SEA STACK DEFINITION: A sea stack is an isolated pinnacle of rock entirely surrounded by the sea at high tide.
SEA STACK FORMATION: Sea cliffs around our coast receive a constant onslaught of erosional processes. The most important is Hydraulic Wave Action, which in simple terms is the process of waves hitting the rocks at incredible pressures and literally `exploding` in the joints and crevasses. Softer rock is exploited at a faster rate than harder rock and eventually the cliff will fall away unevenly to form headlands and bays.
Waves will continue to batter the headlands from both sides and in softer rock a sea cave will form with a single opening. The opening will continue to be exploited until the cave opens all the way through to form an arch.
Even with the formation of the arch the erosional processes do not stop. The waves continue to widen the gap until the rock above can no longer be supported. At this point the top portion collapses into the sea below. This leaves the furthest arch support standing out in the open sea as a SEA STACK.
If its summit has a larger diameter than its height, then it is an island.
Not all rock types are even in their hardness and so the coast at the Portland Bill is very `jagged` due to different areas being worn away at faster rates than others. It just so happens that we have a slightly harder structure in Pulpit Rock and hence a Sea Stack could form.
SO WHY IS PULPIT ROCK AN ARTIFICIAL SEA STACK & NOT A NATURALLY OCCURRING COASTAL FORM?
Pulpit Rock is all that remains of a large natural SEA ARCH called “WHITE HOLE” which was removed by quarrymen in 1870. If the quarrymen had not removed the arch rock it is likely that the sea stack would have eventually formed by the erosional processes described above. However, they simply accelerated the time frame by many thousands of years!
ISLE OF PORTLAND GEOLOGY: A SIMPLE EXPLANATION!
So now we have had a detailed geomorphological (earth surface processes) explanation of how sea stacks are formed in the coastal environment, no visit to Portland Bill would be complete without a very quick look at the geology.
The Isle of Portland is a limestone island connected to the mainland of the UK by a coastal beach form known as a Tombolo (Chesil Beach). It is approximately 6 kilometres (4 miles) long by 2.4 kilometres (1.5 miles) wide and the topography declines from a height of 151 metres (495 ft) near the Verne in the north to just above sea level at Portland Bill. The island is best known for its PORTLAND LIMESTONE, which was made famous by Sir Christopher Wren, the architect and MP for nearby Weymouth. He used six million tons of white Portland limestone to rebuild destroyed parts of London after the Great Fire of London of 1666. Well-known buildings in the capital, including St Paul's Cathedral and the eastern front of Buckingham Palace feature the stone. In the 20th century many thousands of tons were used throughout Western Europe as headstones for fallen Commonwealth soldiers of both World Wars.
In very simplistic terms there are four main types of rock found on the island. These dip down from north to south of the island with Kimmeridge Clay at the bottom overlain in the order of Portland Sand, Portland Stone and finally outcrops of Purbeck stone.
Portland stone is an Oolitic limestone from the Portlandian formation (Jurassic) and is an excellent building material. Hence, the numerous quarries that can be found scattered throughout the island.
For those of you that would like an extremely detailed Portland geology lesson then please visit the University of Southampton website by Dr Ian West: http://www.soton.ac.uk/~imw/Portland-Isle-Geological-Introduction.htm
PLEASE NOTE: I CANNOT ACCEPT JOINT ANSWERS FROM GROUPS OF GEOCACHERS AS I DON'T HAVE THE TIME TO SORT OUT WHO HAS SENT WHAT FOR WHOM!. IF YOU WANT TO LOG MY EARTHCACHE PLEASE EXTEND ME THE COURTESY OF MESSENGING ME YOUR OWN ANSWERS INDIVIDUALLY. THANK YOU
HOW TO LOG THIS EARTHCACHE?
To log your visit to the Earthcache you must complete a task:
MESSAGE me (via THE LINK at the top of this page) the answers to ALL of the following SIX questions. Do not put them on the `logged it` page. Metric or Imperial is acceptable.
1. Estimate the height, width & length of Pulpit Rock Sea Stack.
2. Estimate the distance between Pulpit Rock Sea Stack and the main cliff edge. Please do not get too close to the edge when doing this!
3. Estimate the height, width & length of the block of Portland Limestone leaning against Pulpit Rock on its northern side.
4. Using the measurements of the block estimate how much it weighs if Portland Limestone is around 2.4 tonnes/m3. To work this out simply use height x width x length x 2.4 = Approximate weight of the block.
5. Standing on the Limestone Platform immediately in front of Pulpit Rock WHAT FOSSILS DO YOU SEE everywhere in the rock and ESTIMATE THEIR SIZE?
6. In the bottom right hand corner of the Limestone Block there is a small brass plaque. What is the name of the person and the two dates listed here?
OPTIONAL ADDITIONS TO YOUR LOG
If you would like to take a photo of Pulpit Rock with you and your GPSr in the foreground and add it to your log that would be great as it is always nice to see geocacher`s at the location.
WARNING!:
Although there appears to be steps and hand holds cut into Pulpit Rock DO NOT be tempted to climb up on to it`s top as it is very dangerous. Keep away from all cliff edges and remain on the Public Footpath area at all times.
ALL DOGS MUST BE KEPT ON A LEAD and CHILDREN MUST BE SUPERVISED AT ALL TIMES. THIS IS A DANGEROUS LOCATION. You have been warned!
Extensive pay parking for the earthcache can be found next to Portland Bill Lighthouse, just a few hundred metres away. Currently (2011) 80p for up to 1hour and £1-60 for up to 2 hours
YOU MUST LOG AT THE SAME TIME AS SENDING ME YOUR ANSWERS VIA A GEOCACHING MESSAGE. If you can`t do both at the same time then wait until you can, otherwise I will just delete your LOG and you will have to do it all again as I DO NOT have the time to chase after you. AND be warned, I do check every one! Unfortunately, a few geocacher`s try to log the earthcache without completing the task and so I will maintain the right as the earthcache owner to delete any logs that do not fulfill the requirements or I entries that I believe to be false. I will ONLY contact you if there is a problem AND I will delete any logs for which I don`t receive answers or LOGS that I believe to be false.
Pulpit Rock is an excellent example of a Sea Stack found on the British coastline and if you go there on a stormy day and high tide you will experience the waves crashing over and exploding onto it. You will experience the power of the sea and just how it`s mighty force of water pressure actually erodes even the hardest of rocks.
ENJOY YOUR VISIT TO THIS STUNNING LOCATION!
SNAKE PLISSKEN -
Kind permission for the establishment of this earthcache has been given by Mr Philip George, Crown Estate Local Agent & Bailiff