history, vast areas of sand dunes
developed in central Australia.
Today, inland Australia is well-vegetated, covered with
Acacia woodland, shrubs and grasses.
In central Australia, remnants of the old rainforests, like
the cycads and palms, are found only in small, isolated refuges
like Palm Valley. |
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Palm Valley taps into a huge underground basin of fossil
water, largely unaffected by present-day droughts.
This sandstone is like a sponge saturated with water.
Water moves very slowly through the sandstone, because
of its low permeability.
Most of the water seeping out into Palm Valley, through
cracks and other features in the sandstone, is old water,
although some is from recent rains. |
The Central Australian Red-cabbage Palms
Despite changing conditions, Central Australian Red-cabbage Palms
have survived here in the desert, hundreds of kilometres from any
other palms.
Palm Valley is one of very few areas in the Central Ranges where
there is both permanent water and protection from raging floods.
Water
that fell as rain thousands of years ago, when central Australia
was much wetter, now seeps down through a great mass of fractured
sandstone, towards Palm Valley and beyond.
High valley walls protect the palms from drying winds.
The gentle
slope of Palm Creek means that the palms' shallow roots are protected
from scouring by great deluges of water.
The MacDonnell Ranges Cycad
The MacDonnell Ranges Cycad (scientific name Macrozamia macdonnellii)
only grows in the Central Australian ranges, although other forms
of cycad are found elsewhere in Australia.
These cycads grow mainly on rocky slopes where they get some shelter
from the hot sun, and where their roots can penetrate deep into
moisture-trapping rock crevices.
The cycad is also a very slow growing plant.
Its trunk consists of the densely packed remains of every leaf
the plant ever grew.
By counting these leaf bases, scientists estimate that some of
the local cycads are 300 years old.
So these individuals were alive and growing here even before captain
Cook landed in Australia. |
