# Geologists...Help! **URL:** \ **Category:** Factual Questions\ **Created:** [February 1, 2019, 1:45am UTC](https://boards.straightdope.com/t/geologists-help/828759 "2019-02-01T01:45:32Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![ZurBob](https://avatars.discourse-cdn.com/v4/letter/z/71e660/32.png) [@ZurBob](https://boards.straightdope.com/u/ZurBob)\ **Post date:** [February 1, 2019, 1:45am UTC](https://boards.straightdope.com/t/geologists-help/828759/1 "2019-02-01T01:45:32Z")
What determines the layers as seen in this photo of the dover cliffs in England: [https://www.bing.com/images/search?view=detailv2&iss=VSI&form=ENAIMG&q=imgurl:https%3A%2F%2Fwww.bing.com%2Fth%3Fid%3DABTAD5FCAED6BB252274903455B414994DFB39A6A43186EE8DFC14C0A20A7F6A8D4%26qlt%3D90%26pid%3DInlineBlock&ptitle=©%20Nomad%20/%20eStock%20Photo&ajaxhist=0&selectedindex=1&id=2B518D6AB28750A9615C7EC4C39B1CE98C4C6519&mediaurl=http%3A%2F%2Fcdn.desktopwallpapers4.me%2Fwallpapers%2Fnature%2F1920x1200%2F4%2F40221-high-rocky-shore-at-the-ocean-1920x1200-nature-wallpaper.jpg&exph=1200&expw=1920&vt=0&ccid=SC%2BB3S%2B%2B&simid=608029559351019591&thid=OIP.SC-B3S–0-wg\_cyVDGpbLwHaEo&pivotparams=imgurl%3Dhttps%253A%252F%252Fwww.bing.com%252Fth%253Fid%253DABTAD5FCAED6BB252274903455B414994DFB39A6A43186EE8DFC14C0A20A7F6A8D4%2526amp%253Bqlt%253D90%2526amp%253Bpid%253DInlineBlock&eim=1,2,6](https://www.bing.com/images/search?view=detailv2&iss=VSI&form=ENAIMG&q=imgurl:https%3A%2F%2Fwww.bing.com%2Fth%3Fid%3DABTAD5FCAED6BB252274903455B414994DFB39A6A43186EE8DFC14C0A20A7F6A8D4%26qlt%3D90%26pid%3DInlineBlock&ptitle=%C2%A9%20Nomad%20/%20eStock%20Photo&ajaxhist=0&selectedindex=1&id=2B518D6AB28750A9615C7EC4C39B1CE98C4C6519&mediaurl=http%3A%2F%2Fcdn.desktopwallpapers4.me%2Fwallpapers%2Fnature%2F1920x1200%2F4%2F40221-high-rocky-shore-at-the-ocean-1920x1200-nature-wallpaper.jpg&exph=1200&expw=1920&vt=0&ccid=SC%2BB3S%2B%2B&simid=608029559351019591&thid=OIP.SC-B3S--0-wg_cyVDGpbLwHaEo&pivotparams=imgurl%3Dhttps%253A%252F%252Fwww.bing.com%252Fth%253Fid%253DABTAD5FCAED6BB252274903455B414994DFB39A6A43186EE8DFC14C0A20A7F6A8D4%2526amp%253Bqlt%253D90%2526amp%253Bpid%253DInlineBlock&eim=1,2,6)…Specifically, the thickness of the layers, and the color variations. The aforementioned link is merely one example, there are innumerous variations worldwide. ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [February 1, 2019, 3:00am UTC](https://boards.straightdope.com/t/geologists-help/828759/2 "2019-02-01T03:00:58Z")
Firstly - those are the Cliffs of Moher in Ireland, not the (famously, White) Cliffs of Dover in England. The biggest colour variations are caused by differing rock types - lighter sandstones, darker shales. As to why the thicknesses vary, there are lots of reasons - different depositional environments, different depositional climates, different erosion rates - [no one reason.](https://en.wikipedia.org/wiki/Stratum) Moher’s generally considered a [delta](http://2.bp.blogspot.com/-rTEu2EyEScY/VkHlmHvTxFI/AAAAAAAACWc/pjlun81jNL0/s1600/6.18.jpg)-[type](https://www.geol.umd.edu/~jmerck/geol342/lectures/10b.html)[sequence](https://en.wikipedia.org/wiki/River_delta#Sedimentary_structure), so there’s a lot going on there, from river sandstones to [turbidites](https://en.wikipedia.org/wiki/Turbidite) at the delta edge.The middle part of the cliffs are Tullig Formation cyclothem - read more about that [here](http://www.sepmstrata.org/page.aspx?pageid=726). [This cite](https://core.ac.uk/download/pdf/4833902.pdf)goes into a lot of detail about that part of the stratigraphy of the cliffs, from page 187 onwards.