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Evolution of Evolution — Text-only | Flash Special Report
Darwin’s Missing Rock and the Increasing Precision of Earth Time

Darwin's understanding of geologic time helps zero in on elusive rock layer

By Kirk R. Johnson

Darwin began to conceive of the immensity of Earth’s time when he observed the rate of coastal erosion and compared it with the vast thickness of geologic strata. “What an infinite number of generations which the mind cannot grasp, must have succeeded each other in the long roll of years! Now turn to our richest geological museums, and what a paltry display we behold” (“On the Origin of Species,” p. 287). At the same time, he realized how few of the lives that have been lived were actually preserved as fossils. Time is long, the Earth is large and only locally, and occasionally, is the subsiding coffin of sedimentary deposition in operation.

As a museum guy, Darwin’s words still ring true to me 150 years after they were written. I am, as he was, greatly impressed by the thickness of geologic strata, the amount of time represented by the strata, and even more so, the time that passed while unconformities formed between the depositions of subsequent formations. Darwin sought to explain why intermediate forms were not usually seen in geologic formations and came, in part, to the conclusion that species lasted longer than it took for formations to accumulate and that formations themselves were relatively sparsely distributed in time. He also realized that time within formations was not uniform: “It would seem that each separate formation, like the whole pile of formations in any country, has been intermittent in its accumulation” (Ibid, p. 295).  Both of these insights, like many made by Darwin, were strikingly modern.

Now flip this around and ask the question, what do you do if you want to find a rock layer that was deposited in a specific year, let’s say 65,510,023 years ago? This is a problem that did not trouble Darwin but it troubles me because I search for the K-T boundary, an event that appears to have happened one day at the end of the Cretaceous period. It is amazing that we have found the K-T boundary at all. The fact that we can search for and find it in specific spots like the Denver Basin or the seafloor says a lot for our vastly increased resolution of geologic time. A stack of 65,510,023 pennies would be 56 miles high. Imagine the challenge of finding a specific penny in that stack and you have a concept of why it is not easy to find the K-T boundary. Darwin said little about dinosaurs, and his discussions about extinctions never imagined ones that happened in a single afternoon, yet he truly understood the ramifications of deep time and the fact that tiny changes over huge time make for huge change. When confronted with the immensity of time it is sometimes easy to forget that abrupt moments occasionally leave their mark and things that happened a long time ago may have happened really fast.

In some places we can find the K-T boundary because of its trademark mix of cosmic particles and properties; however, we are still unable to precisely measure its age. To be sure, the 500,000-year-error bars of the 1980s have been replaced with 50,000-year-error-bars, and cyclostratigraphy and high-resolution uranium-lead analyses are now pushing toward errors less than 20,000 years.

Darwin ended his chapter on the imperfection of the geological record with a nod to Lyell’s metaphor of geologic time as a giant multivolume history of the world, where only a few scattered words from a smattering of random pages were preserved. It is amazing that we are now able to begin to place those few words in their proper position in Earth time.

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Kirk R. Johnson is chief curator and vice president of research and collections at the Denver Museum of Nature & Science.  He is a paleobotanist who has been on a 25-year, worldwide search for the K-T boundary, trying to understand its impact on plants.  In the process, he has learned that geologic time is simply time that is hard to measure and even harder to conceive. A summary of his efforts (with colleague Douglas Nichols), “Plants and the K-T Boundary,” was published by Cambridge University Press in 2008.  The National Science Foundation supports his research on vertebrate fauna from the late Cretaceous (Campanian) of Utah and on critical transitions in the history of life in and around Denver, Colo.

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Please see the Resources section for the Bibliography/Additional Reading list for this essay.

 

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