Last April I had the pleasure of leading a Science Festival excursion to the Hermitage of Braid and Blackford Hill, in the course of which Agassiz Rock in Blackford Quarry was visited, and its international significance explained. Unfortunately this significance is not mentioned or even implied on the commemorative plaque placed on the rock in 1993 by Scottish Natural Heritage, nor was it mentioned in the two previous plaques that had been put up there. The wording on the current plaque is as follows:
 

The earliest plaque was fixed on a post just in front of the rock, in about 1920, by our Society. It read:
 

Some time about 1960 a second plaque was put up, also by the Society (the earlier one having disappeared), which read:
 

As an example of ice action in smoothing, striating and (in this case) undercutting the crag, Agassiz Rock is a good example, clear and interesting. But better sites abound in Scotland, many of which were seen by Louis Agassiz in his 1840 visit. What makes the Blackford Quarry rock face so important and significant, not merely locally or even nationally, but internationally, is that it marks the first recognition in the world of the reality of former ice sheets where now there is no ice.

It is often forgotten how revolutionary was the Agassiz hypothesis of an ice age with countrywide, if not continent-wide, ice sheets: a revolution in geology on a par with plate tectonics. We are now so familiar with the concept of an ice age that it takes some imagination to realise that, before the work of Agassiz, the concept had not even been thought of. Apart from valley glaciers, all the phenomena such as boulder clay, roches moutonnÈes, erratics, eskers, kames and striated bedrock, which now we recognise as due to ice action, were then ascribed to catastrophic floods with rushing waters thousands of feet deep. This near-impossible idea was only acceptable by appealing to scriptural warrant in Noah's flood.

Louis Agassiz (1807-1873) was born in northwestern Switzerland, at Motier. He is reported to have had a charming personality, radiating optimism and friendship. Being fluent in six languages helped his studies at Lausanne, Zurich and Heidelberg. Following an invitation by the explorers Spix and von Martius to study their Amazon fish collections, he specialised in the study of fish, both fossil and living, becoming a world authority in this field.

This brought him to the attention of William Buckland (1784-1856) who, in 1813, was appointed Professor of Mineralogy at Oxford University and, six years later, Professor of Geology. He was one of the great eccentrics, but likeable, candid, good-humoured and free from prejudice. He emphasized the importance of the love of truth in unbiased, factual observations and the deductions to be made from these. In 1834, he invited Agassiz to study the fossil fish collections in Britain and in the course of this visit, he met Hugh Miller and studied his Old Red Sandstone fishes.

As a relief from fish, Agassiz spent some summer vacations exploring Swiss glaciers, where it was common knowledge that they did not extend as far down their valleys as formerly. The ice-free portion of the valley yielded ample evidence of former ice in smoothed and striated rock surfaces, erratics and moraines.

In 1838, Buckland visited Switzerland where he examined this glacial evidence in company with Agassiz, and quickly realised that he had seen similar evidence in ice-free Scotland. When Agassiz was told this, he made the crucial intellectual leap from mere valley glaciers to the idea of countrywide or even continental scale ice sheets.

Buckland quickly accepted Agassiz's thesis, probably with great relief after trying to explain the evidence by appeal to impossibly great floods. As one wag put it:
 

All was darkness once about the flood, till Buckland rose and made it clear as mud.

Two years later, in 1840, Buckland invited Agassiz to Scotland for the British Association for the Advancement of Science meeting in Glasgow, where Agassiz gave papers on ice and on fish. Afterwards, the two friends toured the country through Argyll, the Great Glen and eastern Scotland, everywhere noticing the evidence of ice action.

At Fort Augustus on October 3rd Agassiz wrote a letter to Professor Jameson in Edinburgh outlining his theory of a former ice sheet countrywide across Scotland, for publication in the Edinburgh New Philosophical Journal. As the current number had just gone to press, Jameson, who was sympathetic to Agassiz's idea, and immediately recognised the significance of his theory, passed the letter to Charles Maclaren, editor of The Scotsman and amateur geologist, who published it on October 7th, not only appreciating its geological significance but also that it gave him a fantastic journalistic scoop as first with news of this revolutionary theory. If Agassiz was right then centuries of belief in diluvial theories based on Noah's Flood had to be abandoned: one of the most telling blows against science's reliance on scriptural evidence.

Meanwhile, Agassiz continued his tour, eventually reaching Edinburgh. Maclaren and other Scottish geologists on 27th October took Agassiz to see various smoothed and striated rock surfaces on Salisbury Craigs, Castle Rock, Craigleith Quarry, Corstorphine Hill and finally Blackford Quarry where, Maclaren records, 'we accompanied M. Agassiz [round Edinburgh]; he had expressed doubts as to some other supposed marks of glacial action near this city, but on seeing those at Blackford Quarry, he instantly exclaimed, "That is the work of ice!" '

Despite Agassiz's international standing, it took nearly twenty years before his ice-sheet theory became generally accepted. In fairness to Agassiz's critics, they were, as they saw it, being asked to replace a near-impossible Flood hypothesis with an equally impossible general refrigeration. And it must also be remembered that in the mid-nineteenth century no geologist had studied either the Greenland or Antarctic ice sheets.

Studying Swiss glaciers with Buckland and hearing about the ice-action evidence in ice-free Scotland was the catalyst for Agassiz's concept of extensive ice sheets, and prompted him to make his 1840 tour which finished in Blackford Quarry. So the international significance of this spot is not in its being yet another, though good, example of ice-scratched and smoothed rock, but that it marks the first recognition in the world of the reality of former ice sheets where now there is no ice.