How is it that certain sedimentary rock layers can be traced for hundreds to thousands of miles across large continents, unless these land masses were once covered by a global flood?
Perhaps you never thought to ask, but this question reflects a common misunderstanding among Young-Earth Creationists. Recently, Answers in Genesis revisited the topic on their Twitter feed by featuring a 2008 article by Dr. Andrew Snelling, entitled Transcontinental Rock Layers. Therein, the non-technical article accurately explains that geological formations like the Tapeats Sandstone, Coconino Sandstone, and Redwall Limestone (all of which outcrop in Grand Canyon) can be traced “right across North America”. Additionally, Cretaceous chalk layers span much of Europe, while several Carboniferous coal beds appear on multiple continents. According to Snelling, “God’s judgement is clearly seen” in the widespread deposition of these rock formations, because:
As Noah’s Flood catastrophically swept over all the continents to form a global ocean (described in Genesis 7–8), we would expect the waters to deposit fossil-bearing sediment layers rapidly across vast areas around the globe. And that is exactly what we find—further evidence that the global cataclysmic Genesis Flood was an actual event in history, just as God has told us in His eyewitness account of earth’s history.
To those of us who cherish the message of Noah’s Flood, recorded in Genesis 6–9, it is difficult to escape the confirmation bias evoked by Dr. Snelling’s reasoning. If the famed tale of judgement and redemption was recorded for us in the rocks beneath our feet, then the story becomes accessible in a very tangible way. Suddenly, we can reach out and touch the word of God, so to speak, or—at the very least—use it as a scenic backdrop to our family vacation photos.
What Snelling’s article gratuitously assumes, however, is that extensive sedimentary rock formations are somehow contraindicative to our conventional understanding of Earth history. But if these sediments were deposited over tens of millions of years, then transcontinental rock layers are precisely what we’d expect to find, because sedimentary basins often do cover vast parts of the continent. Let’s take a look at some examples using GoogleEarth:
Beginning close to home (for most of you), take note of the light-blue shaded ocean along the eastern coast of North America. Though covered by ocean, the water is relatively shallow, and this region—called the continental shelf—is still part of continental North America. It also represents what is called a passive margin and is major site of deposition for sand and silt, which is carried to sea by rivers along the east coast. Over time, this margin tends to build out (or prograde) into the ocean, creating vast and continuous layers of sand, silt, and occasionally limestone. Millions of years in the future, these layers could be crumpled into a mountain range or uplifted into a plateau, much like that found near Grand Canyon, given the right sequence of tectonic events.
Within the continent are many areas where no deposition is occurring, such as mountain ranges and high plateaus. However, there are several large sedimentary basins, such as the Mississippi River Valley, which are actively accumulating sediments that can be traced over hundreds of miles as a single geological formation. These basins also tend to be very flat, which explains why many sedimentary strata appear as horizontal layers in the rock record: the accumulation of sediments occurs only within (nearly) flat basins and not along the rugged high ground.
Off the eastern coast of South America, we find a similar depositional area that spans nearly 4,000 kilometers. This coastline is an extension of the passive margin of eastern North America.
Along the Caribbean coastline, we find active deposition primarily of carbonate rocks, such as limestone. Over the past few million years (when the Caribbean Sea took recognizable form), lime and shale sediments have been prograding into the sea to form sedimentary layers that are now miles thick.
Another famous example of a continental-scale sedimentary basin is the Sahara Desert, which is accumulating millions of tons of sand over a continuous layer. This modern basin serves as an analog for understanding formations like the Coconino and Navajo sandstones in the Colorado Plateau. Just like those ancient formations, which contain a mixture of cross-bedded sand and sporadic beds of shale and mudstone, the Sahara is not homogenously covered by large dunes, but is dotted with tiny drainage basins (ephemeral streams and lakes) and oases.
One of the best examples of a transcontinental rock formation in progress comes from Snelling’s own backyard. The Australian coastline is surrounded by thousands of miles of shallow-water continental shelf (light blue regions), which are accumulating sand, silt, and lime mud into horizontal layers for future geologists to dig up. The Great Barrier Reef also provides a classic example of coral-bearing limestones that parallel the shoreline, as we commonly find in Phanerozoic rocks across the world. Finally, much of the inner continent is comprised of vast deserts, in which terrestrial bodies of fossiliferous sand, silt, and limestone are actively forming.
Perhaps the most stunning examples of vast sedimentary basins, in terms of areal extent, come from the Eurasian continent its Arctic/Pacific coastlines. Here, far more of the continental shelf is submerged (especially during interglacial periods), creating a much larger area for sedimentary accumulation. Additionally, some of the world’s largest river systems deliver characteristic sediments to the shelf throughout the year. During glacial intervals, sea level falls and exposes much of the shelf, which results in erosion (flattening of the uppermost layers) and progradation of the river plain out toward the sea.
Inland, much of the Eurasian continent is very near sea level and very flat. Currently, river floodplain sediments are actively accumulating across its surface, which will one day be recognized in the rock record as thick siltstone and shale formations with lenses of cross-bedded sand. During warmer periods in Earth history, however, much of this region was covered by seawater to form broad epeiric seas. If you’ve ever wondered where Russia got its massive oil and gas reserves, now you know!
What do transcontinental rock layers tell us about Noah’s flood?
In a word, nothing, despite that Dr. Snelling presents them as geological evidence #3 in favor of a recent global flood. His claim is wholly unsubstantiated, because conventional geological theories explain these strata perfectly well. Additionally, Snelling misleads his audience by presenting formations like the Tapeats Sandstone and Carboniferous coal beds as somewhat homogenous bodies of sediment, which might be characteristic of a single depositional event. But while these formations can be traced over long distances in the rock record, they are far from homogenous and have left clues that preclude a catastrophic global flood from their long history.
Most of these formations (such as the Tapeats) are elongate, since they parallel what used to be a coastline—much like what you see in modern examples above. Upon close examination, we often find evidence that the water currents were perpendicular to that coastline (e.g. from cross bedding or ripples). Tracing the formation closer to the coast, the sandstone transitions horizontally into shale, because the water became deeper in that direction (toward the left, in the figure above). Tracing the formation in the opposite direction, the sandstone transitions into metamorphic and igneous rocks from which the quartz minerals were being weathered and eroded into sandy sediment. In other words, these transcontinental rock formations commonly represent ancient continental shelves; they are not remnants of a global flood. By interpreting those ancient environments and used magnetic data to plot their positions through time, paleogeographers such as Ron Blakey have compiled time-slice images of Earth’s surface at various stages in its history. As you look through these reconstructions, it will become immediately apparent how extensive sedimentary strata could have accumulated over large parts of the continent, a few million years at a time.
Featured image by Bernard Gagnon (via Wikimedia Commons)