May 29, 1879] 



NATURE 



"5 



when saturated with water, as low as 14° to 22°. The angle of 

 repose of the hill-side debris would, therefore, depend on the 

 relative proportion of the angular materials and their matrix, 

 and on the extent of saturation. The slopes of the hills being 

 on the whole greater than that of the angle of repose of the 

 saturated under-water rubble, this latter, easily set in motion on 

 the settlement of its constituent parts as the water drained from 

 it would, as the level of the lake water fell, tend to slip or slide 

 down with the falling water, and this slip would continue until 

 the disturbing cause ceased, and the momentum of the mass was 

 checked by the inertia of the water gradually coming to rest on 

 reaching the level of the col of e^cape.^ The effect of the 

 arrested slide would be to project the mass more horizontally 

 forward, and form a ledge. This ledge, modified slightly by 

 subsequent .'uljaerial action and weathering, and by the dressing 

 of its slope on the occasion of the next fall of the lake, consti- 

 tutes the "road." 



Although in the case of the other "roads" there is not the 

 same evidence of a minor col-barrier, as the results are alike in 

 all, the causes which led to them must have been the same ; and 

 it is shown that there is nothing incompatible in the features of 

 the ground with the existence of such barriers, or rather that 

 I there is some evidence in each glen, however slight, of water- 

 lines at levels higher than the "roads." 



Comparing the theoretical inferences of structure with the 



I facts, so far as they are known, Mr. Brown's substratum of 



I "clay with boulders indistinctly stratified with thin (lenticular) 



I layers of sand," represents the sliding detrital mass ; the finely 



stratified sand and clay — the sediment which subsided from the 



muddy lake waters after their fall ; and the two to three feet of 



stones with clay — the subaerial fall of cUbris from the slopes 



1 above. In the substratum and overlying sediment, Mr. Brown 



found four species of fresh-w ater diatoms, while he found none 



in the upper bed. This fact serves to confirm the subaqueous 



origin of the body of the ledge, w hile it tends further to disprove 



the marine hypothesis. 



I Although there is in all the cols an entire absence of a defined 



I water channel, such as would be worn by the long-continued 



flow of a river, there are in all of them traces of strong water 



; action, such as might result from the temporary passage of a 



' large and rapid body of water. 



With respect to the main barriers acting as dams to Glen 

 Gluoy, Glen Roy, and Glen Spean, they were due, as already 

 ' pointed out, to the circumstance of an accumulation of ice at 

 these spots so excessive and so hiijh as to last long after the ice 

 generally in the lower tracks had given way. Not, however, 

 that any ice barrier could have been permanent for a great period 

 of time, but this the author's hypothesis does not require. In 

 any case, an ice- barrier in a state of rest will form a more effec- 

 tive barrier than when in motion. 



Passing over the barrier at the entrance to Glen Gluoy, it is 

 shown that the point where the Glen Roy barrier existed is that 

 I where a glacier coming down Glen Roy would meet in opposi- 

 tion the ice from Ben Nevis and the Spean Valley ; and that 

 this glen w as occupied by a glacier is proved by the occurrence 

 of glacial striae on the rocks forming the bed of the valley near 

 Dalriach, and of Till, or boulder clay, lower down the valley, 

 nearer Achavady. But the great mass of the latter lies precisely 

 on the spot where the Ordnance Survey have placed the line of 

 barrier ; it was there heaped up by the same conflicting causes 

 that produced the barrier of ice. That it was originally larger 

 ; and higher is proved by its occurring on the two sides of the 

 valley, the river having worn a channel through it, and by the 

 presence upon it of thick beds of water-worn and water-strewn 

 ' gravel, the materials of which have been derived from the un- 

 derlying deposit, and which was formed, in all probability, by 

 the rush of the waters on the bursting of the barrier, for, in 

 many places the gravel is thrown back and over, as though by 

 downwaid and outward pressure of water in motion. This 

 detrital ma s extends for a length of two miles or more. 



The great bairier needed at the entrance of Glen Spean is 

 precisely on the ground that the great glaciers of Ben Nevis 

 met the ice stream coming down Strathspean. Unachan Hill, 

 ■ which rises immediately behind the line of barrier marked by 

 the Ordnance Survey, together with the rising ground on the 

 flanks of the valley, consist of a thick substratum of till or 

 I boulder clay, with a covering of gravel, the latter formed in 

 greater part, if not entirely, from the destruction of the former, 



' Even nuw considerable slides occasionally take place on the steeper 

 slopes o£ Glen Roy. ^_ ^ J- v 



SO that there is little doubt that the detrital barrier here also was 

 at one time much more important. Still, although the detrital 



' matter formed a considerable element, the author believes that the 

 great mass of ice constituted the essential element in the harriers. 



