I was walking around Guildford recently, looking for inspiration for earthcaches and I thought I would take a short cut through a park, which I had never been into. The park is on a very steep slope however the earthcache, if accessed from the top is not too far down. There is an amazing outcrop of a Lewis Nodular Chalk Formation (Middle Chalk). The Chalk is abundant with flints.
Flint is a cryptocrystalline sedimentary rock (a rock made of crystal without being able to see the crystal form. Flint is formed in a complex process which began in the chalk seas millions of years ago. Organisms like sponges and radiolaria use silica from seawater to make biogenic opal which forms their skeletons. When the organisms die the microscopic silica is dispersed around the seabed and mixes in with the other sediment.
The microscopic silica then gets buried under a lot of sediment it undergoes a process called diagenesis, it is a way that sedimentary rocks can change from one form to another. The silica liquifies into a gooey gel in a complex chemical reacting. The gel can then fill into spaces in the sedimentary rocks which might have been left by burrows or the insides of large shells like sea urchins which explains why flints are often quite knobbly. It then solidifies again into the cryptocrystalline rock. The outside of the flint is often quite cherty, it is where it has been touching the outside of the sediment and hasn't quite had the chance to form into the glassy consistency. The flints in this outcrop are especially numerous.
Questions
- What size are the flints?
- What shape are the flints?
- What is the colour of the inside and outside of the flints
- Look around the edge of the whole park about how many Chalk outcrops are there (just the large ones)
I may not reply to your answers if they are correct because I am quite busy at the moment, however I will reply to you if they are wrong. Assume your answers are correct if after 2 days I have not replied