Glacial Erratic Deposition - At the height of the last Ice Age about 18,000 years ago, ice sheets extended as far south as Cornwall and the northern most tip of the Isles of Scilly. When the ice sheet retreated icebergs broke away (`calved`) from the retreating ice sheet and some beached onto the Cornish coast. On melting they left behind any rocks picked up and which became embedded in the ice as it moved south over the British Isles.
It is at Breageside Rocks, Porthleven that we can find a fantastic example. Here deposited on the `Wave-Cut Platform`to the west of the Harbour can be found a `GLACIAL ERRATIC` known as the `GIANT`S ROCK` or `GIANT`S QUOIT`.
The Geologists Association Guide says that it is garnetiferous gneiss and has yet to be matched with anything else in the UK. It has gneissic banding and crystals of garnet (up to 1cm across). Under no circumstances should anything be hammered or broken off it! It is a huge block, estimated to be in excess of 50 tons (20 tonnes) in weight, found nowhere else in the British Isles. It is so large that even the huge Atlantic storms have been unable to move it from its original point of deposition.
Wave-Cut Platform Formation - It usually forms after destructive waves hit against the cliff face. After a period of time this leads to undercutting of the cliff itself between the high and low water marks. The geomorphological processes involved are erosion and hydraulic power. As time progresses these create a wave-cut notch which eventually enlarges into a cave. The waves then undermine this portion of the cliff until the roof of the cave cannot hold due to the weight bearing down from above. Eventually it collapses, resulting in the cliff retreating landward. The base of the cave forms the wave-cut platform as attrition and further erosion causes the collapsed material to be broken down into smaller pieces. This material is further used in the erosion process whilst some cliff material may be washed into the sea to form an offshore terrace at the seaward side of the platform itself. At Porthleven it is estimated that the platform is eroding at a rate of 0.2mm per year.
PLEASE NOTE: I CANNOT ACCEPT JOINT ANSWERS FROM GROUPS OF GEOCACHERS NOR JOINT PHOTOS 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 SENDING ME YOUR OWN ANSWERS INDIVIDUALLY AND ADD A PHOTO (optional)TO YOUR OWN LOG. THANK YOU
To log your visit you must complete a task:
1. Visit the Giant`s Quoit at Low Tide ONLY and take at least one photo of the rock with you holding your GPS in the foreground and upload this to your log entry.
2. MESSAGE me via the LINK at the top of this page the answers to the following questions. Do not put the answer on the `logged it` page.
a. Measure the Giant`s Quoit dimensions - length, width and maximum height. Metric or Imperial is acceptable.
b. Estimate the distance across the wave-cut platform from the Giant`s Quoit to the cliff.
c.The rock sits in a pool cut in the wave cut platform. What main factor caused the formation of this pool?
Have fun exploring this site. Good Luck!
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.
This is No 1 in Snake Plissken`s Lizard Peninsula Earthcache Series!
SNAKE PLISSKEN -