Posted by: Rob Viens | June 22, 2012

Relative Dating with Darwin II

June 22nd and Darwin wrote his first diary entry since the 19th:

“During these days have been busily employed with various animals; chiefly however corallines: & my walks in consequence have not been extended far from the house.” (June 20/21/22)

But we have a story to finish, so let me take us back to the major principles of relative dating…

The first natural philosopher to really work on this problem was Nicolas Steno (born Niels Stensen, but like all the cool scientists he Latinized his name).  Steno, who was born on January 1, 1638, traveled Europe studying with many of the great scientists of his day.  At the age of 28 he became a professor at the University of Padua, where he was “discovered” by the Grand Duke Ferdinando II de’Medici. The Grand Duke, known for sponsoring the sciences, took Steno in as his official physician and supported his research into anatomy and geology.

Nicolas Steno (probably painted by Justus Sustermans between 166 and 1677)

Nicolas Steno

Interesting, later in his life Steno’s interests changed from natural science to theology. He joined the Catholic Church and eventually became a bishop. In 1938 he was canonized as a saint, and in 1988 he was beatified.  He is the only geologist that I know of that can go by the title “Blessed Saint”. (For a history of Steno, I’d recommend Alan Cutler’s book The Seashell on a Mountaintop.)

Steno made several major contributions to geology, including being the first to really describe the relationship between layers of rock. (More on Steno’s contributions to understanding what fossils where later this year.)
Steno officially started the geologic discipline of stratigraphy – the study of layers (strata) of rock. So the first couple of rules of relative dating, identified by Steno, help us to understand the relative age of layers.  They are incredibly simple, yet the second one in particular, is especially powerful in determining relative age. Steno recognized these principles by observing sediments laid down by successive floods. They include:

  1. Principle of Original Horizontality – This basically states that rock layers start out flat, so if they are found tilted we have to assume that happened after they were deposited.  This also sets the stage for the…
  2. Principle of Superposition – The idea here is that layers of sediment (or rock) are laid down on top of one another, such that the oldest is at the bottom of the pile and the youngest at the top. Hence, if rocks layers are like the pages of a book on its side, then we read them from the bottom to the top.

Steno’s third principle, which does not really apply directly to relative dating is the Principle of Lateral Continuity. This one states that rock layers are horizontally continuous.  So if you see a layer on one side of a canyon, you should be able to find that layer on the other side as well.

Steno’s  374th birthday was honored by a Google Doodle this year, highlighting his contributions to stratigraphy.

Nicolas Steno Google Doodle

The next couple of rules for relative dating were outlined by Darwin’s friend and mentor, Charles Lyell (see Charles Darwin – Geologist).  Lyell (along with the father of geology James Hutton) is credited with defining the next two simple yet powerful principles:

  1. Principle of Cross Cutting Relationships – This one states that if one rock cuts another, then the rock being cut is older than the rock doing the cutting.  If you think about it, this makes perfect sense. The first rock has to exist before the second rock can cut across it. (For example, a loaf of bread has to exist before you can cut it with a knife.  The opposite would make no sense.) This rule applies particularly well to igneous rocks that are injected into other rocks as magma.
  2. Principle of Inclusions – Think about this one like this – if a rock “includes” a fossil, then the fossil itself has to be older than the age of the rock that it is contained in.  In a more general sense, an inclusion in a rock is older than the rock itself. Think chocolate chip cookies – someone had to make the chocolate chips first, before making them into a cookie.

The final rule is often credited to William Smith, the man who made the first geologic map of England in the early 1800’s.  (In the interests of credit, Baron Georges Cuvier was also documenting this principle at the same time in France.)  Smith, a canal engineer, noticed that there was a regular progression in the order of fossils in the strata.  He already knew Steno’s Principle of Superposition, so his suggestion was that fossils changed through time in a predictable order.  Therefore, the relative age of a two (or more) rocks could be determined by the type of fossils they contained. For the sake of completeness, here is the rule laid out in the same format as the first four:

  1. Principle of Fauna Succession – Fossils follow (succeed) each other in a predictable order, making it possible to recognize the relative age of rocks based on the fossils they contain.  Since sedimentary rocks are really the only ones to bear fossils, it is generally applied to them.

For his contributions to geology, Smith is frequently referred to by the nickname William “Strata” Smith. (For a history of Smith, see Simon Winchester’s book The Map that Changed the World.)

William Smith by the painter Hugues Fourau

William Smith

There you have it – with these five simple rules you can determine the relative age of any series of rock units.  Only the last principle requires any specific knowledge of geology to be useful – the others can be applied by anyone.

So with this new knowledge of relative dating , let’s go back to Darwin’s sketch (shown again below slightly annotated):

geologic cross section from Rio – figure 22

In this case, the first rock to form would have been “A” (based on the principle of inclusions), followed by “B”.  Darwin notes that these rocks have the same foliation (“C”).  This suggests that the foliation event happened after “A” and “B” had initially formed.  In other words, they were both metamorphosed at the same time (this is basically the principle of cross cutting – the foliation “cuts” across the other two rocks). Lastly, the igneous intrusion (“D”) cut everything else. We know this intrusion happened after the metamorphism that produced the foliation (“C”) because “D” is not foliated.

Now how rock “A” got into rock “B” is a good question and one that is difficult to answer without more information. It may have been a large fragment that got included in “B” when both rocks were still sedimentary (having been metamorphosed into gneiss later during the time of “C”).  However, if the physical boundary between “A” and “B” is as sharp as Darwin drew it, this idea is difficult to support.  Metamorphism should have at least partially blurred that line.

Whew – I bet that was more than you ever wanted to know about relative dating. But don’t you feel empowered now?  Ready to take on the relative age of any rock layers you happen to see on the side of the road.  That is the beauty of relative dating – anyone can do it! (RJV)


  1. […] helped determine the basic rules of relative dating (see Relative Dating with Darwin Part I and Part II).  Steno was the first real modern thinker to get a handle on what fossils were – essentially […]

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