As a young man aboard HMS Beagle, Charles Darwin was fascinated by erratic boulders. After completing his voyage, he wrote several papers about their origin. Tierra del Fuego was of particular interest, for he found boulder trains at different elevations at a place known as Bahia San Sebastian, which faces the Atlantic Ocean. Darwin actually delayed the survey work of HMS Beagle so he could gather more extensive information. On returning to the UK, he made the boulders the focus of two geological papers published in 1841. The route by which Darwin reached his conclusions is instructive for all of us involved in research today.
Some of "Darwin's Boulders" (Source here)
It is well known that Charles Lyell's writings were a major influence on Darwin. Captain Fitzroy acquired Volume 1 of Principles of Geology for the library of HMS Beagle. Darwin not only read it but afterward said that "it altered the whole tone of one's mind" and that, thereafter, he saw everything in the light of Lyell's ideas. He made a point of acquiring the other volumes as they were published. Lyell's approach to making geology a science was to relate all geological interpretations to the operations of present-day processes. He championed uniformitarianism as a methodological principle - and Darwin drank it all up. He followed Lyell's lead in explaining landscape evolution in terms of gradual, incremental changes of sea level.
"Darwin's thinking was profoundly influenced by Lyell's obsession with large-scale, slow, vertical movements of the crust, especially as manifested in his theory of submergence and ice rafting to explain drift. In turn, Lyell profited greatly from Darwin's observations, including uplift of the Pacific coast of Chile during the Talcahuano earthquake. Lyell celebrated these observations because they supported his idea of uniformitarianism - that continued small changes, as witnessed in the field, could account for dramatic changes of Earth's surface over geologic time."
Lyell had noted how sediments carried along by icebergs could be deposited far from their source, and Darwin extended the observations by documenting the way boulders were transported by icebergs. He then developed an ice-rafting model to explain erratic boulders. In 1845 he pointed out that if these boulders were close to glaciers, they were likely pushed into position, but if far from source, they were ice-rafted.
"Few geologists now doubt that those erratic boulders which lie near lofty mountains have been pushed forward by the glaciers themselves, and that those distant from mountains, and embedded in subaqueous deposits, have been conveyed thither either on ice-bergs or frozen in coast-ice."
Darwin supported this model for the Tierra del Fuego boulders using two arguments: first, that the land surface on which the boulders lay were free of mounds and ridges which might point to glacial action; and second, that the boulders were angular - which would not be expected if they were pushed such a great distance. Darwin "considered the possibility that glaciers could have extended" much further than they do today, but rejected the idea because it departed too much from uniformitarian thinking. Darwin's approach to interpreting landscape anomalies is described by the authors of a recent paper as "inductive reasoning". This is discussed further below.
The new research sets out to revisit "Darwin's Boulders" and to review the causal mechanism. The authors have mapped the Bahia San Sebastian train of boulders on the east coast (which number about 500) and also a second train at Bahia Inutil on the west coast (which number about 1000). All the boulders are medium-grained hornblende granodiorites, several hundred kilometres from the nearest source. The authors write:
"Of the three plausible mechanisms for emplacement of these distal erratics - iceberg rafting, stream-ice rafting, or direct deposition from glaciers - we support the latter. Overwhelming evidence for complete glaciation of Tierra del Fuego, from coast to coast, has been unchallenged for almost a century. It is unlikely that stream ice could have transported such large boulders over hundreds of kilometers while maintaining such a tight distribution, and there is no evidence of a capable fluvial environment in the immediate vicinity of either boulder train."
To expand on the relevant points: mapping of the surface sediments of Tierra del Fuego has revealed that the whole region has been glaciated. The boulder trains at Bahia San Sebastian train and Bahia Inutil have been mapped as resting on moraine crests. The "tight distribution" has been documented and ice-rafting has never been observed to result in anything like this. Direct deposition, however, is observed. Landslides onto glaciers can leave large boulders on the ice which can then be transported however far the glacier extends, leaving a linear train of blocks when the ice melts away.
I want now to return to the "inductive reasoning" comment noted earlier. Inductive reasoning starts with observations and philosophical premises, uses reason to identify patterns, which lead to the proposal of initial hypotheses. These can then be tested and confirmed hypotheses lead to theories. Darwin's observations were of angular erratic boulders, the ability of icebergs to carry large rocks over long distances, and relatively short glaciers in the upland areas. His philosophical premise was uniformitarianism. Put these together and the hypotheses were iceberg rafting or stream-ice rafting. The angularity of the boulders ruled out stream-ice rafting, so Darwin drew the conclusion that the mechanism was iceberg rafting.
