I need a volunteer to do the write up today please. A familiar request from Sheila and one I can usually easily ignore because someone else always volunteers. Anyway, as I had brought Max, my bouncy cocker spaniel, - I needed to keep an eye on him and I hadn't brought a notepad or pen either- so I decided to keep quiet. However, Sheila had her eye on me and lent me her notebook and her pen! So here goes Joy and Max's account of the field trip to Dancing Ledge, Seacombe, Winspit and St Adlhelm's Head.
As we were starting early, 10.00 am instead of 10.30 am, our leader Jon Chaffey gave us the option of finishing early or extending the trip to St Adlhelm's Head. Unanimously, we agreed to extend the trip.
Max is not a very sociable dog when it comes to other dogs. So our first challenge of the day was to get past the unattended German Shepherd dog roaming the field outside the first farm we pasted. My thanks go to Chris and Mick who agreed to help me out in case of trouble, particularly Mick who had a walking stick. Fortunately, Max chose to ignore the dog and trot happily along side us and the German Shepherd also chose to ignore us. So we all arrived at Dancing Ledge in one piece.
Dancing ledge is on the southern tip of the Purbecks and is a north dipping monocline. The dip is towards the Isle of Wight and is about 2 to 30, Looking westwards towards St Adlhelm's Head, John pointed out the medieval cultivation strip lynchetts. There the Portland beds are exposed on the top of the head causing it to be stable with little slippage. However, as we looked around us on Dancing Ledge, we could see the hummocky terrain caused by land slippage. This was due to the unstable Purbeck beds. The ledge's profile was of a slope and wall. The wall was represented by the cliff face of the Portland series. At this point it is the Portland Limestone. Below the limestone is the Portland Sand although this is below sea level at Dancing ledge with the exception of a small area that has been faulted up. However, it is an important feature beyond St Adlhelm's Head.
At Dancing Ledge, Seacombe and Winspit we were to see varying stages of the Portland Succession. The Portland Succession was deposited in shallow marine conditions not unlike those found offshore of the Bahamas today. John pointed out that we were standing on the on the top layer of the bottom series and looking at the bottom layer of the top series. I knew what he meant but Max looked a bit perplexed, even though he has spent many hours with me while I studied and has examined numerous geological maps.
Anyway, at the bottom of the top series is the Freestone Limestone. This has a nice creamy colour, breaks into sizeable chunks that be cut into many different dimensions. This makes it an ideal building stone. There is about 8ft of it and all along the coast to Winspit it has been worked in galleries. It is durable, although with the pollution we have today it is less so.
The succession on top of this is~
o 3 feet of Under Picking Cap. This was removed by blasting to get at the freestone
o 8½ feet of House Cap
Listy bed
o Cherty Bed. This contains silica from sponges
o Pond Freestone. This was never worked at Dancing Ledge. It was worked at Winspit
where it is thicker and easier to get at. Also at Worth Matravers, about 1 '/2
miles inland, it is the first choice for quarrying.
o Titanites beds. So named because it contains fossils of the ammonite Titanites
o Shrimp bed. Now covered by netting to assist the many climbers who frequent these cliffs.
The quarry floor at Dancing Ledge is the top of the second succession called the cherty series. Here we could see the chert in fairly large chunks. This, John explained, is the difference between Flint and Chert. Chert equals large nodules of silica and Flint is small nodules. There is about 20 feet of the cherty limestone extending down to the platform the forms the ledge. The ledge is made up of the Prickle Bed. It is also called Puffin Ledge. Max wished he could see some pufflns because he was feeling a bit peckish at this stage. He had to be content with looking at the inlet that had been formed in the ledge. Last time we visited this site I had been told that it was a swimming pool carved out by a Victorian headmaster for boys at his school. John said there were many legends about it but he didn't know if it was a natural formation or not. He pointed out the cart tracks that had brought the stone to the ships that once moored here. The stone was levered down by cranes into the carts. Max said he liked the story of the swimming pool better.
The prickle beds were described as quite soft and cherty although we couldn't see any chert. We did see large ammonite fossils though. It was conjectured that some silica might have come up from the Portland sand into this bed. John described a famous 1786 shipwreck that happened here. The ship was going to the East Indies with slaves and the cannons of the ship can still be seen.
As we moved along the cliff and passed. Eastington John pointed out where the springs form in winter. It was dry for us but the rushes marked the spot of the springs. These springs are responsible for a lot of slippage. Because of the seaward slope of the ledge there is a sludge with large chunks of rock in it. We were told the name of the sludge but I can't read my writing and Max has forgotten it.
On arriving at Seacombe we could see the first of the big valleys on the Purbecks. It illustrates well what can been seen in all the others. There is a broad outer valley with an inner one. John explained that there were two possible reasons for this.
1. Falling sea level - but he thought this unlikely
2. Coastal erosion which has been fast enough to cut a new inner valley.
In wet winters the water table rises and brings a stream down the valley that cascades as a waterfall onto Puffin Ledge. We were told that it was unusual not to see water in the stream at that time of year. The geology at Seacombe is the same as at Dancing Ledge.
At Winspit the inner and outer valleys are very sharp and caused by marine erosion. The valley is made up of Limestone which is more accessible for quarrying so they did not have to bore into the cliff John explained that this is the only place where the Pond Freestone was worked as an experiment. Quarrying stopped at Winspit in 1945 and at Seacombe in 1930. We saw
large cracks in the rocks and it was explained that this is caused by marine erosion which is breaking up the rocks rather than it being jointing.
Max and I have been to Winspit on many occasions so did not investigate the caves again. For those new to the site the ammonites in the under picking cap were pointed out.
Max was thankful for a rest, some water and some tomato soup. He will be eternally grateful to Debbie's son who fell for the 'I'm a starving dog who hasn't been fed for a week' look. I am pleased to say that the carrotine that caused me to take a black, white and red dog home, soon faded.
We then walked over the cliffs to St Adlhelm's Head. By this time it was a glorious hot late September afternoon. This did not prevent Max from being determined to keep up with John and try to take over as leader of the pack. The walk was well worth the most beautiful view when we reached the head. The sea was like a millpond and the way the light shone on the clay gave it a stunning blue/purple aura. I could have stayed there until the sun went down.
We were all tired by this stage, so all my notes say are:
The stopover at the pub for beer and chocolate was enough to revive us for
the walk back to the car.
Joy & Max Vooght