Clastic Detritus

Canyon Lake Gorge (credit: http://www.canyongorge.org/about-the-canyon-gorge)

In 2002, flood waters from Canyon Lake dam reservoir in central Texas were diverted into an emergency spillway at nearly 200 times the normal flow rate. The resulting flood event, which lasted for six weeks, removed trees and sediment and excavated a 7 m deep and >1 km long canyon into the limestone bedrock.

A paper by Lamb & Fonstad called Rapid formation of a modern bedrock canyon by a single flood event published in Nature Geoscience this week documents the patterns left in the landscape and reconstructs the hydraulics involved in this catastrophic flood.

I encourage those interested in the details of sediment transport and bedrock incision to read the paper. But what I want to discuss in this post is an issue that will come up as a result of this study — an issue that I can almost feel bubbling up as I write this. In fact, I’m certain that someone, somewhere is abusing the results of this study in an attempt to claim that because significant change to the Earth’s surface can occur abruptly then it follows that all geologic change must be the result of abrupt and cataclysmic events. This is a false dichotomy pushed by neocatastrophists that are typically, but not always, associated with the viewpoint that the Earth is ~6,000 years old.

When the history of the science of geology is taught it commonly includes the classic uniformitarianism vs. catastrophism debates of the late 1700s-early 1800s. The uniformitarianists, so the story goes, argued that the changes we see in the geologic record were the result of minor and gradual processes that accumulated over time — from processes that we can see working on the landscape today. Catastrophists believed the same geologic products were the result of cataclysmic events that reshaped the land abruptly.

Studying this historical debate is thought-provoking and provides context for a novice geoscientist but the science has largely moved on from this false dichotomy. Geologists in the 1800s and early 1900s documented features that could only be explained by large-magnitude events*. Geologists realized that, of course, both gradual and catastrophic processes helped shape the landscape. The great paleontologist Stephen Jay Gould wrote a paper in the American Journal of Science in 1965 (he was 24 years old) called “Is Uniformitarianism Necessary?” that succinctly summed up where the science was regarding the concept of uniformitarianism. The incredible clarity of Gould’s writing forces me to quote the entire abstract:

Uniformitarianism is a dual concept. Substantive uniformitarianism (a testable theory of geologic change postulating uniformity of rates or material conditions) is false and stifling to hypothesis formation. Methodological uniformitarianism (a procedural principle asserting spatial and temporal invariance of natural laws) belongs to the definition of science and is not unique to geology. Methodological uniformitarianism enabled Lyell to exclude the miraculous from geologic explanation; its invocation today is anachronistic since the question of divine intervention is no longer and issue in science. Substantive uniformitarianism, and incorrect theory, should be abandoned. Methodological uniformitarianism, now a superfluous term, is best confined to the past history of geology.

In other words, the original notion of uniformitarianism — that there is uniformity in rates — is false. Although the science has discarded this erroneous concept, it is what modern catastrophists use as a straw man in their arguments. And, unfortunately, we see glimpses of it in mainstream science writing and reporting of geologic processes. The nuance of Gould’s paper from nearly 50 years ago is lost within the compelling story of pitting one absolute versus another.

I would argue that rapid and significant processes are included within our current understanding of processes. For example, I study the processes and deposits of turbidity currents, which are essentially submarine avalanches of sediment. The recurrence of such events varies but is typically on the order of hundreds to thousands of years. Moderate to large turbidity current events would surely be labeled “catastrophic” from our point of view. Yet, entire sedimentary basins are filled with the deposits of hundreds of thousands of individual catastrophic events. While each event may be short-lived and cataclysmic, they occur very regularly over time and incrementally stack to produce a stratigraphic succession. We might consider some volcanic systems similarly — each eruption event might be catastrophic, but over time this is how the volcano is incrementally constructed.

This doesn’t take away from the insights from Lamb & Fonstad’s study. What they document here is just how rapid significant Earth-surface modification can occur given a certain set of conditions. In this case, they explicitly make the point that characteristics of the bedrock are important within the context of their results:

We suspect that well developed vertical and horizontal joints at Canyon Lake Gorge define blocks of bedrock that have little interlocking along their boundaries, rendering their behavior similar to an alluvial bed when critical stress for mobility is surpassed. … Thus, it seems plausible that erosion of well-jointed rock by large floods might be extremely rapid, such that canyon formation is limited by the capacity of the flood to transport plucked blocks rather than by the plucking process itself.

In other words, the bedrock which was eroded during the flood was already slowly eroding through the formation of joints (a type of fracture) in the rock. The high-energy flood event took advantage of this weakness and literally plucked large boulders of bedrock from the floor and walls of the canyon. In this case, the slow and gradual processes of joint formation worked in concert with the catastrophic flood event to produce this result.

There will be some who will use this paper to attempt to tear down uniformitarianism. Not only will they fail to mention the nuances of this specific study but they will be tearing down an idea that has long since been discarded by geology.

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Lamb, M., & Fonstad, M. (2010). Rapid formation of a modern bedrock canyon by a single flood event Nature Geoscience DOI: 10.1038/ngeo894

Caltech press release: http://www.eurekalert.org/pub_releases/2010-06/ciot-cgi061810.php

Scientific American: http://www.scientificamerican.com/article.cfm?id=canyon-lake-flood

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* see this post at 4.5 Billion Years of Wonder for more commentary about this paper within the context of Bretz and the origin of the Channeled Scablands of Washington.

update: check out Geotripper’s post about a similar flood event involving the Tuolumne River of California in 1997; also check out before/after images of the event described above at Pathological Geomorphology