Wednesday, July 5, 2017

View from Alice's Tardis


(20170706):-

Flatness, and the Relentless Imperative of 'Beaches'.


(Meaning, .. how flat can land be, when it is not by the sea?)
Australia's experience of the Great Regression - the time when the inland seas and lakes ran of the land into the developing ocean basins.


















(Agnew, Western Australia) [Picture = Agnew pub.]

In which we compare and contrast two erosional land surfaces that developed over Archaean granites , and note the very slight topograpical difference that can lead to very pronounced incision - even when rainfall in the continental interior is near-desertous.

One surface is old laterite and flat-as-a-tack (darker brown in the upper part of the figure), the other is younger, much incised and is cut into the underlying lighter coloured saprolite.  The pin marks some old, degraded sand dunes on the older surface and may be older than the younger drainage radiating off the high ground). 

The land drops about thirty metres over the step of the breakaway, formed as the younger erosion cannibalises the high ground, and a further thirty metres to the lower boundary of the figure (~ 7km).

The breakaway offers a good profile through the saprolite.   Not quite sure what to make of the contrast as regards climate variability.  Laterite forms in tropical and subtropical climates implying high rainfall, but drainage over the older laterite describes a profile that is almost as sheeted and flat as it is possible to be.  Further afield than the figure the breakaways seem to develop on what small slope differentials that do exist.

(The Agnew pub?   Just a blast from the past, ... )

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