Swanage fault line : geology

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 Dogwatch 02 Dec 2020

Anyone who has climbed at Swanage will have walked/scrambled/climbed along the fault line, the semi-continuous ledge that is around 10 feet above sea level at Subluminal, dropping as you move west to around 4 feet by Cattle Troughs. The rock below the fault line seems darker, rougher and more eroded but it is also more exposed to wave action to I'm not sure if it is really different.  I'm curious about the geology, I've tried looking it up but either cannot find an explanation or am failing to understand what I'm reading. Is anyone here able to explain it to a non-geologist or point me at a link?

 scott titt 02 Dec 2020
In reply to Dogwatch:

Ian West is THE man for Purbeck geology have a look at http://www.southampton.ac.uk/~imw/

You are right in thinking the rock below the "fault line " is different, it was deposited earlier, and in different sea conditions.

The "faultline" is a band of deposition (mostly sea shells) that is softer than the layers above and below.

The faultline doesn't drop as you move west in a general sense; it is much higher in the Ruckle, and even higher at St Aldhems Head. The differences are due to local faulting, some minor (well seen at the entrance to Benny) and some major (Buttery Corner).

 ianstevens 02 Dec 2020
In reply to scott titt:

You're right to put "fault line" in brackets - from what you describe it's not a fault line at all, rather a weaker sedimentary layer which has been eroded between two different units of limestone.  For something to be a fault, it needs displacement to occur (i.e. movement of the two units relative to one another).

OP Dogwatch 02 Dec 2020

Thanks, keep the comments coming. "Fault line" is the term I have seen used by climbers for this feature, accurate or not!

 hang_about 02 Dec 2020
In reply to Dogwatch:

Have you tried the classic landform guides?

 wbo2 02 Dec 2020
In reply to Dogwatch: I haven't climbed at Swanage for 30 years so only vaguely remember this, but as above I think it's a weaker unit on top of a hardground in the limestone?  An unconformity, break in deposition?  That would explain the number of seashells fossils and surface texture , plus beaks in limestone deposition normally allow early cementation to occur and for the surface to form a hard ground.

OP Dogwatch 02 Dec 2020
In reply to hang_about:

> Have you tried the classic landform guides?

I hadn't and it would be a thought. However the East Dorset guide seems to be out of print and 2nd hand are rather pricy. 

 mutt 02 Dec 2020
In reply to Dogwatch:

the BGS Report

"Winspit and St Alban's Head, Isle of Purbeck, Dorset. Fully exposed in sea cliffs, 37m thick, divided into Portland Chert and Portland Freestone members"

limestone is laid down as reefs during periods of submersion. At the start of the Jurassic period the supercontinient Pangaea was split into North and South Gondwanaland. needless to say there was a lot of geological change over those 50 million years. Each formation of purbeck and portland limestone is indicative of a period when the band was below the sea surface accumulating sea shells. The formation ends when the band is uplifted out of the sea by geological processes. I imaging therefore that the divide between Portland Chert and Portland limestone relates to a period of uplift and therefore the upper band was never really part of the lower. There should therefore be no expectation of continuity and particularly as the limestone is soluble water would and probably still does penetrate the divide and dissolve both formations from within.

 jkarran 02 Dec 2020
In reply to Dogwatch:

I've never been to Swanage but as far as I'm aware it's horizontally-ish bedded limestone, what you describe sounds more like a bedding plane than a fault (though it could be).

jk

 apwebber 02 Dec 2020
In reply to Dogwatch:

This is the relevant page on Ian West's site. You can see a labelled photo showing the name of the various beds and a couple of different geological columns showing the relationship between the different units. As far as I can see all the layers are conformable, meaning they were laid down one after another without a significant time gap, and they haven't been faulted either.

http://www.southampton.ac.uk/~imw/Anvil-Point-Blackers-Hole.htm

OP Dogwatch 02 Dec 2020
In reply to apwebber:

Good stuff, thanks. So http://www.southampton.ac.uk/~imw/jpg-Anvil-Point/7ANV-Lighthouse-climbers.... shows the "fault line" very clearly, describing it as "Puffin Ledge" or the "Prickle Bed".

https://ukfossils.co.uk/2008/03/26/dancing-ledge/ says:

From the sea, the lower part of the cliff is a natural cliff and sea ledge exposue of the Cherety Series of the Portland Chert Series (Dancing Ledge Member). The Prickle Bed, a conspicuous, thin and nodular rock lies within this and is often referred to as Puffin Ledge, as puffins nest here.

http://www.southampton.ac.uk/~imw/jpg-Anvil-Point/7ANV-Subluminal.jpg also shows the horizontal continuity of the Prickle Bed and that it has a visible vertical thickness. I love the old steamer too. 

By all means it isn't a fault in geological terms, however that's what climbers have called it for many years e.g. https://www.climbers-club.co.uk/information-summary-list/new-routes-archive... contains "fault" 53 times.

On the other hand http://www.southampton.ac.uk/~imw/jpg-Dancing-Ledge/10DAL-Boulder-Ruckle-We... showing the continuation from Boulder Ruckle towards Cattle Troughs shows vertical discontinuities in the Prickle Bed which I guess would be properly called "faults".

 Nic 02 Dec 2020
In reply to Dogwatch:

 

> On the other hand http://www.southampton.ac.uk/~imw/jpg-Dancing-Ledge/10DAL-Boulder-Ruckle-We... showing the continuation from Boulder Ruckle towards Cattle Troughs shows vertical discontinuities in the Prickle Bed which I guess would be properly called "faults".

Yes, the two bold lines with half arrows either side on the right of the photo are faults - the "throw" as labelled is the amount of displacement

Nic (B.Sc. Geology!)

 mutt 02 Dec 2020
In reply to Dogwatch:

nice! pickle bedding formed in shallow lagoons where the water was  hypersaline, and mixed with sand which explains why Boulder Ruckle is very often sandy. Which has often vexed me as there  there isn't a beach for several miles.

 apwebber 02 Dec 2020
In reply to Nic:

Yeah so this coastline has a really nice structural history, perfect for undergrad field trips. It was once part of the Wessex basin, meaning normal faults are quite common as they allowed the thickness of sediments to accumulate in the basin. Then later the basin was inverted, and those normal faults were reactivated into thrusts, uplifting the strata and forming traps where hydrocarbons can be found. There are oilfields at kimmeridge and near Poole.

Anyway, yes faults are to be expected in this area but this particular feature is not one of them.

OP Dogwatch 02 Dec 2020

So reading on, the large sea level platform at Dancing Ledge is also part of the Prickle Bed. Lots at  http://www.southampton.ac.uk/~imw/Dancing-Ledge.htm

http://www.southampton.ac.uk/~imw/jpg-Dancing-Ledge/10DAL-Giant-Ammonites-P... shows ammonites, which are plentiful in places. For some reason http://www.southampton.ac.uk/~imw/jpg-Dancing-Ledge/13DAL-Dancing-Bathing-P... from the 1950s appeals to me. Even though, no, I'm not quite that old.

Thanks to everyone who has added to the knowledge here. 

Post edited at 21:33

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