197 



PARALLEL ROADS OF GLEN ROY. 



PARALLEL ROADS OP GLEN ROY. 



198 



gravel-terraces are seen both at the summit and at the foot of Glen 

 Roy, and in each ease appear intelligible as the result of fluviatile action 

 at higher levels than the present water-channel. 



The lines of Glen Roy and the other valleys are not short protruding 

 terraces connected with the several lateral affluents, nor declining 

 planes of gravel, but almost perfectly continuous shelves, sloping 

 toward the valley from a level line, and almost perfectly parallel 

 through all the various windings of their linear extent, and round the 

 hollows and projections of the hills, whether these are small or great, 

 sudden or otherwise ; and each respective range, on one side of the 

 glens, is exactly on the same level with that corresponding to it on 

 the other side. This almost perfect horizontality and continuity oi 

 range is the grand feature of the phenomenon, and though easily 

 admitted on ocular inspection, it is satisfactory to know that Sir T. ll 

 Dick has proved it by careful levelling with Mr. Maclean on each of 

 the shelves in Glen Hoy. With such characters it is not wonderful 

 that these mysterious works of nature should be supposed the effect of 

 art, and the term of Parallel Roads is far from inapt, and is certainly 

 very descriptive. 



The shelves generally appear distinctly and even sharply defined 

 when viewed from below or from a distance ; but when inspected more 

 narrowly, they appear very indistinct, and one may be actually tread- 

 ing on a shelf without being aware of it. This arises in a great measure 

 from the fact that the surface of the shelves is far from level, but being 

 often a little concave above and a little convex below, easily on a near 

 view loses itself in the general slope of the mountain. 



Sir T. L. Dick represents the general figure of a section of the shelf 

 thus : - 



Dr. MacCulloch gives many representations on a small scale, which 

 show the really sloping character of the shelf or terrace, the angle of 

 declination from the horizon varying from 12 to 30, while the 

 breadth of the terrace part varies from 10 to 70 feet. Where the 

 slopes of the lines are steepest, their surface is narrowest ; and gene- 

 rally where the ground slopes the least the terraces are the broadest. 

 A real profile of one part is given below by the black line. 



According to Mr. Darwin's view of the relation between the actual 

 form and the original outline of the fundamental rock, one part will 

 appear to be formed by excavation of the rock, and another part by 

 accumulation of detritus, very slightly projecting beyond the slope of 

 the mountain. MacCulloch's drawings indeed represent the shelves as 

 hardly in any case projecting into a mound. Another part is drawn 

 to represent a valley terrace composed of gravel, to which the shelves, 

 (where they cross valleys) occasionally unite on a level, and thus form 

 broad expansions. 



" The shelves entirely disappear when crossing any part of the 

 mountains in which the bare rock is exposed ; for loose matter cannot 

 accumulate there, and the rocks themselves, from their laminated 

 structure, do not readily become worn into any regular form. They 

 likewise disappear when crossing any part which is greatly inclined ; 

 for their own slope then coincides with that of the alluvial covering, 

 mnot be distinguished from it" (Darwin.) 



The actual surface of the shelves is very irregular : they are com- 

 posed of the same kind of alluvium with that covering the whole sur- 

 f;ice of the mountain. (Darwin.) They commonly contain rounded 

 pebbles, and are in many places covered with large masses of etone, 

 some of them many tons in weight, lying for the most part quite 

 detached on the surface, and having their neuter angles rounded off in 

 the greater number of instances ; in short, in every respect resembling 

 those fragments generally found strewed on the margin and ou the 

 xhallnw edge of alpine lakes. In some places, where the stones are 



large and the shelf narrow, a single block covers its whole breadth. 

 When rock appears anywhere on a shelf, its angles are also rounded. 

 (Sir T. L. Dick, from whom the above description is taken, states that 

 the soil above the highest line of shelf is materially different from that 

 below, which has more gravel, pebbles, and alluvial clay ; but Mr. 

 Darwin's subsequent observations do not confirm this.) The cases 

 are few in which the solid rocks are much worn away in this manner ; 

 indeed they are not visible on the shelves. Some of the blocks of 

 stone on the shelves have fallen from the heights above ; others belong 

 to far-removed sites, and are in fact erratic boulders. Among these 

 latter are large granite blocks. 