The problems with this start with the philosophical premises. Once uniformitarianism was accepted as essential to science (as Lyell argued), Darwin felt honour-bound to adhere to it. His thinking became constrained. He was only prepared to work with hypotheses that were compatible with uniformitarianism - all else would be regarded as speculation or even antiscience. This led him to overlook data that was right in front of him: the "tight distribution" of the boulders that was inconsistent with the hypothesis. It also delayed the recognition of the glacial features that covered Tierra del Fuego.
The problem goes back to Francis Bacon, who wanted to move away from the deductive methodology of the Aristotelians and establish something more grounded in empiricism. Induction was a key stage in his methodology - but he underplayed the human dimension. Researchers have to bring philosophical premises to bear on their work. How can we avoid becoming slaves to our adopted premises? Uniformitarianism lasted a century before researchers accepted that catastrophism was just as viable as a philosophical presupposition. What matters is that we use these philosophical approaches to generate testable hypotheses. Multiple working hypotheses are to be commended as long as ways are found to put them to the test.
Charles Darwin never escaped uniformitarianism. It pervaded his geology - as is apparent from the example before us here. It entered his thinking about biological transformation: the natural selection of small incremental variations. (Unfortunately, this constraint is still with us today, as Darwinians are unwilling to concede anything significant to the theory of punctuated equilibrium or to evo-devo.) Darwin missed out in understanding heredity, because he was looking for gradual change rather than discontinuous variation. (For more on why Darwin did not discover the laws of inheritance, go here).
Philosophical premises are of crucial importance. We cannot afford to leave discussion of this to the philosophers. Scientists bring philosophical premises whether they know it or not - and, as Darwin demonstrated, it matters. This is why the issues raised by Intelligent Design are of such importance. Should design inferences be part of science? Those who say 'no' are united in the belief that design cannot be inferred in the natural world. They 'know' this, not because they have empirical evidence to show it, but because their philosophical starting point is naturalism. All causation must be by Law or by Chance, they say. ID advocates have repeatedly pointed out that this stance involves circular reasoning, because they can envisage no scientific test to prove or disprove design. Consequently, disproofs of design are always theological: 'God would not do it that way!' However, philosophical naturalism has within it the seeds of its own destruction: like Darwinism, it is a universal acid that eats up our humanity (consciousness and free agency), our values (all morality is relative and socially constructed) and ultimately our science. That's why ID advocates must persevere until the urgently needed changes come.
Enigmatic boulder trains, supraglacial rock avalanches, and the origin of "Darwin's boulders", Tierra del Fuego
Edward B. Evenson, Patrick A. Burkhart, John C. Gosse, Gregory S. Baker, Dan Jackofsky, Andres Meglioli, Ian Dalziel, Stefan Kraus, Richard B. Alley, Claudio Berti
GSA Today, December 2009, 19(12), 4-10.
Charles Darwin considered himself to be a geologist and published extensively on many geologic phenomena. He was intrigued with the distribution of erratic boulders and speculated upon their origins. In his accounts of the voyage of the HMS Beagle, Darwin described crystalline boulders of notable size and abundance near Bahia San Sebastian, south of the Strait of Magellan, Tierra del Fuego. Influenced by Charles Lyell's reflections upon slow, vertical movements of crust, submergence, and ice rafting to explain drift, Darwin proposed that the boulders of Bahia San Sebastian were ice-rafted. Benefiting from 170 years of subsequent study of the glacial history of Tierra del Fuego, petrography, and terrestrial cosmogenic nuclide measurements, we revisit the origin of "Darwin's Boulders" at Bahia San Sebastian. We suggest that they, as well as another train of boulders to the west, at Bahia Inutil, represent rock falls of Beagle-type granite from the Cordillera Darwin onto glacial ice flowing into the Bahia Inutil-Bahia San Sebastian lobe. These supraglacial rock avalanche deposits were subsequently elongated into boulder trains by glacial strain during transport and then deposited upon moraines. The cosmogenic nuclide exposure dates support the correlation of Andean glaciations with the marine oxygen isotope record and the glacial chronologies recently proposed for Tierra del Fuego.
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