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Swallow hole (sink, sinkhole, ponor) Reseto and Vodonos on Cerknica Polje (Cerknisko polje

[GEOMORPHOLOGY] This is a swallow hole. A steep-sided, enclosed depression in a karstic region, normally found at a location of increased joint density.  It enlarges by solution.  A shaft may be present, leading to a cave system below.
Image credit: JDL. Location Cerknica Polje, Slovenia.



A rock specimen has been obtained 300 km from the oceanic ridge and was dated at 9 Ma.

i. Calculate the rate of sea floor spreading in centimetres per year.

ii. Identify the location of the ridge axis on the diagram.

iii. Explain why the boundaries between normal and reversed polarity are irregular.


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Rock of the Month (October)
The rocks that make Oxfordshire

Horsehay Sand Formation


The Horsehay Sand, is described as a pale grey and brown to off-white, medium- to fine-grained, quartzose, unbedded to weakly bedded and cross-bedded, locally cemented into calcareous or weakly ferruginous sandstone; with thin dark grey mudstone and siltstone beds in places. Rootlets and lignitic  (fossil wood) debris common. Shells and shell debris very rare.


Image:  Duns Tew Pit, 1960. Crown Copyright.







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[MINERALOGY] Stibnite is the predominant ore mineral of antimony. Antimony compounds have been known since ancient times and were powdered for use as medicine and cosmetics, often known by the Arabic name kohl.    
Image: © US Department of Interior/USGS. Credit: Scott Horvath.

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October 2021

       The precipitous destination for Alejandro this month is Guillemot Ledge, Swanage.


OGG's Membership Secretary is a keen rock climber. and the climb he is sharing with us this October is a dramatic  looking face exposed to the English Channel. In this picture you can see that he's scaling the Portland Stone Formation that forms the sea cliffs west of Swanage, at a spot called called Guillemot Ledge.  The formation is (late/upper Jurassic Period, Tithonian Age (approximately 145.0 - 152.1) in the Titanites anguiformis ammonite zone.  


The Portland Stone Formation was deposited in a marine environment, on the floor of a shallow, warm, sub-tropical sea.  It would have been a proximal location as fossilised wood and other terrigenous and  ferruginous material can be found in the rock.


When shallow water is warmed by the sun, its capacity to carry dissolved gas is reduced; as a result one such gas, carbon dioxide, is released into the atmosphere. Once the carbon dioxide is removed from the water, calcium carbonate precipitates out, tiny particles of which rain down on the seafloor as a blizzard of calcite. The accumulation of millions of tiny carbonate crystals, together with other substrate material, forms a lime mud - a micrite.


The lower part of the formation is a calcilutite, described as a pale brown or grey bioturbated, massive limestone that is composed of more than 50% clay-size, or both silt-size and clay-size, detrital carbonate grains with peloids, sponge spicules, shelly detritus and nodules of chert. The upper part of the formation is a thick bedded, ooidal, shelly calcilutite.  Bioclastic material is abundant towards the top of the formation, with small patch reefs formed by algal mats of Solenopora. You can see the image in the centre panel, above, is showing the detail of the micritic limestone.

"Guillemot Ledge is a large and diverse area that is home to the highest section of cliff along the whole of the Swanage coastline. The big multi-pitch trad routes on the West Face are amongst the most challenging in the south, and almost all of them give tremendous climbing in wild and exposed situations. Across the two areas of the West and East Faces, there is a reasonable grade spread from HS to E6, but Guillemot Ledge should be recognised and treated as a serious cliff where experience of committing and difficult climbing is essential. The rock is usually sound, and the presence of a quarried top on the East Face will calm fears of potential loose finishes when topping-out. A number of the climbs in this area rely on fixed protection from pegs - these should be carefully inspected and backed-up " © UKC



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