Published 9:27 am Wednesday, April 7, 2021

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Part 8: The Sands of Time.

Burt Carter

James Hutton – Soil Scientist

When James Hutton realized his farm was washing down streams, and determined that it was headed to the sea, he also began wondering where it came from in the first place. Examination of the sediment moving down his rills and small streams showed it to be suspended mud, with sand and fine gravel bouncing and rolling along the bed, just as we see in Providence Canyon today. Examination of his soil showed it to be precisely the same material, all mixed together.

Clearly the wasting of his land and fertility had been going on for time out of mind and yet his soil, probably noticeably thin in places anyway, was not completely gone. Is there a way in which soil is made in one place, to replace what has eroded and gone to some other place?

He found his answer at the village of Jedburgh (Figure 1), south of his lowland farm Slighhouses. Here, in a roadcut leading south out of the Jedwater valley, he saw layered bedrock, a rock he called “schistus”, at the bottom of the cut. (This was not the metamorphic rock schist. The root simply means “layered” or “split”). At the top of the cut was a horizontally layered rock. In between was a crumbly transition zone of clay, sand, and gravel-sized pieces of schistus. Examination of the schistus showed it to be hard, alternating layers of shale (made of mud) and sandstone (made of sand, of course). The rock fragments in the transition zone were softer than their counterparts in the lower part, and both sediment types crumbled easily in the hand to mud and sand, just like the soil on his farm. The rock in the bottom part of the exposure was exactly that – hard rock. The top of the schistus was “weathering” to soil.

Clearly exposure of the rock to the elements led to loss of cohesion and production of loose, finer grains. Hutton could “see”, or at least infer, the process entirely in this one outcrop. As far as I know, Hutton was the first westerner to consider and describe the process by which our most precious resource (after the rainwater that produces it) arises.

This discovery must have been a relief to Hutton, for a time at least. He didn’t have to worry about all of his soil washing away because the rain that did the washing also made new soil. But then he realized that this only partly solved his problem.

The rain did not just take away soil, it also took away elevation. Even though the farm looked the same to him as it did when he was a child (and presumably his father, and his father, back many generations could say the same) Hutton realized that you can’t take sediment away from a place and leave it the same, just as the removal of sediment from Providence Canyon has left a huge hole in the ground. At the surface of Earth the loss of mass means the loss of elevation as well. It is the top of the soil that leaves each spring, sure as the seasons. The farms were getting a few millimeters closer to sea-level every year. Projecting forward there would come a time when there would be no Scotland, nor anywhere else, unless there was also a way to create elevation; a way to uplift the land. Hutton had a new thing to look for, and he already had part of an answer, thanks to Nicholas Steno!

James Hutton – Historical Geologist

Recall Steno’s Principle of Original Horizontality. Sediment is deposited initially in unlithified, flat layers, then later hardened to rock, a process Hutton also first studied. But if you look back at Figure 1 you will see that the layers of schistus at Jedburgh are definitely not flat now. Something has stood them almost exactly on end!

Let’s consider the consequences of such rotation. The originally flat rocks sat upon solid earth, presumably all the way down as far as we wish to consider, just as a flat book, full of pages, sits on a table or floor. If we then rotate the book so that it and its pages are vertical, like the layers of schistus, then the highest part of the book is higher than its original position. There has been “uplift” of the book in the experiment, or the land in the world it was meant to model.

Hutton would point out that the book is not an ideal model for the process because its pages already have edges, cut into it before we ever laid it on the table and started the experiment. The schistus, as Steno had pointed out, would have to be eroded to the edges of the beds after it was uplifted, implying even more original elevation than is immediately obvious in the experiment. Not only did Hutton “see” the soil forming at Jedburgh he “saw” the history of it forming and washing away back to the time when the beds were in their original, laterally continuous condition. Time and history were written into the outcrop by the processes that they allowed to happen.

But what caused the uplift? Hutton had an idea for this as well, though it flew in the face of the prevailing notions about rocks – that they are all sedimentary and that they could be used to make a time scale.

The time scale in use at the time (Figure 2) was both very simple and thoroughly flawed. It was based solely on the Principle of Superposition and an incorrect interpretation of how the rocks in the Alps appeared to “obey” it. The “Primary” rocks, what we would now call igneous and some high-grade metamorphic rocks, were thought to have been precipitated like salt from an ocean originally much different in its chemistry than the one we know. We understand this to be impossible now – water simply cannot hold enough of the right elements to have done that. But people at the time were not particularly sophisticated chemists and they did not understand this.

By the “Secondary Period” much of that odd chemistry had been exhausted, in the conventional wisdom of the day, and so the rocks look like modern sediments, but much harder, or even metamorphosed to low grade. The waters could still precipitate strong cements to lithify rocks well.

By the “Tertiary Period” even that was on its way out, and the rocks did not get nearly as hard. I have often used a small handsaw to cut “Tertiary” limestone down to a manageable size – an idea I got from the early residents of Marianna, FL who made their chimneys out of limestone blocks cut in just such a way from local quarries. This could be done to a “Primary” limestone, but I wouldn’t want to waste either the time or the sawblades.

Today, in the “Quaternary Period”, sediment is not lithified at all. Or so people thought at the time. Spark plugs and Coke® bottles have been found in beachrock in the Bahamas, placing the timing of lithification firmly in the 20th century.

Hutton the chemist had a different idea. He knew that precipitation is not the only way to make the crystals of a rock. Freezing a liquid would work too, but the liquid from which the minerals of igneous rock formed would have to be very hot indeed. They would have to be so hot that they, like the vaguely hot air in a hot air balloon, would create buoyancy that could uplift the overlying rocks. Being a good scientist he treated this as hypothetical and went looking for a way to support or disprove it.

He found it on a friend’s farm at Glen Tilt (Figure 3). A large mass of granite in the deepest part of the valley had sent thin stringers up into the overlying sedimentary rock: intrusions of “Primary” granite cross-cutting the overlying “Secondary” rocks, and therefore younger than they. Pieces of the “Primary” rocks were included within the Secondary. The Principles of Cross-Cutting Relations and Inclusions apply, not Superposition. Hutton had found what he thought was his uplift mechanism, though it turns out that the igneous rocks and the uplift are both consequences of a more fundamental process. He also destroyed what people had been assuming was a useable geologic time scale.

It was not the only revolution in our thinking about time that he caused.