This is a simple EarthCache highlighting a remnant sea stack at the port of Seaham Harbour which is also a great place to view the local geology. It allows you to step back in time by 250 million years and picture what it might have been like all those years ago. No previous geological knowledge is needed, simply read this EarthCache, part of which is partly based on an information board you will find near the listed coordinates, although there is more detail in this listing. Then you need to observe the cliff when you arrive.
The Zechstein is a unit of sedimentary rock layers of Middle or Late Permian age located in the European Permian Basin which stretches from the east coast of England to northern Poland. The name Zechstein was formally used as a unit of time in the geologic timeframe, but nowadays it is only used for the corresponding sedimentary deposits in Europe.
Here you are standing in front of Red Acre Point. What you see here is a huge, remannt sea stack that was lucky to escape destruction when the port of Seaham Harbour was constructed for the Londonderry family in 1828, for the export of coal from their inland collieries. Instead it was utilised to help shelter the harbour.
A sea stack is a geological landform consisting of a steep and often vertical column or columns of rock in the sea near a coast, formed by wave erosion. They are formed when part of a headland is eroded by hydraulic action, which is the force of the sea or water crashing against the rock. The force of the water weakens cracks in the headland, causing them to later collapse, forming free-standing stacks and even a small island. Without the constant presence of water, stacks also form when a natural arch collapses under gravity, due to sub-aerial processes like wind erosion. Erosion causes the arch to collapse, leaving the pillar of hard rock standing away from the coast—the stack.
When the town's first lighthouse was built on top, it was known as Lighthouse Rock, and lime kilns were constructed on the east and west faces.
Two hundred and ninety five million years ago this site was part of an immense estuary that reached as far as Perm in Poland and as a consequence the rocks laid down in this period are known as Permian Rocks. Local cliffs and Red Acre stack contain 'beds' of Magnesian Limestone, created from lime-rich mud built up in layers on this ancient sea floor. After a period of time an event would occur that would interrupt the mud from being deposited (perhaps a drought for example) and the bed would be complete. Then a fresh bed would begin forming on top of the previous one creating the various layers of rock you can see in the cliff today. You can see these horizontal layers in front of you here. Some of the beds have been affected by chemical changes forming 'concretions'.
Concretions are commonly misunderstood geologic structures. Often mistaken for fossil eggs, turtle shells, or bones, they are actually not fossils at all but a common geologic phenomenon in almost all types of sedimentary rock, including sandstones, shales, siltstones, and limestones. A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Concretions are often ovoid or spherical in shape, although irregular shapes also occur.
Limestone of different origin, chemical makeup and age are found in other places on the planet, but the limestone here contains concretions with shapes and textures that are unique in Europe, and probably the world.
Questions to Answer (Logging Requirements)
Please try to send the answers at the same time as logging, or soon after your visit. If your answer is not received within a reasonable timeframe, your log may be deleted. Please answer questions by sending me a message or email via my profile. There is no need to climb onto the sea stack, please make your observations from the information board where you have a clear view of the stack.
1) Firstly, please describe the limestone in terms of colour, texture and hardness.
2) Have a look at the beds that have formed. Are they all the same thickness? How high is the biggest bed compared to the smallest? Where do you see the bigger beds and where do you see the thinner beds? How does the texture and hardness differ between the thicker and thinner beds? How do you think that helped the stack survive the erosion of the waves?
3) Look for the 'concretions' in the limestone. Describe their shape, size and relative hardness to the limestone beds. Where they are located in relation to the beds of limestone?
4) The limestone beds do not continue all the way to the top of the stack. How high up the stack do the beds stop? Describe the difference in geology where the bedding stops. Does the top of the stack look stronger or weaker than the limestone beds at the bottom?
Please feel free to include photos of yourself with nearby statues or the harbour, but please avoid including spoiler photos of the sea stack that would give away the answers. Please enjoy your visit to Seaham Harbour.