It must be evident, from the foregoing description, that the Parallel 

 Roads of Glen Roy, and the lines resembling them or connected with 

 them in the neighbouring regions, are marks of the ancient'level of 

 water filling the valleys to their summits or nearly so, and extending 

 towards their mouths till within a few miles of the sea, or the great inte- 

 rior valley of Scotland. As many lines as we see, so many successive 

 effects are there of water oscillating about particular levels through 

 all the extent of the lines. This is freely admitted by all writers and 

 all observers on the subject. Sir T. L. Dick, who gives abundance 

 of illustrations of similar shelves being actually formed on. the steep 

 sides of modern lakes, adopts the hypothesis which most easily and 

 obviously suggests itself, and ascribes the lines in all the valleys to the 

 operations of a lake whose waters were successively lowered. Dr. 

 MacCulloch discusses very ably two other modes of watery action 

 whereby the lines may be supposed to have been formed ; first, by a 

 sort of local deluge filling the valley ; secondly, by the excavating 

 power of water in a valley previously filled by alluvial sediments to a 

 particular height ; and shows each hypothesis to be unsuitable and 

 untenable. 



On the view of the lacustrine origin of the parallel roads it is to be 

 conceived that a lake had existed at the uppermost level of Glen Roy 

 for so long a period as to have accumulated on its margin that allu- 

 vium which now forms the uppermost of the lines in question, and 

 that by a subsequent siukiug through two successive and similar 

 periods, the two lower ones had been formed in the same manner. 

 (MacCulloch.) 



Mr. Darwin explains the phenomenon of these roads ou the same 

 principle as Sir Charles Lyell explains the formation of the parallel 

 roads of Coquimba in South America. Sir Charles says" The three 

 parallel roads were formed by the waves of the Pacific, and not by 

 the waters of a lake ; in other words, they bear testimony to the suc- 

 cessive rise of the land, not to the repeated fall of the waters of the 

 lake." 



From a series of appearances below the lowest shelf of Glen Roy, 

 continued into Glen Spean such as the nature and distribution of 

 detritus, the form and position of many short terraces, some related to 

 the shelf or ancient beach, others unconnected with the actions of the 

 streams now running Mr. Darwin concludes, apparently with reason, 

 that every appearance in the sides of Glen Roy and Glen Speau, below 

 the lowest shelf, will be explained on the supposition of a gradually 

 subsiding sheet of water which received detritus from lateral stream- 

 lets. By observations of accumulated sand and pebbles in other 

 valleys, as the sides of Loch Ness, Loch Dochart, &c., Mr. Darwin 

 endeavours to extend this inference to a large portion of the Highland 

 valleys, where no horizontal shelves make it a matter of certainty. 

 Having rejected the hypotheses of separate and connected fresh-water 

 lakes, on account of the difficulties attending them, which even in a 

 single case are great (but if extended to many of the Highland valleys, 

 insurmountable), Mr. Darwin adopts the alternative, that it was the 

 sea-water, in the form of narrow arms or lochs, such as those now 

 deeply penetrating the western coast, which once entered and gradu- 

 ally retired from these several valleys. 



That there has been a change of level between the land and sea in 

 Scotland, to the extent of several hundred feet, is proved by Mr. 

 Smith's and Mr. Prestwich's observations at Banff and near Glasgow 

 and other places ; and, thus fortified, Mr. Darwin traces in hypothesis 

 ;he effect of an elevation of the land equably or by intermitting effects, 

 showing that by this means the main phenomena of the Loch.iber 

 valleys would certainly be occasioned. 



Objections to this speculation easily arise. Why, for example, are 

 shelves, the marks of intermission in the elevating forces, which 

 affected a large area with extraordinary equality, confined to a few 

 valleys ? Why, after encircling in complete belts the upper ends of 

 .he valleys, do they cease toward the outlets ? Why has Glen Roy 

 ,hree shelves, and the neighbouring Glen Gluoy but one, and that at 

 a different level from any in Glen Roy ? Why, if the sea for so long 

 a time laved the mountain sides, are no marine shells found except at 

 comparatively small elevations ? 



Mr. Darwin answers to these objections : first, that the conservation 

 and even the original formation of benches (for such thu shelves 

 certainly are) might rather be the exception than the rule, requiring 

 >articular slopes, loose materials, and freedom from erosive currents ; 

 secondly, that the difference of level between the Glen Gluoy shelf 

 and the highest in Glen Roy may be explained as nn effect of unequal 

 idal oscillation; thirdly, that instances of the absence of marine 

 shells from the upper parts of marine deposits occur in Sweden, 

 Scotland, and elsewhere. 



