37.1: Uniformitarianism vs. Catastrophism

Figure \(\PageIndex{1}\): A rectangular canyon, called a ‘coulee,’ in the Channeled Scablands. (CC-by; Photo by Pictoscribe, via Flickr)

We have to start our story with the backstory. This was a philosophical debate, the tension between two competing ideas: uniformitarianism and catastrophism.

Before geology was established as a scientific discipline in western Europe in the mid- to late-1700s, the prevailing intellectual paradigm was based on a literal reading of the Bible, specifically the book of Genesis. Many people will be familiar with the story of Noah’s flood, one of many similar flood myths in different cultures. A global flood would be a great catastrophe —indeed, that was the point— and the thinking went that it might be responsible for moving a lot of sediment around from one place to another. Attributing geologic change to a flood (specifically to “The” Flood) was the ideology called catastrophism. This was palatable to those thinkers who anchored themselves firmly in the Christian faith, which was almost everyone in western Europe during the 1700s. Because of this, catastrophism is frequently invoked alongside creationism, the idea of a divine origin for the world. It is a favorite go-to explanation today among young-Earth creationists, those who think the Earth is far younger than most geologists do. For our purposes, the key thing to note is that broadly construed, catastrophism argued that geologic change was sudden and violent.

Figure \(\PageIndex{2}\): Portrait of James Hutton by Sir Henry Raeburn (Public domain via Wikimedia Commons.)

Then came the new discipline of geology. Early geologists found evidence that was inconsistent with all change being catastrophic. Foremost among them was James Hutton, a Scottish intellectual and farmer. Hutton saw the creeks draining his farm turn brown and turbid after a heavy rain, and inferred that some of the soil on his fields was being lost to the sea. He envisioned what must happen to that sediment after it coursed out of sight. When the creek flowed into the sea, that sediment would settle out and make a thin layer over the seafloor. Repeated thousands or millions of times, this simple, inefficient process could eventually denude the land and carpet the bottom of the ocean with vast layers of sediment. He inferred that the many layered sedimentary rocks must have formed in just this way, a little bit at a time, in enormous horizontal sheets. The present day’s slow and gradual change wasn’t just the ‘calm after the storm Flood,’ it was the way that change had happened over eons, producing and erasing the geologic record through slow, gentle, gradual processes playing out over vast amounts of time. Repeated cycles of uplift and erosion at a given location were interspersed with times of deposition, burial, and lithification.

Figure \(\PageIndex{3}\): The angular unconformity at Siccar Point shows vertical Silurian graywacke strata overlain by Devonian redbeds, themselves at a non-horizontal angle. (Callan Bentley photo.)

Taking the next step from this insight, Hutton articulated the significance of the angular unconformity at Siccar Point, Scotland, where two packages of stratified sandstone are juxtaposed at two different orientations. Hutton realized that it would take six steps to make this distinctive outcrop: (1) deposition of gray mud and sand strata horizontally on the seafloor, (2) lithification of those muddy sands into graywacke followed by rotation of the layers into a vertical position, (3) erosion of some of those layers — presumably above sea level where erosion is easiest to accomplish, (4) a new round of deposition of horizontal sediment, this time highly oxidized red mud and sand — but still horizontally, (5) lithification of those new sediments into solid sandstone and shale, followed by rotation of those layers slightly, and (6) modern erosion of both rock types. That’s a lot of detail to extract from one outcrop, and Hutton reckoned that going through all those steps must have taken a good long while. He thus established that geologic time must be fairly vast, fairly “deep.” Siccar Point holds a special place in the hearts of geologists, since it was the place where “deep time” was first fully appreciated.

Since Hutton’s insight, geologists have successively pushed our sense of the age of the world back further and further. William Whewell dubbed Hutton’s big idea uniformitarianism, and Charles Lyell promulgated uniformitarianism far and wide in his best-selling textbook The Principles of Geology. Charles Darwin read Lyell’s book, and used this newfound sense of an immensely old Earth as a necessary pre-condition for his own ideas about natural selection driving evolutionary change in living organisms.

Figure \(\PageIndex{4}\): Uranium/lead ratios contained in tiny crystals called zircons are used to date ancient rocks. (Photo: Michael John Cheadle, University of Wyoming)

Later still, the discovery of radioactivity finally gave geologists a “clock” they could use to quantitatively determine when certain minerals formed. Before this technique was a few decades old, it had put an age on the planet that exceeded a billion years. Modern estimates place the planet’s formation at 4.566 Ga: plenty of time for slow, gradual change to dominate the geologic record. Uniformitarianism now had all the wiggle room it needed to explain Earth history. Geologists were now almost unanimously uniformitarian in their outlook. Catastrophic hypotheses smacked of young-Earth creationism, and that was plainly unacceptable to professional geologists. (An excellent exploration of flood mythology, creationism, and uniformitarianism may be found in the book The Rocks Don’t Lie, by David Montgomery.)

This was the situation that J Harlen Bretz walked into, and he had some unwelcome news for his colleagues: he had found geologic evidence of an actual catastrophic flood.