Erosion theory explains uniform patterning of landscapes.
For more than a century, geologists have been aware that some landscapes bear seemingly uniform patterns, such as evenly spaced ridges and valleys. Taylor Perron, a geologist at the Massachusetts Institute of Technology in Cambridge, became intrigued by this ridge–valley 'wavelength' as he observed Earth's topography from aeroplanes, particularly over the central California Coast Ranges between San Francisco and Los Angeles. “Geologists,” he says, “have a marked preference for window seats.
Perron and his colleagues, Jim Kirchner and Bill Dietrich at the University of California, Berkeley, decided to examine how erosional processes create these patterns by comparing a computational model with precise measurements of various landscapes. “We agreed that if we were going to try to explain how landscapes form, we'd have to be able to explain these patterns,” says Perron.
The first step was to scout for sites — which they did with satellite images and Kirchner's Cessna aircraft — where they would be able to gain access on foot. “If you want to write the equations that describe how the topography evolves, it's important to stand there and see what's happening,” says Perron. But accessing such sites required delicate negotiations with landowners, especially on agricultural and ranching lands, where soil erosion is a sensitive topic.
In the Salinas Valley of central California, where rows of lettuce, artichoke and other crops stretch as far as the eye can see, Perron and his team found a ranching family that granted them access to a section of Gabilan Mesa. This is a swath of land with relatively uniform valley spacing that covers thousands of square kilometres.
There, Perron and colleagues took a close-up look at the physical processes sculpting the landscape, including gully erosion, which slowly deepens valleys by scouring soil, and 'bioturbation', such as ground-squirrel burrowing, which gradually smooths the landscape by stirring the soil and moving it downslope. “The basic mechanism is a competition between one process that incises valleys and another process that tends to smooth them out,” says Perron.
This mechanism was also found to occur at four other sites the team examined, with differences in climate and rock type accounting for variations in wavelength from site to site.
Next, the authors turned to laser altimetry, a technique that relies on the Global Positioning System and aircraft-mounted lasers to produce precise digital topographic maps, even in landscapes covered by dense vegetation. Using this, the team precisely measured the ridge–valley wavelengths at the five field sites, and tested whether the measured wavelengths matched the predictions of a computational model built from their field observations. To Perron's excitement, they did (see page 502).
Having developed a computational model that accurately predicts the natural patterns in landscapes, geologists can learn more about how factors such as climate have shaped Earth's topography. And the model could potentially be applied farther afield — on Mars, for example, where evenly spaced ridges and valleys have formed on steep slopes such as those within impact craters. Knowing that similar topographical patterns occur on other planets, says Perron, “can tell us many things about those landscapes, even if we can't get there on the ground.”
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Taylor Perron. Nature 460, 434 (2009). https://doi.org/10.1038/7254434a