An Ancient  Landform 

 

Way back in the beginning of time for the Valleys of the Blue Mountains,  the land was once a  vast flat plain that sometimes was under the cover of an ocean and sometimes not. 

The shoreline of Sydney  extended out much further than  today   Even at the time of the last  ice age  of  12,000 years ago  the coast line of Sydney was at least 12 miles further out and  it was a period of fairly rapid sea level rise  the tides wre massive and ebbed and flowed  according the whims of the Moon.

 With the major remodelling of a landmass like that involved in the formation of Pangea, oceans are closed and water currents drastically modified.  Ocean currents are important temperature regulators because they redistribute heat around the planet, so a change in ocean current circulation inevitably leads to a change in the climate.

 

During most of the Devonian Period, the climate was relatively mild and the continents  straddled the equator at the time, set the scene for the spread of tropical and equatorial forests. The position of this land mass also meant that many communities of vertebrate animals were concentrated near the equator where the warm climate encouraged their growth and evolution.

410 million years ago the influence of the moon was much closer that today. But in the past,  the Moon is now twice as far from Earth.  Our planet was therefore turning faster in the past, such that each year consisted of more, but shorter, days  just 22 hours and the tides were seven times higher than todays tides.

 

 
'Ideal view of a marshy forest in the Coal Period'
 

 

The crust of the earth in the Cox Valleys

Ancient Ranges and Peneplains.  From very early times the crust of the 
earth in this area has been in a state of unrest. The Silurian and Devonian beds 
are unconformable, and have been greatly crushed and folded. Since the close of 
the Carboniferous there has been relatively little folding, the earth movements 
having been more of a vertical character. 
At the c lose of the Devonian there was a great period of mountain-building, 
With the movement of uplift dominant, the land was kept above the sea, 
and a great cycle of erosion resutlted in the reduction of the folds, and the 
formation of a peneplain. So great was the amount of erosion, that areas of the 
intruded granite were exposed. The harder rocks, such as Gangerang and Mt. 
Walker, persisted as peaks, up to 2,000 feet above vhe softer rocks of the pene- 
plain, which formed gently-rolling plains. Some of the peaks exist at the present 
time. 
Until the close of the Triassic, the eastern part of the area continued to be depressed, 
but the western part appears to have been lifted above the sea at the close of the Permian.
Since the deposition 
of the Hawkesbury, Narrabeen and Wianamatta beds  the latter only occurring on 
the eastern margin of the area  the major movement has been one of uplift. The 
earliest uplifts have all been obscured by those coming subsequently, and it is 
not until the period of basalt flows, probably later than Middle Tertiary, that we 
get a distinctive peneplain preserved.
 
The great Tertiary Peneplain, which now forms the plateau surface, was 
probably formed in the latter part of the Tertiary period. The history of the land 
surface subsequent to this peneplanation, is most readily traced by physiographic 
methods. 
the rock structures 
of the area, one is struck by the great resistance to erosion shown by the majority 
of the rock types
these uniformly-sloping ridges were once 
more level than now, and have been tilted by warping, thus giving the Cox its 
asymmetric valley.

 

 
 
After the streams in an area have reached "base level", lateral erosion is dominant as the higher areas between the streams are eroded. Finally, the upland is almost gone: the stream floodplains merge in an area of very low to no topographic relief. The resulting flat plain is the ultimate stage in the cycle of erosion or geographical cycle.

The streams within a peneplained region show extensive meandering and braiding. If the area is subsequently uplifted due to adjacent orogenic processes, without internal deformation within the peneplain, the streams will again begin downward erosion - creating incised meanders, water gaps, and other unique geomorphic features.

A peneplain can be mistaken for a depositional plain. However, the rocks beneath a peneplain have been folded and tilted by tectonic forces, while the rocks beneath a depositional plain lie in horizontal layers.