<p>Flint formation is complex, and the details still subject of research and debate.</p>
<p>The main factor is not a change in the relative proportion of siliceous and carbonate plankton, but the influence of chemical processes in the sediment after deposition. The flints precipitated where carbonate was dissolved. The dissolution occurred at the boundary between oxygenated water moving from the surface downwards and anoxic water rich in hydrogen sulphide from decomposing organic matter (the redox boundary). The reaction between oxygen and sulphide makes the water acidic at the boundary and this allows the carbonate to dissolve. Silica, which is in solution throughout the chalk, is in a state where it is precipitated from water with high carbonate concentrations, so flint forms at the redox boundary, typically between 5 and 10 metres below the sea bed.</p>
<p>The repetitive layering of bands seen in the cliffs may be influenced by cycles in the rate of sedimentation, with bands forming when sedimentation paused and the position of the redox boundary was stable.  A secondary factor may be changes in the chalk matrix that influenced flow rates of water, and hence the redox boundary location. These changes are the result of changes in sediment supply, and bioturbation by burrowing animals at the sea bed.</p>
<p>The process is described in a paper by <a href="https://books.google.co.uk/books?id=4IjCPNkBnqMC&amp;pg=PA43&amp;source=gbs_toc_r&amp;cad=3#v=onepage&amp;q&amp;f=false" rel="nofollow noreferrer">Clayton, 1986</a>.</p>

CC BY-SA 4.0
2022-02-18T06:46:31.960Z
Thanks for the answer! Your formation explanation makes much more sense. As for layering, what might cause sedimentation to pause?
John Leonard
CC BY-SA 4.0
2022-02-18T07:06:47.077Z
Changes in chalk matrix sounds plausible. My layman's guess is that there is a tipping point where the redox action is more likely to occur (maybe due to sediment density), flint begins to form and acts as a barrier preventing further upward movement of the hydrogen sulphide. The flint layer can thicken until the sediment below the barrier finishes decomposing, and the chalk can thicken above the barrier until there is enough decomposition to create the conditions for the tipping point again. Total conjecture of course!
John Leonard
CC BY-SA 4.0
2022-02-18T16:24:31.227Z
The cyclical variation in sedimentation is probably due to changes in the local climate; warmer drier conditions favoured chalk formation, and cooler wetter conditions  favoured marls.
Andy M
CC BY-SA 4.0
2022-02-18T19:49:36.300Z
It should be possible to back-calculate the (theoretical) total height of the cliffs if that carbonate had not dissolved....
Knob Scratcher
CC BY-SA 4.0
2022-02-17T16:54:01.413Z
2022-02-18T16:32:03.343Z
2022-02-18T16:32:03.343Z
Andy M
<p>The White Cliffs of Dover are mostly chalk but there are regularly spaced horizontal lines of flint.</p>
<p><a href="https://i.sstatic.net/3fthX.jpg" rel="noreferrer"><img src="https://i.sstatic.net/3fthX.jpg" alt="White Cliffs of Dover, chalk with lines of flint" /></a></p>
<p>I understand how the chalk and flint layers are formed independently (from <a href="https://en.wikipedia.org/wiki/White_Cliffs_of_Dover#Geology" rel="noreferrer">Wikipedia</a>) but I haven't been able to find any information on why they formed in these regular stripes.</p>
<p><a href="https://i.sstatic.net/JDmlw.jpg" rel="noreferrer"><img src="https://i.sstatic.net/JDmlw.jpg" alt="flint layer close up" /></a></p>
<p>Would coccoliths alone pile up for a long period of time, then sponges/siliceous plankton pile up for a much shorter time, then back to the coccoliths alone piling up and so on? ... seems very implausible.</p>
<p>Sorry if this is a very basic question but this has been bugging me ever since I visited over 10 years ago! :D</p>
<p>Thanks</p>

CC BY-SA 4.0
2022-02-17T03:17:09.803Z
I observed the same kind of flint bands in the chalk cliffs of [Møns Klint](https://en.wikipedia.org/wiki/M%C3%B8ns_Klint) in Denmark.
njuffa
CC BY-SA 4.0
2022-02-17T16:48:53.430Z
When I have found flint in limestone has been lumps rather than layers.
blacksmith37
<geology>
How did the Cliffs of Dover form flint in regular stripes?
John Leonard
# How did the Cliffs of Dover form flint in regular stripes?