[ The Till, although accumulated in larger masses in the above- 



\ named sites, is found in places all up the valley, generally in the 

 form of terraces covered by gravel, as at Inverroy, Murlaggan, 

 Inverlaire, and elsewhere. There is one feature common throuCTh- 

 out, namely, the levelhng and terracing of this glacial dibris by 

 subsequent water-action, which could not have been effected in 

 the still waters of a lake bed, but probably took place on the 

 bursting of the main barriers and during the rapid outflow of the 

 waters. The levelling of the original glacial mounds having been 

 effected at the time of the drainage of the lakes, and having been 

 then covered and masked by gravel, the terminal slopes were 



, either formed at the same time by the outpouring waters as they 

 fell to a l>wer level, or subsequently by wearing back by the 

 present streams. 



I An objection may occiu" to the foregoing hypothesis, in that, 

 with elements so variable as the problem has to deal w ith, the 

 parallelism of the "roads "with the horizon, which has been 

 remarked on by all observers from Macculloch downwards, could 

 not have been maintained. For the detritus of the hdl sides 



j vary, however slightly, in the relative proportions of rock frag- 

 ments and soil, while the slopes above and below the "roads" 

 vary also in their angle, so that, as these conditions varied, so 

 would the momentum of the sliding mass vary, whilst the resist- 

 ing force of inertia would remain the same. The consequence 

 would necessarily be, that the slide would continue at some 

 places to a lower level than at others, and the line of the "roads " 

 could not be throughout parallel with the horizon. 



There is no doubt that, to all appearances, the " roads " are 

 perfectly level, and such was the author's first impression ; but 

 afterwards, on referring to the elaborate 6-inch Ordnance maps, 

 he found the levels there given clearly show that the "roads" 

 are neiiher at the exact height of the cols nor are they perfectly 

 parallel with the horizon, after allowing for the variable inclina- 

 tion of the "roads," and for the observations being made in 

 their centre. 



Instead of a perfect level water line, the "roads" are really 

 slightly waved, the difference between the highest and lowest 

 point being in the four "roads" taken in descending order, 15, 

 II, 15, and 12 feet ; and while the level of the higher "roads " 

 is in most instances below those of the several cols of escape, that 

 of the lower "road " is in all instances above it. Thus in Glen 

 Gluoy, the "road," which is never more than i foot above the 

 level of the Turret Co), is in places 14 feet below it. The (Jlen 

 Roy "roads," Nos. 2 and 3, rise 2 and 4 feet above the col of 

 the Spey, and sink 7 and 13 feet below it; whereas No. 4 

 "road" is never less either in Glen Roy or Glen Spean than 

 2 feet, and rises at places to 10 and 14 feet, above the Pass of 

 Makoul. Further, as the curves formed differ for each "road," 

 the variations cannot be due to a common cause, such as subsequent 

 movements of the grmind, but must be owing to differing condi- 

 tions in each case. Nor do the levels on the two sides of the 

 valleys correspond ; they often vary as much as 7 or 8 feet. It 

 is therefore not possible to reconcile these variations with the 

 hypothesis of the "roads" being lines of water level due to shore 

 action ; nor is the very variable inclination of the "roads " them- 

 selves compatible with that view. 



In the higher "roads" the lower level of the curve is possibly 

 due to the steeper slopes, whilst the fact that the lower " road," 

 No. 4, remains above the level of the Pass of Makoul, may be 

 due to the circumstance either of the slopes being less, or more 

 probably to the fact of the lake having been so very much 

 larger, the escape of the waters was more prolonged, and the 

 fall slower. 



Various phenomena in connection with the great ice-sheet in 

 Lochaber, and their connection with the gener 1 question, are 

 next considered. Tlie author objects to the term of « raine pro- 



fonde to describe the drift of sub-glacial origin, as apt to lead to 

 misunderstanding, although Hogard and other geologists have 



I'sed it in a wider sense. It is evident that the old ice- sheet 

 acted under very different conditions to an ordinary glacier, and 

 it is better to use such terms as moraine detritus or sub glacial 

 detritus for the sum total of sub-glacial products of the former, 

 than employ a term which was originally and still is generally 



restricted to a single and comparatively small product of the 



modern glacier. 



Besides this sub-glacial dihris, there is the larger quantity of 



