Chalk is a type of limestone, which has been formed from the skeletons of coccoliths (marine micro-organisms), squeezed and eventually fossilised on the sea-bed. This process continued for 20 million years and to a thickness of more than 300 metres.
The chalk contains flint formed from another type of marine micro-organism called radiolaria. The exact details of the formation are not well understood, but it occurs during diagenesis (the process by which rocks form from sediments) where minerals are mobile enough to concentrate around some kind of nucleus. This nucleus could be a sponge, a burrow or something else.
In order to log this Earthcache please carry out the following tasks and send me the answers through my Geocaching profile.
1. Around you and all along the South Downs you will find plenty of examples of chalk and flint lying on the ground. Find an example of each and describe the main differences including texture and colour.
2. Look around you. Can you see any valleys or gulleys? Can you see any water flowing? What formed these valleys?
3. Chalk and flint form in similar ways but are primarily derived from two totally different elements in the periodic table - what are they?
Finally, an optional task. Please take a photo of yourself or GPS at the waymarker at GZ
I hope you enjoyed this Earthcache! Published with Permission of the National Trust