May 29, 1879] 



NATURE 



"3 



Glen AA-aig blocked up Glen Gluoy, the glacier from Glen 

 Treig formed a barrier to Glen Roy. 



The "roads" were, he considers, formed by long-continued 

 shore action at each successive level of the lake, that level being 

 determined by the height of the cols over which the lake waters 

 escaped. 



To these views it has been objected, by Mr. Milne-Holme and 

 others, that it is difficult to conceive the glens to the north of the 

 Spean Valley to have been filled with water while at the same 

 time those on the south were filled with ice, and he advocates a 

 detrital barrier formed of clay, sand, and gravel, by marine 

 origin, when the sea stood some 3,000 feet higher than at 

 present. 



Prof. Nicol, also, has pointed out that, had lakes existed in 

 Glen Gluoy, Glen Roy, and Glen Spean for the length of time 

 required to form the "roads" by erosion, and to accumulate the 

 deltas, the cols by which their surplus waters escaped during 

 those periods must have cut a channel in the rocks in the same 

 way that the rivers (which now represent the same drainage, or 

 probably less) have since excavated their channels in the present 

 valleys ; whereas, although there are indications of water-wear 

 in the passes, nothing like a defined river channel exists. Prof. 

 Nicol attempted to explain the facts on the theory of the 

 ' ' roads " being sea beaches. But the absence of coiTcsponding 

 beaches outside those glens — the limitation of the highest 

 "road "to Glen Gluoy — and of the second and third to Glen 

 Roy, and the total absence of marine remains in any of the 

 various drift beds, renders the marine hypothesis inadmissible. 



Sir John Lubbock, looking at the form of the "roads" which 

 has been described by MaccuUoch as parallel layers applied in 

 succession to the sides of the hills, contends that such a form is 

 incompatible either with the heaping up of materials on a shore 

 line, or with their removal by erosion, as in the one case a notch 

 and in the other a projecting ledge in the hill side would be 

 formed, whereas, with one exception of one superior talus 

 pointed out by MaccuUoch, no such structure exists. Sir John 

 points out that a parallelism between the slopes may, however, 

 have been formed by wavelet action, in consequence of the 

 detrital matter taking, as it successively fell and was removed, 

 the same angle of repose as that which the detrital slopes origi- 

 nally had, that angle being the same in water as in air. 



Besides these objections to Mr. Jamieson's hypothesis, which 

 the author considers valid, he points out the difficulty of conceiv- 

 ing that the Arkaig glacier could have ascended the hUls at the 

 entrance of Glen Gluoy to a height of not less than 1,200 feet, 

 while at the same time a pass existed at the head of the glen 

 only 500 feet high, which presented a [ready outlet to the west 

 coast. 



It is a question also whether active glaciers such as Mr. 

 Jamieson requires coiild have formed permanent dams to the 

 large bodies of water pent up in Glen Gluoy and Glen Roy. 

 Glacier lakes are occasionally formed in the Alps, as in the 

 instance of the Margelen See ; but they never last many seasons. 

 The glacier is constantly on the move, and so long as it presents 

 an unbroken front to the lake, so long is the barrier efficient, 

 but when in the progress of the glacier a fissured mass of ice 

 comes forward, the water at once escapes with greater or lesser 

 rapidity, and cannot again accumulate until the defective ice has 

 travelled past or the leak is repaired by winter frosts. 



Equally difficult is it to imagine the existence of such vast 

 glaciers as those of Glen Arkaig and Glen Treig,"while the oppo- 

 site glens of the Gluoy and Roy remained free from ice. The 

 difference in the height of the hills is too slight to allow of so 

 gp-eat a variation in the level of the snow line, and the cause sug- 

 gested by Mr. Jamieson, viz., a great difference in the rainfall 

 such as it now obtains in tliis district, can scarcely be maintained, 

 for, although the annual fall at Fort William is 86 inches, and 

 at Laggan 46 inches, the rainfall at Roy Bridge has now been 

 found to be as much as 62 inches. Further, for the argument 

 to be of any value, it should be shown that in the country further 

 eastward, where the .rainfall is much less, there was the like 

 absence of glaciers during the second period, whereas Chambers 

 and other geologists, including Mr. Jamieson himself, have 

 shown that during that period local glaciers descended from 

 every ^mountain range approaching or exceeding 3,000 feet in 

 height — a height attained by the hills to the north as well as by 

 those to the south of the Spean. 



