Professors reorient researchers, history with discovery

By Denise Jones
February 10, 2011

Two professors emeriti from the Department of Earth Sciences have helped add another chapter to the story of a historic geological landmark.

In a paper published in the February edition of the journal Geology, Western's Glen Caldwell and Grant Young, together with Pierre Jutras of Saint Mary's University, reinterpret the classic Hutton Unconformity found on the Isle of Arran in southwestern Scotland. These outcrops of rock inspired Hutton, in the late 18th century, to propose the idea the geological record contains evidence the Earth is extremely ancient. This notion of "deep time," which ran contrary to current opinion, strongly influenced the conclusions of later scientists such as Charles Lyell and Charles Darwin.

The geology of Arran first became famous when, in 1787, Hutton set out to prove igneous rocks were derived from molten material from Earth's interior.

In the mountainous northern part of Arran, he succeeded in finding veins of granite cutting into older rocks, but Hutton also noticed something else near a place called Newton Point. It was already well known that sedimentary rocks were laid down in horizontal layers, but Hutton saw some of the sedimentary rocks there were in a vertical position, cut off and overlain by younger rocks that were almost horizontal.

After seeing this he realized he was looking at the roots of mountains that had been pushed up so their formerly horizontal rocks were deformed into a near-vertical position and the subsequent younger rocks themselves had been slightly tilted and subjected to a lesser cycle of deformation and erosion. The complex series of events resulting in production of an "unconformity" must have involved a very long time.

This was the first formally recognized unconformity, although Hutton did not use that term.
As significant as this discovery was, Hutton did not leave a detailed sketch of the locality. The situation grew more complicated when the third volume of Hutton's "Theory of the Earth" was lost for 100 years or so. It was published in 1899, edited by the director of the British Geological Survey, Sir Archibald Geikie, who added a diagram showing the unconformity lying in an erosional gully at Hutton's locality in northeastern Arran.

The problem: The location of the unconformity was wrong and generations of geologists were misled.

Caldwell and Young had both studied geology together at Glasgow University and were familiar with Hutton's work. Their association with the geology of the area surrounding Arran goes back more than 50 years.

In the 1960s both of them emigrated to Canada. Caldwell took a position in Saskatchewan while Young came to Western. Eventually, Caldwell came here as well to take on an administrative role. When they both retired, Caldwell invited Young to go back and look again at some of the rocks in the Firth of Clyde area.

That was the beginning of an ongoing research project spanning 10 years.

Young already had a hint something didn't quite fit at Hutton's unconformity when he visited Arran in 1974 with a group of graduate students from Western. He saw that up to a metre above the position of Geikie's unconformity the "grain" of the underlying, near-vertical, Dalradian rocks was visible.

Other geologists had suggested that there was development of calcrete in the vicinity of the unconformity. Formation of calcrete involves replacement and cementation of pre-existing rocks by carbonate minerals that are in solution in groundwater but are precipitated because of evaporative concentration, usually in semi-arid climates.

It is similar, but not identical, to how water and cement can turn loose gravel into concrete, but it involves much more time and the chemical processes are different. Formation of calcrete can mask important geological relationships.

As part of their post-retirement work in the Firth of Clyde area, Caldwell and Young visited Arran in 2003, 2006 and 2007 and wrote a paper describing Hutton's unconformity in detail. Their work showed Geikie had placed the unconformity about a metre too low. They also found a second, less ambiguous exposure of the unconformity about 300 metres to the west.

When Young was acting as an external examiner for a PhD candidate at Dalhousie University, he met Pierre Jutras and found they had a shared interest in sedimentary rocks from the Early Carboniferous period (330-360 million years ago) and in the development of calcrete.

Even though Jutras had worked mainly in the Maritimes region in North America, it just so happened that in the Carboniferous period, the areas that are now Western Europe and the Canadian Maritimes were neighbours.

Jutras and the two Western emeriti really had common interests.

Jutras twice visited southwestern Scotland, bringing his specialized knowledge of calcrete formation to bear on the question of Hutton's unconformity. He was able to tease even more subtle complexities from Hutton's type locality and determined that it involved not one but two unconformities. The problem in unravelling the details was due to the development of an unusual massive variety of calcrete known as phreatic calcrete hardpan.

Even though the Geology paper has provided a more sophisticated and complicated interpretation of Hutton's discovery, Young and Caldwell maintain that the fundamental scientific importance of these discoveries has not changed. Hutton's observations and insight provided evidence for long breaks in geologic time and opened the door to an appreciation of the vast amounts of time involved in the evolution of our planet.

This finding, in a remote corner of a small Scottish island, played a pivotal role in the development of subsequent scientific ideas that influenced the world, perhaps the most significant of which are those of Charles Darwin.


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