Still reading a course on datation over multiple stratums that may have setting up the Earth's crust, I'm meeting for the first time the principle of original horizontality Steno.

Is this theory safe?

Logically, if multiple stratums squeeze, the last one to squeeze will be the youngest. It makes sense to me. But isn't Earth's crust affected by inner perturbations that may disrupt the stratums pile? Is it safe to date stratums this way then?

  • 1
    $\begingroup$ I think you may be confusing the principle of original horizontality (the first part of your question) with the law of superposition (your last paragraph). You might want to ask two questions, or edit this one? Either way, see my answer. $\endgroup$
    – Matt Hall
    Nov 19, 2014 at 2:03
  • $\begingroup$ earthscience.stackexchange.com/q/118/725 might be related. $\endgroup$
    – Gimelist
    Nov 19, 2014 at 7:26

1 Answer 1


No, Steno's principle of original horizontality (I'll call it the POOH) is not 'safe'. It is not a useful theory.

Don't confuse it with the law of superposition (also due to Steno), which is useful. That law is, in a general sense, a foundation stone (ha!) of stratigraphy... with some caveats due to resedimentation and other processes.

Why is the POOH dodgy? On small and large scales, the concept of 'layer-cake' stratigraphy does not hold up. Indeed, it's probably rather unhelpful. Brian Romans wrote a nice essay on this subject in 52 Things... Geology (edited by me, 2014) — see the link below. In it, he refers to Bailey (1998), who described sedimentary bodies as 'lenticles' with complex 3D geometries. And you just have to look at some seismic reflection data from the continental shelf to appreciate that this principle applies to even an outwardly rather continuous environment (excerpt of a line in the Virtual Seismic Atlas):

Seismic section from CGGVeritas, shared by Julien Moreau on VSA


Bailey, R (1998). Review: Stratigraphy, Meta-Stratigraphy, and Chaos. Terra Nova 10, 222-230. DOI: 10.1046/j.1365- 3121.1998.00192.x

Romans, B (2014). Strata are not flat. In: M Hall, ed., 52 Things You Should Know About Geology. Agile Libre, Nova Scotia, 132 pp. ISBN 978-0-9879594-2-3. Available online.

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    $\begingroup$ +1 for the acronym. $\endgroup$
    – stevej
    Nov 19, 2014 at 2:37
  • $\begingroup$ I don't necessarily agree. At the end of the day, when you're standing in front of an unfolded and unfaulted outcrop, the younger strata are on top and the older are on the bottom (in the law of superposition). This POOH is also sometimes true, depends on scale. $\endgroup$
    – Gimelist
    Nov 19, 2014 at 7:21
  • $\begingroup$ Thank you kwin for this. I'll try to edit my question for better coherence. $\endgroup$
    – Chirac
    Nov 19, 2014 at 11:49
  • $\begingroup$ @Michael: Sorry, I tried to be clear: the law of superposition is useful, the POOH is not. The POOH may 'sometimes' hold, but if it usually doesn't then it's not really a principle. I will edit my answer to be clearer. $\endgroup$
    – Matt Hall
    Nov 19, 2014 at 12:38

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