(Asked by: username_0 on 2022-02-16)

The White Cliffs of Dover are mostly chalk but there are regularly spaced horizontal lines of flint.

[![White Cliffs of Dover, chalk with lines of flint](https://i.sstatic.net/3fthX.jpg)](https://i.sstatic.net/3fthX.jpg)

I understand how the chalk and flint layers are formed independently (from [Wikipedia](https://en.wikipedia.org/wiki/White_Cliffs_of_Dover#Geology)) but I haven't been able to find any information on why they formed in these regular stripes.

[![flint layer close up](https://i.sstatic.net/JDmlw.jpg)](https://i.sstatic.net/JDmlw.jpg)

Would coccoliths alone pile up for a long period of time, then sponges/siliceous plankton pile up for a much shorter time, then back to the coccoliths alone piling up and so on? ... seems very implausible.

Sorry if this is a very basic question but this has been bugging me ever since I visited over 10 years ago! :D

Thanks

**Tags:** geology

> **Comments:**
> **username_1**: I observed the same kind of flint bands in the chalk cliffs of [Møns Klint](https://en.wikipedia.org/wiki/M%C3%B8ns_Klint) in Denmark.  
> **username_2**: When I have found flint in limestone has been lumps rather than layers.  

---

## 1 answers

### Answer by username_3 on 2022-02-17. Score: 5 | Accepted answer

Flint formation is complex, and the details still subject of research and debate.

The main factor is not a change in the relative proportion of siliceous and carbonate plankton, but the influence of chemical processes in the sediment after deposition. The flints precipitated where carbonate was dissolved. The dissolution occurred at the boundary between oxygenated water moving from the surface downwards and anoxic water rich in hydrogen sulphide from decomposing organic matter (the redox boundary). The reaction between oxygen and sulphide makes the water acidic at the boundary and this allows the carbonate to dissolve. Silica, which is in solution throughout the chalk, is in a state where it is precipitated from water with high carbonate concentrations, so flint forms at the redox boundary, typically between 5 and 10 metres below the sea bed.

The repetitive layering of bands seen in the cliffs may be influenced by cycles in the rate of sedimentation, with bands forming when sedimentation paused and the position of the redox boundary was stable. A secondary factor may be changes in the chalk matrix that influenced flow rates of water, and hence the redox boundary location. These changes are the result of changes in sediment supply, and bioturbation by burrowing animals at the sea bed.

The process is described in a paper by [Clayton, 1986](https://books.google.co.uk/books?id=4IjCPNkBnqMC&pg=PA43&source=gbs_toc_r&cad=3#v=onepage&q&f=false).

> **Comments:**
> **username_0**: Thanks for the answer! Your formation explanation makes much more sense. As for layering, what might cause sedimentation to pause?  
> **username_0**: Changes in chalk matrix sounds plausible. My layman's guess is that there is a tipping point where the redox action is more likely to occur (maybe due to sediment density), flint begins to form and acts as a barrier preventing further upward movement of the hydrogen sulphide. The flint layer can thicken until the sediment below the barrier finishes decomposing, and the chalk can thicken above the barrier until there is enough decomposition to create the conditions for the tipping point again. Total conjecture of course!  
> **username_3**: The cyclical variation in sedimentation is probably due to changes in the local climate; warmer drier conditions favoured chalk formation, and cooler wetter conditions  favoured marls.  
> **username_4**: It should be possible to back-calculate the (theoretical) total height of the cliffs if that carbonate had not dissolved....  

---

