Science in the Scottish Enlightenment
Science in the Scottish Enlightenment

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Science in the Scottish Enlightenment

5.4 Hutton's geology: The Jedburgh unconformity

One concrete example from the Theory of the Earth will perhaps indicate the way in which Hutton could read features of the landscape as evidence of the action of forces acting over immeasurably long periods. He had been geologising in the valley of Jed Water, near Jedburgh, in the Borders area between England and Scotland. From his observations in the neighbouring Teviot valley, he expected the Jed to be running over a bed of horizontally laid, soft strata which were sometimes exposed as sections alongside the river. However, in his own words:

I was surprised with the appearance of vertical strata in the bed of the river, where I was certain that the banks were composed of horizontal strata. I was soon satisfied with regard to this phenomenon, and rejoiced at my good fortune in stumbling upon an object so interesting to the natural history of the earth, and which I had been long looking for in vain.

… above those vertical strata, are placed the horizontal beds, which extend along the whole country.

(Hutton, [1795] 1959, vol. 1, p. 432, my italics)

What Hutton had found was what is now known as an ‘unconformity’: a junction between sets of rocks of quite different types, formed at quite different epochs. The Jedburgh unconformity was sketched by Hutton's travelling colleague, John Clerk, and appeared as a delightful engraving in the Theory of the Earth (see Figure 5).

Figure 5
(Reproduced by permission of the British Library Board.) ©
British Library Board
Figure 5: Unconformity near Jedburgh (From Hutton, Theory of the Earth, vol. 1, Edinburgh, 1795, plate 3.)

How was the unconformity to be explained? Hutton proceeds, in the Theory of the Earth, by eliminating what he considers to be unsatisfactory explanations. For example, it is difficult to imagine that the upper, horizontal strata could have been laid down before the vertical strata beneath them: this would entail the subterranean building of vertical strata which somehow were ‘cut off abruptly’, in a straight edge, at the level where they met the overlying horizontal strata. Hutton rejects a number of other possibilities and then advances his own explanation. The strata which are now vertical were, like nearly all rocks, laid down horizontally, beneath the sea. As they were upheaved to form land, they were twisted into the vertical. Then

by the effects of either rivers, winds, or tides, the surface of the vertical strata had been washed bare; and … this surface had been afterwards sunk [beneath the sea] below the influence of these destructive operations, and thus placed in a situation proper for the opposite effect, the accumulation of matter prepared and put in motion by the destroying causes.

(Hutton, [1795] 1959, vol. 1, p. 435)

That is to say, the upheaved vertical strata had been planed down by erosion, and had sunk again to the bottom of the ocean to become the bed upon which a new set of horizontal strata began to accumulate. Hutton fortifies this suggestion by pointing to the layer of boulders and stones that occur at the intersection of the two sets of strata: they are, he claims, fragments of the lower, vertical series, which became detached during the long period of erosion.

Now, this may all look a bit confusing to a reader unfamiliar with geology, or commonplace to a reader who knows the basics of the science, but it is worth spelling it out, in order to show the confidence with which Hutton could, with perfect equanimity, contemplate the building and erosion of huge landmasses.

In the case of Jedburgh, he postulated the following sequence. There was once an ocean, where Jedburgh now stands, in which collected both the detritus of the neighbouring landmass and the detritus of tiny marine organisms. Horizontal beds of rock, composed of this detritus, were consolidated at the bottom of the ocean. Then, there was a period of upheaval which twisted and raised these beds vertically above the sea, where they were exposed to weathering and erosion for sufficient time for them to be planed down to a level.

A period of subsidence followed, during which the rocks sank below the ocean again. A new sequence of horizontal sedimentary rocks consolidated on the base of the old, subsiding rocks. Lastly, the whole mass was upheaved yet again. Finally, the unconformity revealed itself to Hutton in the spectacular section cut by the humble river Jed. ‘Finally’, though is the wrong word to use, for Hutton said that there is ‘no prospect of an end’, the forces that wrought these titanic changes are still at work and will eventually drastically remodel the Borders landscape.

It took the best part of a century for Hutton's vision, transmitted through later geologists, to be sanctified, as it were, by the elite English culture and embodied famously in the verse of the English Poet Laureate Tennyson in the most widely read poem of the nineteenth century:

There rolls the deep where grew the tree.

O earth, what changes hast thou seen!

There where the long street roars, hath been

The stillness of the central sea.

The hills are shadows, and they flow

From form to form, and nothing stands;

They melt like mist, the solid lands,

Like clouds they shape themselves and go.

(In Memoriam, 1850, section 123, lines 1–8)

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