zeugen v yardang v rock pedestal

[kevincooper777]

Hi folks, we are having a discussion on the new OCR A GCSE ning as to the finer intricacies of the difference between these things - various internet and textbook sources (including Waugh!) have contradicted each other.

Anyone confident enough to attempt a clear explanation of the difference between them?

Or have a copy of Small (or failing that, Collard's Physical Geography of Landscape)?

This sort of thing drives me barmy!!

I know that the reality is, surely, that the explanation of their formation and clear understanding of the processes is the important thing - there's no way surely the examiners would expect GCSE students to know the finer detail of the differences between zeugen, yardangs and pedestal rocks? And in any case, real landforms show far more variety than textbooks suggest - with spectrums and grey areas - e.g. how elongated does a pedestal rock have to be before it qualifies as a zeugen (or yardang as the case might be!!)?

But I still want to know!

[Inyourface]

 I found my old copy of Small. It says about yardangs -
 " Probably the only landforms of deserts that can be confidently ascribed to wind abrasion alone are the comparatively unimportant "yardangs" and allied "ridge and furrow" features. These comprise elongated depressions, developed by deflation along lines of weakness in the rock and commonly orientated in the direction of the prevailing winds, which are seaparated by upstanding or isolated masses of more resistant rock, the yardangs proper. These are rarely more than 8-10m in height, and frequently less, and show distinct signs od basal abrasion on the upwind side only."

Zeugens aren't mentioned in my copy. Hope this helps. Sometimes I appreciate you just want to know these things.

[annelogie]

If anyone still has acopy of Bunnett there were some good diagrams in there to explain the difference. I think it was to do with the arrangement of the rock strata - zeugens horizontal, yardangs upright and parallel???

[Blue Square Thing]

On the off chance this isn't made up (honestly, never heard of them...) the talk page on wiki says:

From A Dictionary of Earth Sciences: "Zeugen: Mushroom-shaped rock that has been eroded by the abrasive action of windblown sand. The undercutting effect is concentrated near ground level, where sand movement is greatest, and is enhanced in areas of near-horizontal strata when the lowest bed is relatively weak."

http://en.wikipedia.org/wiki/Talk:Mushroom_rocks

[kevincooper777]

Thanks folks, knew someone would have a go...!

Anne - I used Bunnett when I did Geography O-level (the very last year of O-level I hasten to add!!!)!

I think I'm there with the understanding now:

Zeugen and yardangs are both elongated ridges of rock caused by preferential erosion of less resistant rock between the ridges - but zeugen involve more-or-less horizontal layers, yardangs involve near-vertical layers.

See http://www.scribd.com/facts-on-file-sl-earth-science-handbook/d/24694507 and search for the two terms to see diagrams

Rock pedestal / mushroom rock = obvious = like a mushroom shape! - less lateral extent than zeugen or yardang - and can be formed even in homogeneous rock due to concentration of sandblasting / abrasion at lower levels.

Unless someone is going to correct me...! (please do if needed)

[Graham]

'General Geography in Diagrams' (Bunnett)
Me too Kevin... except they were the stabdard 'O' Level text when I started teaching!!! I just love those mutliple choice questions as a form of assessment!

I can send you a copy of Bunnett for old times sake.. I kept a pile of them...I knew someone would need them some day!

[annelogie]

Rock pedestals

Rocky mass formed of alternate rock layers of hard and soft rock. The abrasion is greatest at ground level.

Zeugens

Wind abrasion turns a desert a desert area which has a surface layer of hard rock underlain by a layer of soft rock into a ridge and furrow landscape. The ridges are called zeugens which may be as high as 30 meters. Ultimately the are undercut and gradually worn away.

Yardangs

Bands of hard and soft rocks which lie parallel to the prevailing winds in a desert region are turned into another type of ridge and furrow landscape by wind and abrasion. The belts of hard rock stand up as rocky ribs up to 15 meters high and they are fantastic shapes Yardangs are very common in the central Asian desert and in the Atacama desert.

[kevincooper777]

t y everyone

as ever, honour to the SLN community!

Graham - that's a kind offer - especially if it's the old green-covered version!! (though the more modern glossy yellow cover would be OK too!!)

I did have one of each, but I stupidly discarded when I stupidly left teaching to work for CGP a while back...

Thanks Anne - that's a really clear and convincing explanation and the e.g.s are good too, not least because I should be able to locate some web images better that way

[Graham]

Kevin- no problem.. the old green version will be on its way after half term.