March 5, 2004 | David F. Coppedge

Rethinking the Geological Layers

One of the most formative ideas in Darwin’s intellectual journey was the concept of gradualism, the principle of “small agencies and their cumulative effects.”  This idea became a dominant motif in his philosophy of life.  Describing how the assumption of gradualism permeated his last book (on earthworms) shortly before his death, Janet Browne, in her acclaimed biography of Darwin, describes how the idea grew:

He [Darwin] believed that the natural world was the result of constantly repeated small and accumulative actions, a lesson he had first learned when reading Lyell’s Principles of Geology on board the Beagle and had put to work ever since.  His interpretation of South American geology had been based on Lyell’s vision of little-and-often, and his theory of coral reefs too, each polyp building on the skeletons of other polyps, every individual contributing its remains to the growing reef.  Most notably, he had applied the idea of gradual accumulative change to the origin of species, believing that the preservation of a constant process of minor adaptations in individuals would lead to the transformation of living beings.  His work on barnacles, plants, and pigeons all supported the point.  No one, not even Lyell himself, or any of Darwin’s closest friends and supporters, accepted as ardently as Darwin that the book of nature was about the accumulative powers of the small. 1

It was the record of the rocks that led to Lyell’s uniformitarian principle, and from there, Darwin extended it to all of nature.  But do the rocks actually record a process of slow and gradual accumulation?

In this month’s journal Geology, an earth scientist from the Netherlands makes a startling proposal: the record in the rocks is fractal, not necessarily gradual.  In fractals, a pattern on a small scale can look the same on large scales.  In other words, he seems to be saying, a large stratigraphic record might not be the gradual accumulation of small layers, but a fractal pattern on a large scale that could represent a rapid accumulation of a large quantity of material.

Wolfgang Schlager2 first debunks the conventional wisdom as being only, well, conventional – but not necessarily wise: “Orders of stratigraphic sequences are being used loosely and with widely varying definitions,” he says.  “The orders seem to be subdivisions of convenience rather than an indication of natural structure.”  He proposes that rock layers may not indicate so much about time as about quantity of material.  He calls it a “well-known fact” that “sediment architecture is largely scale invariant over a wide range of scales in time and space.”

Schlager criticizes the conventional method that defines orders of strata by duration, even though the practice is “almost universally followed.”  Thus, he seems to be proposing a radical reinterpretation of the record:

This essay presents a critique of the concept of orders in sequence stratigraphy and argues that the succession of sequences is fractal rather than a hierarchy of orders.  The argument rests on four components: (1) The duration of the presumed orders varies widely, even within one publication.  (2) Exposure surfaces and flooding surfaces as unit boundaries are both common in a wide range of temporal scales.  (3) Extensive studies on sea-level fluctuations and sedimentation rates have shown that the principal trends of both are fractal.  (4) Limited data on shelf edges that prograde and step up and down in response to sea level indicate that these traces, too, are fractal.

He provides examples of discordant measurements when geologists assume the rocks represent “categories in time.”  The confusion does not seem to dissipate with more examples, he says: “Moreover, the values do not seem to converge with time and improving data.”  But if the size of the deposit is a fractal rather than a measure of the passage of time, it could mean that giant deposits could have been laid down in short order, provided enough material were available:

Sedimentation and erosion, the processes that are ultimately responsible for the sediment record, operate in the same fashion over a wide range of scales.  It is characteristic of hydrodynamics that flow properties are largely determined by dimensionless ratios, and few characteristic scales enter in the analysis.  Depositional patterns have been found to be scale invariant over a wide range of time and space.

Schlagel points to examples covering a wide range of presumed depositional times, and strata that represent “energy-dissipation patterns that are scale-invariant over the range of centimeters to hundreds of kilometers.”  His model allows for slow and gradual deposition as well as fast and catastrophic, of course, but he suggests it is not always easy to tell:

In many sequence data sets, the impression of a hierarchy of cycles is very strong.  The model does not imply that this impression is false.  It is characteristic of fractals that the same pattern is repeated at finer and finer scales.  Consequently, any snapshot of the fractal taken at a certain resolution will show a superposition of coarser and finer patterns.  The crucial difference to an ordered hierarchy of cycles [which he disputes] is their lack of characteristic scales.  The fractal model proposed here predicts that the sequence record, like many other natural time series, has the characteristics of noise with variable persistence and thus variable predictability.

He seems to be saying it will be harder to claim that a large depositional unit would have necessarily been a function of long ages.  It’s just a proposal at this point, he admits: “The model is meant as a conceptual framework to steer future data analysis and to provide a basis for statistical characterization of sequences.”  He only speculates about the origin of the fractal patterns.  Nevertheless, this new way of looking at the rock record might cause rethinking of Lyell’s assumption that huge layers necessarily represent huge passages of time:

Stratigraphic sequences are essentially shaped by the interplay of rates of change in accommodation and rates of sediment supply…. As both rates show fractal properties, it is not surprising that the resulting sequence record inherits this attribute.  At a more fundamental level, it may be the complexity of depositional systems and their tendency to evolve toward conditions of self-organized criticality that give rise to fractal features in sequence stratigraphy.

The fact that Schlager’s proposal was published in the world’s leading geology journal indicates that other geologists are taking it seriously.

1 Janet Browne, Charles Darwin: The Power of Place (Princeton, 2002), p. 490.
2 Wolfgang Schlager, “Fractal nature of stratigraphic sequences,” Geology Vol. 32, No. 3 (March, 2004), pp. 185-188, doi: 10.1130/G20253.1.

Although this is a technical subject for mathematically-inclined geologists, it seems to represent a daring break from conventional wisdom.  Some creationist geologists have already demonstrated with experiments that layered deposits can be laid down rapidly in horizontal fashion, forming what look like fractal patterns, in one stage (see the work of Guy Berthault, for example).  Similarly, fine-grained laminations have been found in thick deposits at Mt. St. Helens, where the rates of deposition were known (e.g., one day!).  The old thinking was that each layer represented a long passage of time.  Now, we have observed examples that this is not necessarily true.

Schlager is clearly not proposing a young-age geology; his article assumes millions of years for some deposits.  Nevertheless, his model seems to reinforce the notion that a pattern in the rock layers, no matter how thick, could be a function of “rate of change in accommodation and rates of sediment supply,” not necessarily a long, gradual passage of time.  In simple, creationist-geology terms, were the layers of Grand Canyon laid down by a little water over a long time, or a lot of water over a little time?

Look at the philosophical baggage that Lyell’s vision of gradualism generated.  It appeared intuitively obvious to him, and then to Darwin, that the rock layers must have required many millions of years for their formation.  Darwin’s philosophical voyage from Christianity to agnosticism floated on this belief, which subsequently flavored all his investigations and writings.  Now we see geologists questioning the basic assumption.  The Titanic had a lot of baggage, too.  When the hull was breached, it no longer mattered how ornate the furnishings.

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