Different rocks: the birth of geology

 Around 1670, Nicolaus Steno (1638 – 1686) spelled out a set of basic principles of geology which spread fast: by 1671, there was an English translation available. Here is a modern version that conveys the two laws Steno left for us:

The Law of Superposition: in a sequence of strata, any stratum is younger than the strata on which it rests, and it is older than the strata that rest upon it.

The Law of Original Horizontality: strata are deposited horizontally and then deformed to various attitudes later. That is, undisturbed true bedding planes are nearly horizontal.

Cross bedding Malabar, Sydney,
beds laid down in a sandbank at
a ~30º angle.

(It would seem that Steno never saw cross bedding like that shown on the right.)

When James Hutton found an angular unconformity at Siccar Point in Scotland in 1788, the sloping beds he saw had once been horizontal. This is a place where one set of horizontal sediments had been uplifted, folded and eroded, carved away, before other sediments were laid down over them. Below, you can see Hutton’s unconformity: the upper layer is the famous Devonian Old Red Sandstone, sitting unconformably on Silurian greywacke. The two rock types were different.

Hutton’s Siccar Point Unconformity,
Siccar Point, Berwickshire,
Scotland. [Wikimedia Commons].

An unconformity is a place where there has been a break in time, where the upper rocks fail to conform to the ones below. Seeing this led Hutton to believe that the earth was very old, but on theological grounds, he rejected the idea that a divine Creator would make an earth which would wear out, so he looked for a mechanism of renewal. In his view, the planet was some sort of perpetual motion machine. And so we got the uniformitarian principle, the idea that the forces now operating to change the earth’s surface have always operated in the same way. There were no catastrophes, said Hutton, just slow, steady change.

The result, therefore, of this physical inquiry is, that we find no vestige of beginning, no prospect of an end.
—James Hutton, Theory of the Earth, 200.

So what does an unconformity look like? As part of the work for another book (Mistaken for Granite), I set out to locate points where the bottom of the Sydney Basin (Triassic and Permian rocks) sat unconformably on the underlying older rocks. I know several places where the boundary can be seen. One is at Myrtle Beach, south of Sydney, where you can see the tilted metamorphic rocks below, and more or less horizontal rocks above. The gap is from Permian above to Ordovician below.

To set the scene, Australia is old, and at some time before the Permian, the surface of the land was Ordovician and Devonian rocks that had been heaved up, pushed around and eroded. Then during the Permian, part of the continent sank below the sea, and sediments started to be dumped on the old rocks below. Unlike the old rocks being buried, the Permian rocks still keep their horizontal strata that they were laid down in, and so were the Triassic rocks that later covered the Permian beds.

To geologists, this hand on the rock at Myrtle
Beach spans a gap of about 200 million years.

So how big is the gap? The Ordovician era, according to the geological time scale, was 485 to 444 mya (million years ago), while the Permian was 299 to 251 mya. So if the Ordovician rocks beneath were laid down at the close of business on the last day of the Ordovician, and the Permian rocks were laid down on the first morning of the Permian, the gap is 145 million years. At the other extremes, the gap might be 234 million years: on average, it is probably a gap of some 200 million years.

Budawang Ranges: the top is Permian conglomerate, over Devonian metamorphics, tilting ~20º to the left.

There is also another place inland, in a valley of the Budawang Ranges, where you can reach the absolute bottom of the Sydney basin. The Devonian era was 416 to 359 mya, while the Permian was 299 to 251 mya. So if the Devonian rocks beneath were laid down on the last day of the Devonian, and the Permian rocks were laid down on the first morning of the Permian, the gap is 60 million years. If we take the other extremes, the gap might be 165 million years: on average, it was probably a gap in the record of around 100 million years.

In 1785, James Hutton discovered a number of pink veins of granite, pushing their way up into the dark schist above, and this was the first record of dikes. All the igneous rocks form when magma cools: granite cools slowly, deep down and forms large crystals, basalt cools faster near the earth’s surface and has no visible crystals. Dikes arise when molten rock pushes up into cracks in the rocks above, and that is contrary to the idea that rocks are laid down in horizontal layers. Charles Lyell made much of this.

Dyke near Mt Etna, from Charles Lyell’s
Principles of Geology (1834), volume 3.

Suddenly, about 200 years ago, the world of rock-hounds was hit by a flood of apparent contradictions, observations that demanded a wholesale rethink. Just like climate change, new ideas were suddenly there—though was climate change really such a surprise?

We'll come to that next time, but rest assured, scientists are very good at spotting differences!