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geology

Plenty of Peneplains?

Submitted by jdp on Thu, 05/21/2015 - 09:04 am

 

In the late 19th and early 20th century, William Morris Davis popularized the concept of the peneplain, an extensive low-relief erosion surface graded to sea level. Peneplains were strongly associated with Davis’ cyclical model of landscape evolution, which fell out of favor with most geomorphologists decades ago. By association, the discussion and study of peneplains also fell out of favor.

But peneplains are making a comeback. This is best illustrated by a report from the Geological Survey of Denmark and Greenland (Green et al., 2013), though the ideas and evidence are also laid out in a number of journal articles by the various co-authors. The report is concerned with development of elevated passive continental margins (think of, e.g., the Great Escarpment of Africa, the eastern Australian highlands, or the main subject of the report, west Greenland). The arguments are strongly dependent on the identification and interpretation of planation surfaces. As these planation surfaces are low-relief, regionally extensive, and are eroded across geological materials of varying resistance, and because the authors present evidence that they were originally graded to sea-level (they were subsequently uplifted), they can be legitimately referred to as peneplains.

Geoscience Metanarratives -- Part 2

Submitted by jdp on Mon, 04/20/2015 - 11:41 am

 

This is a continuation of a previous post, and this one will be even less intelligible unless you read that one first.

So, even though we rarely use the term, geoscientists have our metanarratives. Metanarrative is something of a perjorative for postmodern (pomo) critical social theorists, but just because because a metanarrative doesn’t really explain everything, even within its domain, doesn’t make it wrong, useless, or even hubris-y. As long we don’t make claims or insinuations, or have expectations, of a “theory of everything,” overarching theories or explanatory frameworks can be evaluated on their own merits or lack thereof—that is, whether a construct can be considered a metanarrative or not is independent of its utility and value.

Geoscience Metanarratives

Submitted by jdp on Mon, 04/20/2015 - 10:26 am

 

At my job I am housed in a building occupied mostly by social science and humanities scholars, many of whom are postmodern, post-structuralist, “critical” social theory oriented. The “critical” is in quotes not to cast aspersions, but because these folks use the term somewhat differently than do scientists, for whom all well-conceived legitimate work is critical in the sense of skepticism, testability, and the potential for falsification.  Anyway, my office location ensures that I am exposed to a good deal of the concepts and jargon of that community.

One of those is metanarrative. According to the Sociology Index web site:

Strat-and-Transition Models II

Submitted by jdp on Mon, 03/09/2015 - 10:48 am

This is a continuation of my earlier post on applying state-and-transition models (STM) to stratigraphic information, to account for the missing bits.

Barrell’s (1917) explanation of how oscillatory variations in base level control the timing of deposition. Sedimentation can only occur when base level is actively rising. These short intervals are indicated by the black bars in the top diagram. The resulting stratigraphic column, shown at the left, is full of disconformities, but appears to be the result of continuous sedimentation. Noted sedimentologist Andrew Miall has used this example in several articles to illustrate the problems of gaps in sedimentary & stratigraphic records.

Graph Theory in Geosciences

Submitted by jdp on Fri, 02/27/2015 - 10:15 am

Wolfgang Scwhanghart, Tobias Heckmann and I have collaborated recently to review applications of graph theory in geomorphology and the geosciences in general. One of our papers, Graph Theory in the Geosciences, was just published in Earth-Science Reviews. The abstract is below. Our other joint paper, dealing specifically with graph theory applications in geomorphology, is still in press (in the journal Geomorphology) even though it was completed and accepted before the ESR paper. Go figure. 

Δ DELTAS

Submitted by jdp on Fri, 01/23/2015 - 01:44 pm

 

Several studies have noted the temporal coincidence between shoreline erosion around some major deltas (e.g., Nile, Mississippi, Ebro), and the reduction of stream sediment loads due to reforestation, soil conservation practices, and trapping of river sediment behind dams. There are, of course, excellent reasons to suspect a causal link, but the link itself has not, in my view, been fully established.

THE CYCLE OF EROSION

Submitted by jdp on Sun, 01/11/2015 - 01:43 pm

 

Out on the trails of Shaker Village at Pleasant Hill, Kentucky, this morning, I got to thinking about William Morris Davis’ “cycle of erosion” conceptual model (also called the geographical or geomorphological cycle). The drive-by, oversimplified version is that landscape evolution starts with uplift of a more-or-less planar, low relief surface. Weathering and erosion goes to work, and results in an initial stage of increasing relief as streams carve valleys, and slope processes operate on the slopes thereby created. Eventually, however, as the streams begin to approach base level, a new stage of decreasing relief begins as hilltops and drainage divides are lowered and valleys infilled. This continues until the entire landscape is about as close to baselevel as the geophysics of mass transport will allow, creating a low-relief, almost-planar surface called a peneplain. At some point a new episode of uplift occurs and the cycle begins anew.

I was thinking of this because many landscapes in the world, like the one I was viewing this morning, do give the impression of a dissected plateau or a low-relief surface into which denudational processes have cut.

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