Re: Valleys in Chinese Karst scenery

From: Carsten Troelsgaard (carsten.troelsgaard_at_mail.dk)
Date: 10/13/04 

Date: Wed, 13 Oct 2004 10:01:24 +0200

"Harvey Rutt" <xh.ruttx@x.ecs.soton.ac.uk> skrev i en meddelelse
news:416c0bd7@news.ecs.soton.ac.uk...
>
>
> > If you need additional info on Karst and related:
> >
> > http://www.lcc.ctc.edu/departments/natural_sciences/links/linksCAVES.xtm
> >
> > In my view, I've done the google that you ought to have done before you
> > asked your question.
> >
> > Carsten
>
> Now thats a complaint I often level at others, not guilty!
> I spent quite some time, on more than one occasion, looking on the web, & in
> the ones I found (floods of stuff, hard to narrow to the question I was
> asking) not one of them really addressed that detailed point of why broad
> flat valleys.
> I'm not saying it isnt there if you know the subject; just that it was non
> trivial to find that specific answer in the flood of obvious general stuff
> on karst.

This comment is somewhat irrelevant: You put the wrong question! If the karst-towers weren't there,
you woulden't ask why the ground is flat. The mail of Jo and George suggest different levels of
drainage which obviously has the potential to make a complex situation, and in a real life situation
a sane geologist would suggest a seismic or electric profiling of a transect of the valley - to get
a true picture.
As a first approximation I would look at the profiles of the uncovered valleys and keep in mind that
soil that builds on top of it, if humid and begrown, has a high CO2 pressure and that eventually
acidifies percolating water for continued subsurface chemical erosion. Any chemical erosion will
follow fissures (chalk giving a different overall pattern than eg. sandstone) and as it is in a wet
tropical or subtropical region, the subsurface chemical breakdown will be very extensive, in
particular becourse the basement is chalky.

> (I even looked in two geology text books! I must be old fashioned!)
>
> Oh & BTW, that site I found & it has so many broken links to what looked
> like what I wanted:
>
> a.. Karst - andforms developed by dissolution of soluble rocks
> a.. Karst - Limestone and Reefs
> a.. Homepage of Ciudadanos del Karso - What is the importance of the karst?
>
> (especially the first) for example I gave up on it!

I did finally locate Guilin & Yangshuo on the geographic map. My suggestion as to sediments derived
from northern loess doesn't seem relevant.

http://www.askasia.org/image/maps/ele_china.htm

The topographic map reveals a large vally extending in from the seaside. Tectonic uplift or
subsidence and global sealevel-stands in combination sets a relative sea-level that basically is a
line that separates functions of erosion and deposition. You could check weather the flat vallies
are within the reach of a seaside inundation.
Suggested google-words: 'chemical erosion'+humid+tropical, 'drainage pattern', and if you are really
serious about geology 'sequence stratigraphy'

Carsten

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