A straightforward EarthCache on the beautiful island of Sark. One of the problems faced by many visitors to Sark is a shortage of time on day trips. The aim of this EarthCache is to show you some of the impressive features the geology of the island has created in conjunction with the erosional force of the sea, but hopefully, it shouldn't take you too long to complete.
The information on this listing has been written especially for this EarthCache, it is not copy and paste from websites so please have a little read before sending me your answers. No previous geological knowledge is required, all answers are observational. Visibility of 100m is required. Binoculars may help you view the features more closely, but are certainly not essential.
Background Information
Sark (including the nearby island of Brecqhou) has an area of 2.10 square miles (5.44 square km). The small island is made up mainly of amphibolite and granite gneiss rocks, intruded by igneous magma sheets called quartz diorite. Recent (1990–2000) geological studies and rock age-dating by geologists from Oxford Brookes University shows that the gneisses probably formed around 620–600 million years ago.
The important fact you need to know about the rocks of Sark is that they are very hard/solid rock types, and so it shouldn't be a surprise that they are withstanding the power of the sea pretty well. This explains why this small island continues to exist so far out in the sea without the sea destroying the land too quickly.
The view south from the coordinates. You need to look north to answer the questions.
Wave Erosion: Caves, Arches, Stacks and Stumps
Wave erosion here is slowly eroding the land into the sea, but whilst it is busy doing that, it can create quite beautiful geological artwork. Once at the given coordinates for this EarthCach, you find yourself standing on a relatively narrow bit of headland, which has been formed by the land eroding away through a combination of water flowing off the island and the force of the waves pounding the land from below, predominantly the latter. Over time, the headland will shrink and eventually any of the below could become the possible future here.
The lower coast of Sark is full of sea caves. The Gouliot Caves, the Boutique Caves, The Red Cave, The Dog Cave, The Gull’s Chapel Caves, Sark Caverns and the Derrible Head Caves are all listed as recommended as places to visit whilst on the island, some are quite spectacular. But of course there are many more that aren't accessible by tourists that provide important habitats for marine life.
These caves are created by the power of the sea. Wave erosion is a specific type of erosion. It is caused by the power of tides and waves projecting sea water and the sediment within the water at the cliffs and rocks found on the shore. Although the sea contains chemicals and salts, it is not believed that chemical erosion plays any real part in creating the type of features you see here today. There are two main processes responsible, and these are:
- Hydraulic Action: where the motion of water against a rock surface produces mechanical weathering. Most generally, it is the ability of moving water (flowing or in this case waves) to dislodge and transport rock particles.
- Abrasion: the occurance of mechanical scraping of a rock surface by friction between rocks and moving particles during their transport by wind, glacier, gravity, running water or in this case, waves. Rocks and sediment are found within the sea and the power of the waves brings them into contact with the rock on the shore.
As already explained, caves are a common feature of this island, so it stands to reason that arches, stacks and stumps may also appear - but this is certainly not guaranteed. These features may seem to be different visually, but they are all simply different points of a particular erosional process.
As waves pound the cliffs here, the following processes may happen.
- Cracks: once a crack occurs, it will become widened in the headland through the erosional processes of hydraulic action and abrasion as explained above.
- Caves: over a period of time, the waves continue to grind away at the crack, and it slowly opens up getting bigger and it may then form a cave. A cave is simply a hole in the rock and is usually defined as such when it becomes big enough for a human to enter.
- Arch: again, over a period of time the cave becomes larger and eventually breaks through the headland to form an arch. However, the arch could collapse immediately or very quickly depending on the type of rock
- Stack: as the sea continues to erode the rock the base of the arch continually becomes wider, until its roof becomes too heavy and collapses into the sea. This leaves a stack (an isolated column of rock).
- Stump: the final part of the process sees the the stack becoming undercut at the base until it collapses to form a stump.
Obviously this can't happen just anywhere, if a cave appears below a large cliff the only thing that will happen once it gets big enough is that the cliff above it will fall down. But if a cave were to appear on a narrow headland, such as the one you are currently standing on - for example - directly below where you are standing, it could eventually become an arch - there could already be one that you can't see - if you have time to go to waypoint 2 have a look and see what is beneath you!
Logging Requirements (Questions to Answer)
The vast majority of people who visit Sark only come for a day trip and only have a few hours on the island and so for most people, there isn't time to climb down to the coastline and explore all the caves and see the fascinating geology up close. This EarthCache aims to show you a great place to view some of Sarks spectacular geology from a great viewpoint.
As with all EarthCaches, we ask you to answer a few questions in order to earn your smiley. The answers to these questions should be sent to me, the EarthCache owner, at the time of logging or very shortly afterwards. If you don't do this, please don't log a find.
There are five questions with sub questions but I only require a few words for each answer so don't be put off. It is much quicker and easier to answer a greater number of clearly defined questions than to have a couple of vague questions where you end up writing an essay for each because you aren't sure what the co is looking for in the answer.
You need to reach the given coordinates which are up on the rocks on the top of the headland. There is a path to the location, the terrain isn't particularly difficult but be careful on a windy day - you need to continue further on from the bench and go up onto the top of the rocks, but you don't need to go near the edge. You cannot see all the features from the bench. You need good enough visibilty to see about 100 metres up the coast, so if you visit in the sea fog you may have to give it a miss. You don't need to look beyond Saignie Bay to answer any questions.
Remember as you look out here at all these features - these were once headlands, peninsulas and points much like the one you are standing on now. It is fascinating to stop and think about how the landscape is constantly changing.
1) Look north from the given coordinates and observe the coastline to the north of Port du Moulin beach. Look for the obvious cave.
a) Tell me where you see it, b) approximately how big is it, c) is it constantly being eroded by the sea or only certain times d) does it have the potential to ever turn into an arch - if not, what will happen when it gets too big to support the weight above it?
2) Still looking north from the given coordinates, identify the sea arch.
a) Tell me where you see it, b) approximately how big is it, c) is it constantly being eroded by the sea or only certain times, d) given the rock is quite hard, do you think it is likely to collapse in the next hundred years?
3) Still looking north from the given coordinates, have a look for sea stacks.
a) how many sea stacks can you see, b) how tall and wide are they, c) do any look like they are in threat of collapse within the next hundred years?
4) Again, looking north from the given coordinates, take a look for stumps. (Look for stumps that are 10 metres diameter or more, we aren't concerned with any small boulders)
a) how many stumps can you see, b) how tall and wide are they, c) do you think they will all last the next 100 years?
5) Look at the cliffs just to the north of Port du Moulin beach. You will see an obvious place where there has been a recent collapse just before the publication of this EarthCache.
a) what colour do you see? b) How does it look different to the rocks that are beneath your feet up here and how might that have contributed to the collapse?
6) Please include a photo of yourself / GPSr / personal item looking south or out to sea from the given coordinates. However, please don't include views looking north that would give away the answers.
I strongly recommend going to the second waypoint and looking at the cave and sea arch more closely if you have the time, it is a beautiful place, but this bay is tidal so I have not made it a logging requirement.
Answers can be sent via the email link on my profile, or via the message center. Acknowledgements are more likely if you email as I can reply directly from my inbox. I read all answers as all messages sent to me via messenger are forwarded to me by email, however, I cannot directly reply to them without logging into messenger so I may only contact you if there is a problem if you message me that way. Please enjoy your visit to Sark's Port du Moulin.