Have you ever wondered why sedimentary rock layers all seem so flat? Either you have visited places like the Grand Canyon, or you have seen the pictures: relatively flat geological strata extend for miles in every direction. Yet when we look at the Earth’s surface today, we find canyons, river beds, drainage basins, and more. What’s the deal?I’ve heard the argument several times that such flat layers in the geological record are evidence that a singular, worldwide catastrophe was responsible for their deposition. If each of these layers represents several thousands or millions of years, shouldn’t we find more topography?
This argument sounds feasible on the surface, because it appeals to common experience. From a geologist’s perspective, however, the question is rather misguided. While it is true that the Earth’s surface is very rough, sedimentary basins (i.e. where sediments are actually being deposited) are typically very flat and smooth. If you would like to explore this topic, GoogleEarth is a wonderful tool. Take a look, for example, at the Gulf of Mexico and the coastline of the southeastern United States. Silt, sand, and carbonate are accumulating as we speak, actively forming a new sedimentary layer along the continental shelf. The continental shelf (light blue) is a relatively flat surface that extends for miles and miles out into the ocean.
If sea level were to drop a few hundred meters, and a river (the Mississippi?) carved a canyon into the newly exposed shelf, how would it look? Well, very much like the Grand Canyon! You would find flat layers of sediment/rock extending for miles in every direction. So the ‘flatness’ of geological strata is entirely consistent with the conventional picture of Earth history.
More examples can be found from around the world. Every major river valley (Mississippi, Volga, Nile, Mesopotamian, etc.) is extremely flat in places where deposition actually occurs—i.e. the floodplain, where the river starts to meander and produces regular floods. Coastlines, as they build out into the ocean, also produce extensive, horizontal layers. Alluvial and rift basins (Great Basin, Death Valley, etc.) are also very flat, despite the high topography surrounding them. Even deserts, when inundated by the ocean, are preserved as extensive, horizontal layers.
I should be careful how I use the term flat here, because I don’t want to be disingenuous. Every example I cited contains some topography, such as river channels and the like. But these features are also common in the geological record. Channel cuts, karst topography, and even small caves and canyons make regular appearances in geological strata. When you look close enough, no geological layer is truly ‘flat’.
So what about the rest of Earth’s surface—the mountains and canyons that attract hikers, bikers, and rafters from around the world? Did they all disappear from history? Well, in a sense, yes—they did. Large-scale topography only forms in places where erosion is actively occurring. The Grand Staircase (including Grand, Zion, and Bryce canyons), for example, is a product of erosion, and when erosion is removing sediment, the topographical features will not be preserved.
For something like Grand Canyon to be preserved in the geological record, sea level would have to rise at such a rate that the landscape would not flatten out before it became a coastal river plain—a very unlikely situation. If you are skeptical about this process, consider the California coast. Although it is very rocky, wave action is currently ‘smoothing’ out the landscape rather than burying it for preservation.
A counterexample from the North Atlantic
Some parts of the Earth have changed rapidly enough for ancient topography to have been preserved in the geological record. The North Atlantic is the site of an active spreading zone (Mid-Ocean Ridge) as well as a mantle ‘plume’ or ‘hot spot’ (currently fueling Icelandic volcanoes). Heat from this spreading center was sufficient in the early Cenozoic, around the Paleocene-Eocene boundary, to cause an ancient landscape (pictured below) to be rapidly inundated by water and covered with sediment. The ancient river drainage basin is now buried beneath a kilometer of marine sediments.
The landscape was discovered by seismic survey, and reported earlier this summer in Nature Geoscience (article here*). For those unfamiliar, seismic exploration works much like an ultrasound: sound waves are bounced back from the subsurface when a change in rock type occurs. Hundreds of seismic lines (seen in Figure a) are pieced together to reveal a 3D surface (Figure c).
I need not comment further here—the picture really does speak for itself!
*Special thanks to A.H.-R. for bringing this article to my attention.
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