Friday, 30 November 2007

"Switch back" topography

There were a couple of nasty surprises for Mark today - the condition of his wheel was the first but having repaired that, he then had to face what he describes in the diary as the 'switch back' hills to the east of Adelaide. I have done a bit of investigation about the hills and here's what I have unearthed. You can see on the aerial view above that the southeastern suburbs of Adelaine end fairly abruptly along a line which coincides with some accentuated relief. It is so marked that I would go as far as to say that the city is hemmed in by the range of hills. These are the Mount Lofty Ranges which separate Adelaide and the Adelaide Plains from the lowlands of the River Murray to the east. And yes, it does look very 'uppy downy' as you can see when you zoom in a little.

The problem with aerial views like this when you want to look at relief is that they flatten the hills. You have to look for clues like the hairpin bends and tunnels to really understand what you are looking at. Clearly, from Mark's description of being two hours in low gear, the road across the Mount Lofty ranges is very steep!

The summit of Mt Lofty provides some pretty good views back down to Adelaide and gives an indication of just how high Mark had to climb today ... It will be no consolation to Mark that the east side (downhill for him) is much less steep. There is a good geological explanation.....
The Mount Lofty ranges form the eastern rim of a large syncline (downfold or basin of sedimentary rocks). However, the topography is complicated by the fact that there are also a number of active faults in the Mount Lofty Ranges and these have allowed sections of the crust to slip downhill as in this diagram. It is apparently a very complicated fault zone in which there is still regular minor seismic activity. This type of faulting results in a series of steps in the landscape - downthrown blocks, some with reverse tilt which give the 'switch back' topography described by Mark. And below.... one of those steps .....

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