Anglesey contains some of the oldest rocks on the planet, and is much studied by geologists. The rocks at GZ are Precambrian, and are much older than the mountains of Snowdonia, which are visible on the horizon from the location. The Precambrian period of the geologic time scale spans the formation of the Earth about 4.6 billion years ago to the beginning of the Cambrian period around 540 million years ago. The Cambrian period is named after Cambria, being the classical name for Wales where rocks from this age were first studied.
Rhoscolyn is one of the island’s most interesting locations from a geological standpoint, as many complex, folded rocks are on display. As a result it has been designated a site of special scientific interest. A geological fold occurs when a stack of layers of rock which were originally flat, such as sedimentary strata, are bent or curved and permanently deformed as a result of stress or pressure. Sedimentary rocks are typically formed by minerals being carried by water settling and accumulating, or minerals precipitating from a water body in which they were dissolved, and building up in layers. Over maybe millions of years the weight of the sediments on top squeezes the water out of the sediments below, causing crystals to form which cement the sediments together to form rock. The sedimentary rock strata in Rhoscolyn have often been folded several times since their original formation, with folds here occurring on many different scales, from small to huge. It may seem odd that rock can be folded - in the forms that we normally experience rock in it is brittle and tends to break or shatter if put under enough stress. However, when subjected to the levels of heat and pressure involved in the processes that were at work at a time when these rocks were far below the surface, the rock becomes more malleable and flexible, allowing the formation of folds as it is compressed.
The local feature which is probably the most famous among geologists is the Rhoscolyn Anticline, a huge arch-shaped fold in the rocks. The term anticline refers to a fold that is convex up (ie the rock strata dip away from the crest of the fold) and has the oldest rock beds at its core. The core of the Rhoscolyn Anticline runs beneath the Coastguard Lookout around 500m east of GZ, and from the sea it can be seen exposed in the faces of the nearby cliffs. Studies of the deformed sedimentary rock sequence that can be seen here have enabled a greater understanding of the geology across the whole of Holy Island.
Here are some of the folded rocks that can be found at GZ
In order to claim this cache as a find, firstly make your way to GZ. Be very careful with children and pets when walking close to the cliff edges! Once there you will find many superb examples of folded rocks. Take a look around and find the specific rocks shown in the pictures, and draw a field sketch of the folds (further guidance below). Once done please message me via the message centre with a photo of your field sketch (you can use the "Message this user" link at the top of this cache page - please do not email, as you will not be able to attach your field sketch). Please do not include any field sketches in your log.
There is no need to wait for a reply before you log the cache, but I will contact you if there are any issues with your answers. Note that the field sketch is necessary in order to log a find, this is reflected in the D rating.
Field sketch guidance - this presentation should be very helpful: http://pcwww.liv.ac.uk/geo-oer/index_htm_files/Field%20sketches%20&%20how%20to%20draw%20them.pdf. Note the requirements on p13. Your field sketch should include at a minimum a title, rough estimated measurements and a very brief description in your own words of how the folds were probably formed.
Many thanks to Cyfoeth Naturiol Cymru / Natural Resources Wales for permission to publish this cache.