This Earthcache was originally about the Seven Sisters cliffs and about their formation. Most of the information was obtained from an information board, so it had a D1 rating. Unfortunately the information board has been replaced, meaning I have had to change the focus of the cache whilst still maintaining its existing D/T rating. For this reason, much of the information you need is contained in the text below, although you will also need to rely on your own observations at GZ too.
The posted coordinates will take you to the new information board, but close by you will see a metal staircase leading down to the sea. You do not have to go down these steps, but you do need to go on to the top level so you can turn and observe the cliff face. Whilst you are there remember to have a look at the Seven Sisters themselves, to the east – there is a great view of them from here.
The cliff faces here have stood for millions of years. They were formed during the Late Cretaceous Period, 60-90 million years ago. However, comparatively speaking they are now nearing the end of their life. Soon they will fall into the sea.
Rocks are formed in many different types of environment. These can be on, or beneath the Earth's crust. There are three types of rock, and each is formed in a different way.
Sedimentary rocks often start as sediments carried in rivers and deposited in lakes and oceans. When buried and impacted, these sediments lose water and cement together to form rock
Metamorphic rocks are formed inside the Earth by temperature and pressure changes that affect existing rocks.
Igneous rock is formed within the Earth’s crust, or on it’s surface. It is formed by the cooling of magma or lava (molten rock.)
The white cliffs are formed of chalk, made from the calcium-rich skeletons of trillions of plankton, called coccoliths, which died and settled here. These plankton with their calcium-rich skeletons literally rained down onto the seabed, forming a white ooze of sediment.
However, you can see that the cliffs also have thin dark bands in them. These dark lines are flint, which comes from quite a different source altogether. These bands are known as “flint horizons”.
The flint deposits here are believed to have come from silica-rich marine sponges, as well as from silica-rich plankton called diatoms and radiolarians. The silica formed into pockets, eventually dehydrating and hardening into quartz crystals that form the flint.
The question, though, is why the silica concentrated in the way it did, in flint “nodules”. The answer to that question is that nobody really knows. What is known is that the formation occurs during “diagenesis”. This is the name given to the process during which rock forms from sediment. During diagenesis the silica is mobile enough to concentrate.
Perhaps ithe silica was concentrated in burrows in the solidifying ooze, made by another animal. Nobody knows for sure. If the flint was formed by silica concentrating around burrows dug by other animals, then this would suggest that there was a pause in chalk deposition which allowed a “hardground” to form – a firm surface which formed between intermittent deposits of sediment. This then allowed an animal to burrow into that ground, and leave that burrow intact for a period. Then, in later deposits of sediment, the silica would have had time to penetrate the burrows and concentrate enough to form the flint.
A similar theory is that the hardgrounds provided purchase and anchorage for various siliceous marine sponges, corals, diatoms, etc, for the interval between the depositional events. These communities were then lost with the next depositional event, and their decomposing material is chemically reduced to its fundamental silicate minerals. These fluid chemical reductions “sink” through the porous chalk and gather in spaces (“bloats”) formed by the gases of decompositional microbial activity. From here, nodular bodies of fine chert are left resident in the chalk host, and this is how the nodules are created.
One alternative theory is that the Milankovitch Cycle is to blame. Milankovitch Theory deals with the oscillating changes in the orbital cycle of the Earth. It is measured in thousands of years, and affects climatic patterns. Every 20,000 years, for example, the rotational axis of the Earth changes. Would that, then, account for a change in the way sediment settles and allow flint horizons to form? One problem with this is that Milankovitch Cycles are driven by changes to the Earth’s ice volume, and during the Cretaceous Period there were no ice caps at either pole.
Hmmmmm.
So what could it be? Were there pauses in the precipitation of calcium-rich coccoliths to allow siliceous planktonic micro-organisms to concentrate during diagenesis? Or did Milankovitch oscillations in axial tilt determine climatic changes by orbital forcing, thereby causing layers in the upper-chalk formations of the Late Cretaceous Period?
The best thing, I find, is to try not to think about it and just enjoy the pretty patterns in the cliffs.
In order to claim this cache please send me an email or message with answers to the following:
- How many million years ago was the chalk and flint formed here?
- Of the three different rock types mentioned on this cache page, which type do the flint and chalk belong to? Explain why.
- Have a look at the cliff face nearest to you. Do the lines of flint run in a horizontal or vertical direction? Why is this?
- To prove you were here, look at the information board at the posted coordinates. Find the “You are here” arrow. Just to the left of this is a white number in a purple background. That number corresponds to an insert picture and description. In the description what is the first animal mentioned (two words, one of them a colour)?
Taking a photo of you/your GPS with the Seven Sisters in the background is optional, but would be appreciated, if only to provide photographic evidence over the next few hundred years of how the cliffs recede!!!
This Earthcache has been placed with the kind permission of Natural England.