On the Marine Origin of the Parallel Roads of Glen Roy
Authors: Watson R. Boog 
Published: October 2009

The Parallel Roads of Glen Roy have been described by several observers, who have also offered explanations of the manner of their formation. In the author's opinion, Mr. Robert Chambers has solved this problem in his ‘Ancient Sea Margins,’ and in this paper he supported the view there advocated, and offered some objections to the theory of an ice-dam, originally put forward by Prof. Agassiz, and recently illustrated by Mr. Jamieson.

After noticing the strong points of the ice-dam theory, Mr. Watson remarks that, though Prof. Agassiz and Mr. Samieson agree in supporting it, yet they differ so much respecting details that they mutually weaken their cause*. He then considers the specific objections to the theory, especially the existence of terraces similar to the Parallel Roads, though less perfect, at all levelsroundour coasts, as well as inland, along every fjord in Norway, and across the whole of Sweden; these, he says, cannot be accounted for by the ice-dam theory, which therefore treats them as something distinct, whereas they are specifically the same, differing from the Glen Roy Roads only in being less perfectly preserved. Mr. Watson also remarks that, even in Lochaber, there are banks of water-sorted gravel at various heights and unconnected with any “col”—a fact which cannot be accounted for by the ice-dam theory; and he mentions particularly two short fines between the two highest roads in Glen Roy, and one in Glen Gluoy at 960 feet, which do not correspond with any “col,” also several instances in and near Glen Collarig. He then describes an extraordinary series of terraces on Ben Clinaig, and draws attention to certain gigantic flat-topped heaps of watersorted gravel, whose levels do not correspond with those of any of the terraces. Mr. Watson therefore asks, What can the ice-dam theory do with all these ? He also states that if the cold was severe enough to supply the ice needed for such a dam, it was too cold in Lochaber for lakes, and vice versâ. Glaciers, he remarks, are mere tongues of ice projecting beyond the snow-line, and the area which these tongues occupy is minute compared with that occupied by the body—the snow-field which contains them; but the ice-dam theory reverses this relation, and makes the snow-body minute and the tongues gigantic. Mr. Watson also considers that it was impossible for the ice to be present at the places indicated, as no glacier from Ben Nevis could reach Glen Roy, and no independent glacier could be formed in Glen Spean. Another objection is, that no place can be found for the ice-dam theory between the glacial epoch and the present day—a consideration which the author discusses in detail, and he concludes his objections by urging that, as the sea has been all over the Lochaber district to a height of 2000 feet, there is no reason for seeking any other agency for the formation of the terraces, nor for introducing such a complex machinery as the ice-dam, where none is needed.

In supporting the “Marine” theory, on the grounds that the sea has been on the spot, and is capable of performing the work required, Mr. Watson gives the following sketch of the manner in which he conceives the "Parallel Roads" to have been formed:— “We have the land standing probably, in the first instance, rather higher than now, but gradually sinking, though perhaps with pauses. The climate resembled that of North Greenland. The land was swathed in thick ice, which was ever settling downwards to the sea, under that law of regulation which gives to ice its true viscid or plastic character. The whole surface of the rock was being moutonnéed and striated. The boulder-clay resulting from the destruction of the earlier soils and looser weathered rock-surfaces was being shot out in heaps beneath the half-floating ice-foot. The finer clays and older shell-beds are the produce of the somewhat later and more land-sunken time, when the comparatively cleared surface of the rock and the shrunken glaciers no longer supplied the same mass of débris as before. The gales of spring from the S.W. came to break up the ice of winter where it lay on the level shallows, such as the long flat of Glen Spean, east of Loch Treig valley, and piled up the blocks of rock which the ice contained into those strange and monstrous sea-beaches (see Jamieson's map)which have been erroneously described as moraines. And, finally, dependent on the sheltered nature of the locality, on its exposure to the prevalent winds, on the character of the hill face, on the supply of débris from above, on the extent to which particular spots at the sea-level were already clothed with detritus under the influence of the varying currents pouring backwards and forwards over the cols at the glen heads—perhaps, too, to some extent, acted on by the ice-cake—subject to all these influences the terrace-lines were formed along the slopes. Sometimes they were swept away again almost before formed, or were spared only for later destruction; but, in the case of the sheltered fjord of Glen Roy, each terrace in turn was carried down with the subsiding land, and protected from injury below the sea in the quiet of the inland fjord, while the next terrace above it was being formed in its turn.”

That there are some difficulties in the way of the “Marine theory” Mr. Watson does not deny, the greatest, perhaps, being, in his opinion, the marvellous perfection of the Glen Roy terraces when compared with anything similar there or elsewhere. He also discusses two other objections, being all that he thinks need consideration. The first of these is, that the horizontality of the “roads” is opposed tot he idea of their having undergone subsidence and re-elevation; but Mr. Watson observes that there is no reason to suppose that any disturbance of relative levels should be exhibited when areas of thousands or hundreds of thousands of square miles are undergoing secular oscillations, and he cites in illustration the beach-lines along the Norwegian fjords. The other objection has been urged by Prof. Agassiz as his greatest difficulty in accepting the “Marine theory,” namely, that the terraces present “no traces of organic life“ Mr. Watson answers this by quoting several examples of Quaternary marine deposits which are also entirely barren of fossils; and he states that it would be surprising if fossils had been found in the Parallel Roads, as he has noticed that there is, from some cause or another, an extraordinary gap, utterly devoid of life, so far as we know, between the most recent of the old glacial beds and the very oldest of the recent deposits.The general character of the Scandinavian Quaternary beds he has found to correspond with that of the Scotch deposits,“ and they also agree in this, that while true glacial shell-beds are to be found from the sea-level, and below it, up to 500 feet, or a little more, above it, there they cease altogether. The higher stratified deposits are utterly devoid of a single organism, and it is not until we descend again to the height of 200 feet above the sea (in Scotland lower still) that we again begin to find fossils, in beds overlying the former; but in the shells of these later beds the glacial character has disappeared altogether.” He holds, therefore, that the absence of organic remains from the terraces of Glen Roy simply accords with the character of similarly placed beds in every other locality, both in Scotland and in Scandinavia.