I visited Orwell and the surrounding villages a few years ago while completing a large trail of caches around the area - while it was great to add a lot of finds to my tally, the highlight for me was visiting and learning about this geological site through a previous EarthCache. Having noticed this being sadly now archived I thought it worthwhile creating a replacement for any future cachers to the area to find!
ROCK TYPES:
There are three types, Igneous, Metamorphic and Sedimentary.
Sedimentary rocks are types of rock that are formed by the deposition and subsequent cementation of that material at the surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle in place. The particles that form a sedimentary rock by accumulating are called sediment. Examples include limestone.
Igneous rock is formed through the cooling and solidification of magma or lava. An example of this is granite.
Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form".The original rock is subjected to heat and pressure, causing profound physical and/or chemical change. An example of this is marble.
GEOLOGY OF THE PIT:
Orwell Clunch Pit is designated as a Site of Special Scientific Interest, due to its rich chalk grassland flora. The well-drained chalk hillside contains both springs at its foot as well as fertile heavy clay soil in its valley. This site is an ancient pit formed by extensive quarrying of the Upper Cretaceous West Melbury Marly Chalk. It now forms a south facing hollow beneath Toot Hill. Much of the ground is chalk grassland with some exposures of Chalk debris and bedrock.
Of particular note at this site are the chalk streams - rare and of ecological importance - England has 85% of the world's total, equating to just over 200 - and eight of these are in this valley. This is an example of 'periglacial activity', seen in particular when examining the interaction of the chalk with the diamicton (sedimentary rock) at the top of the cliff.
Sites showing clear evidence of this geological feature are fairly uncommon, but here it is very evident in the involutions (a result of ground freezing) of the chalk and diamacton in the top metre of the cliff. There is no other place in the region where this can be easily seen; the only other recognised occurrence was at the Barrington Chalk Pit which unfortunately was removed by quarrying.
Now, the name of, and lesson of this EarthCache is on the Clunch Pit, but what actually is clunch? It's simply a local term for hardened sections of chalk, often featuring fossil fragments, resembling a sandstone texture.
CHALK:
Chalk is a soft, white, porous sedimentary rock, a form of limestone, comprising over 95% calcium carbonate. It forms under relatively deep marine conditions from the remains of tiny marine organisms that lived and died in the seas covering Britain around 100 million years ago. More precisely, it is composed of the remains of coccoliths, a form of algae.
The algae lived floating in the upper levels of the oceans and when they died, their bodies fell to the sea bed and compressed together. Combined with remains of all the other creatures forms the rock we see today.
QUESTIONS:
I suggest entering the Clunch Pit site via the public footpath out of the waypointed car park, you will see an information board on your way around the path which may be useful for some of the questions!
1) Clunch is a form of chalk, which is in which rock type category? And in what geological period was clunch formed?
2) Following the footpath around the top of the chalk streams (waypointed), how would you describe the rock in terms of colour, texture, and if you can, composition?
3) Now, when standing at GZ (the foot of the chalk), compare this section of the rock with the previous question, are there any differences in colour or rock texture? If you are able to, explain why these differences exist.
Optional: add a photo of yourself at the pit, or just any photos in general of the site to your log, although not giving any answers away!