Posted by: Rob Viens | June 21, 2012

Relative Dating with Darwin I

I know what you are thinking….a lot of people know that Darwin ultimately married his first cousin Emma Wedgwood.  But this post is not about that type of “relative dating”.  Darwin also used what geologists call relative dating to understand the geologic relationship between some of the rocks that he observed on his journey.  In this case, he found an interesting outcrop near Botafogo Bay that illustrates the principle quite well.  Here is a sketch from his field notes that Darwin updated for the Geologic Observations on South America:

geologic cross section from Rio – figure 22

He describes it as such:

“On a bare gently inclined surface of the porphyritic gneiss in Botofogo Bay, I observed the appearance here represented.

A fragment seven yards long and two in width, with angular and distinctly defined edges, composed of a peculiar variety of gneiss with dark layers of mica and garnets, is surrounded on all sides by the ordinary gneiss-granite; both having been dislocated by a granitic vein. The folia in the fragment and in the surrounding rock strike in the same N.N.E. and S.S.W. line; but in the fragment they are vertical, whereas in the gneiss-granite they dip at a small angle, as shown by the arrows, to S.S.E. This fragment, considering its great size, its solitary position, and its foliated structure parallel to that of the surrounding rock, is, as far as I know, a unique case: and I will not attempt any explanation of its origin.” (Geologic Observations on South America)

Darwin may not be willing to explain its origin, but we’ll try. But first we’ll need a geology lesson…

One of the things geologists are interested in (obsessed with in some cases) is the age of rocks.  Since rocks tell a story of the past, their age helps us to make sense out of that story. (Ripping all the pages out of a book and then reading them in random order would not make much sense, right?)

Now there are two basic ways in which geologists “date” rocks – relative dating and absolute dating.  The first method describes the way we put geologic units/events in relative order, without actually assigning them a specific (numeric) age. The second method (absolute dating) refers to the methods we use to determine an actual (numeric) age for a rock or fossil. Not surprisingly, relative dating is relatively easy and cheap, while absolute dating is time consuming and expensive.  The trade-off is that the second method gives us a more precise answer to the question of “how old is this rock?”.

Here is an example I use in my classes using cars.  Look at the picture below showing several models of cars.  Can you place these cars is relative order from oldest to youngest?  When you think you know the answer, click on the image to see if you are right.

Relative Dating - Cars

Chances are that most people who look at this picture can more or less put these cars in relative order.  We use our general knowledge of the “look” of the cars to tell what looks old vs. new. This is relative dating.

Now a harder question – what is the model year for each of these cars?  See if you can guess and once you are ready, you can see the answer here.

To answer this question requires a lot of knowledge about cars and automobile history.  If you don’t know cars you have to spend some time researching the images to find out a specific answer.  This is absolute dating, and as you can see, it is a bit more difficult.

Now, in some cases, you can use advantages of both methods together to get a pretty good idea of the age of rocks.  For example, if you put those cars in relative order and determined the absolute age of the oldest and youngest car, you’d know that all the cars in between fall between 1930 and 2005. Again, this is often what we do in geology – use the best of both methods to get a good idea of rock ages.

Before Darwin’s time, “natural philosophers” (no one called themselves a geologist in the 1600’s), determined some basic principles that allowed scientists to put geologic events in relative order.  These natural “laws” include the principles of:

  • Original Horizontality
  • Superposition
  • Cross Cutting Relationships
  • Inclusions
  • Faunal Succession

These “laws” and the interpretation of Darwin’s sketch, will be the subject of tomorrow’s post.  Stay tuned… (RJV)


  1. […] 1600′s, 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 – […]

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