With respect to the so-called "deltas" of the Turret in Glen 

 Roy, and of the Gulban in Glen Spean, which are supposed to 

 have accumulated during the long time that the lakes filled the 



valleys, the author shows that the structure of the former is not 

 in accordance with the bedded structure of deltas, but on the 

 contrary, that it is formed of unstratified moraine debris 50 to 80 

 feet thick, with a thin coating of gravel water-worn and recon- 

 structed from the underlying mass, and that the angle of terminal 

 slope is not that of original deposition, but is due to wearing 

 back of the terrace by the Roy, and the fall of the dcl/ris by 

 weathering. 



These are the objections of the author to the hypothesis of 

 Mr. Jamieson, but while objecting to this exposition of the 

 glacial theory, he considers that that theory affords the most 

 satisfactory solution of the problem, only that he would suggest 

 a different interpretation in explanation of the phenomeiva. 



Dismissing the hypothesis of local glaciers of the second period 

 of glaciation, the author falls back upon the original idea of 

 Agassiz with the development acquired by more recent research, 

 and assigns the Lochaber lakes to the close of the first period of 

 great glaciation. He considers the phenomena are due to the 

 peculiar physiogra^hical conditions of the district, and shows 

 that owing to the configuration of the country, the drainage of 

 the Ben Nevis range is diverted into the lower part of the Spean 

 Valley and the Great Glen near Fort William. These conditions 

 which now give this area an excess of water drainage, must in 

 the like manner, during the glacial period, have there led to an 

 exceptional accumulation of ice. 



The observations of MacLaren, Chambers, Milne-Home, 

 Jamieson, James Geikie, and others, sufficiently prove the great 

 thickness of the ice covering in this part of Scotland during the 

 first period of intense glaciation. On the flanks of the Ben 

 Nevis range, glacial stria; extend to a height of more than 

 2,000 feet, while everywhere the rocks in the lower parts of 

 Glen Spean are intensely glaciated, as are also those at the 

 head of Glen Roy on approaching the Col to Glen Spey, and 

 around the Cols of Glen Glaster and Makoul. At the same 

 time, the erratic blocks, with ^the beds of sand and gravel of 

 foreign origin, which have been found widely distributed over 

 the hills around Glen Roy to heights of from 800 to 2, 100 feet, 

 afford confirmatory testimony of the depth of the land ice which 

 then covered the country. 



With the incoming of this glacial period, local glaciers must 

 have descended from every mountain range, and so long as the 

 glacier of one steep glen became confluent with another of the 

 same chain flowing in the same general direction, so long would 

 their course be uninterrupted, and the propelling and abrading 

 force maintained, as in the Alps at the present day ; but when, 

 emerging from these glens into valleys of small gradients divid- 

 ing the several mountain chains, they met with glaciers descend- 

 ing from these other ranges, their progress was not only subject 

 to be checked, and their forces neutralised, but their course 

 diverted, for if the lines of natural drainage were barred, the 

 ice took those of least resistance, although such might be up- 

 hill and against the lines of drainage. This, however, could 

 not be effected vidthout excessive pressure and heaping up of the 

 ice at the points of junction. 



These interferences must have been especially frequent in the 

 valley of the Spean, On the one side, the glaciers descending 

 the steep ravines of the Ben Nevis range, would issue into Glen 

 Spean and project across it to the Glenroy hills opposite. Below 

 to the west, the great Nevis Glen glacier emerged into the valley 

 of the Lochy, while above to the east the great glacier, issuing 

 from Glen Treig, flowed down Glen Spean ; but, meeting with 

 the aforesaid group of glaciers from Ben Nevis, was partly 

 diverted over the flanks of Craig Dhu, and upon the entrance 

 to Glen Roy. 



While the glaciers from this system of mountains were becom- 

 ing confluent in and filling Glen Spean, those from the opposite 

 range of hills were descending Glen Roy, the Rough Bum, and 

 the other ravines of that chain, and coming into collision with 

 those of the Ben Nevis range. In the same way other valleys 

 were focussing their glaciers upon the end of the Great Glen 

 north of Ben Nevis, barring in that direction the passage of the 

 ice down Glen Spean, and diverting it northward towards Loch 

 Lochy and Loch Oich. 



Therefore, the great mass of ice descending Glen Spean, in 

 consequence of meeting with these obstructions, was driven to 

 accumulate in mass in the lower part of that valley opposite 

 Glen Roy, until overcoming further resistance and confluent with 

 the Ben Nevis mass, it wheeled round into the Great Glen at 

 Loch Lochy. 



There is no doubt, also, from the direction of the striae and 



