Sunday, 18 November 2007

Explaining the thin black line....

By sundown today, Mark has reached a point just 50 miles away from the West Australia/South Australia border and is more or less half way 'round' the Great Australian Bight. he overnighted yesterday near Madura which has a geomorphological significance that only became apparent when I took a broader view today.... Hopefully you can see what I'm referring to... that black line which the Eyre Highway crosses at Madura. The line continues eastwards and for all of today Mark has been cycling 'in its shadow' on its southern side but never more than a few kilometres away.

By this evening, Mark had passed Mundrabilla and 'it' was still there. The aerial photo is actually a little deceptive as it gives an impression of a cliff. However, the dark line is in fact trees rather than shading caused by the change in relief. You can see that more clearly when you zoom in close....

Nonetheless, the trees are indeed growing on a break of slope, which a bit of investigation has revealed to be a 100m high limestone escarpment marking the place where the Hampton tablelands give way to the Roe plains. This raises two questions...a) why is the escarpment there and b) why are there trees on it?

It seems that the escarpment is fault guided i.e. subsidence along a crack in the crust caused the coastal plain to be downthrown while the tableland was uplifted. Additionally, at times when sea levels were much higher than they are today, possibly during one of the inter glacial periods of the past, the coastal plain would have been under the sea and waves would have eroded a cliff along the line of what is now the escarpment. Today the same process is being repeated at the current coastline (as we shall see in a day or two). As for why there are trees on the escarpment ..... in a temperate, humid environment, we could assume that trees had been cleared by man above and below the escarpment. Here, however, the sheer scale and the aridity make tree clearance highly unlikely. I think it is possible that since limestone is permeable, any rain which falls and goes to ground, may seep into the structure of the limestone and appear where it is intersected by the relief. This might be sufficient to support tree growth.

I have found this photo on Flickr which is taken from the top of the escarpment near Mundrabilla overlooking the Roe plains. The trees on the escarpment occupy the foreground. Mark has now passed Mundrabilla - where the first sheep station on the Nullarbor was set up there by a Scot in 1872.

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