114 



NATURE 



\May 29, 1879 



ihe position of the transported boulders, that the mass of the 

 Treig glacier struck across the valley of the Spean, and turned 

 down its channel westward ; but that a part ascended to the Col 

 of Glen Glaster, and another passed up the Spean Valley, is 

 doubtful. It is more probable that this glacier, after traversing 

 Strath Spean, met with others coming down the Rough Burn, 

 while these took an easternly direction to Loch Laggan and over 

 the Pass of Makoul. The direction of the stride observed by 

 the author between the Rough Burn and Moy, points, he con- 

 siders, to ice coming down from the hills on the north and join- 

 ing this main east stream. 



The effects of these great conflicting ice streams were not 

 confined only to the piling up and accumulation of the ice. 

 Although glaciers confined by the walls of narrow glens, and , 

 descending steep slopes exercise great abrading power, the 

 observations of Charpentier and others show that when they 

 emerge into broader and flatter valleys, they may pass over beds 

 of loose detritus without disturbance except that of pressure. ; 

 The terminal moraines of the many glaciers emerging into Glen 

 Spean may, according to the varying conditions of the ice, have 

 been pushed forward or rolled over by the ice, while the 

 meeting of conflicting glaciers must have led to the deposition [ 

 and heaping up of the glacial debris at the points of junction. ■ 

 The many checks and blocks that must have occurred during the 1 

 growth of the great ice-sheet — the neutralisation of the ice-force 

 in one place, and the centralisation of it in others — will serve to 

 explain much that is peculiar in the distribution of this sub- 

 glacial dibris or Till, not only in Lochaber, but in other parts of 

 the country, and at ail levels. 



The author then points out the many mounds and terraces in 

 the Spean Valley formed of moraine detritus, though since 

 levelled and often masked by a covering of gravel due to sub- 

 sequent water action. To this cause also he attributes the large 

 accumulation of dttris at the entrance to Glen Roy, between 

 Bohuntine and Glen Glaster, where he shows it to be in places 

 200 to 300 feet deep, and where it rises nearly to the level of the 

 lower parallel road. Mr. Milne-Home has pointed out a similar 

 deposit at the entrance to Glen CoUarig, while the large mass at 

 the entrance of Glen Spean, and now forming Unachan Hill, 

 rising to a height of 613 feet, has been often described. 



The next question discussed is the height of the land in rela- 

 tion to the sea at the period of the great glaciation, as it is not 

 possible to suppose that with the great changes of level which 

 took place subsequently, there is now a return to the status quo 

 ante of the earlier period ; and the author sees rea on to conclude 

 that the land then stood at not less than from 1,000 to 1,500 feet 

 higher than at present, so that the Irish Channel was then above 

 the sea level, and land extended a corjsiderable distance west- 

 ward from the pre^ent coast of Scotland. 



This was followed by a submergence of not less than 1,200 to 

 1,500 feet in central and northern England, Wales, and Ireland, 

 and of 600 feet in the southern part of Scotland, as proved by 

 the occurrence of marine shells at those heights, and assuming 

 for the north of Scotland a submergence, at all events, of 400 

 or 500 feet below the present level, this, added to the previous 

 elevation of 1,000 to 1,500 feet, would estalilish a difference of 

 1,500 to 2,000 feet between the period of great glaciation and the 

 succeeding; period of submergence. 



This difference of level would produce a twofold effect upon 

 the climate — the one resulting from altitude which would be 

 equal to a rise in the mean temperature of from 4° to 6" F., and 

 that caused by the conversion of a continental area into an 

 archipelago. The effects of the two cau-es coulil not be less 

 than from 12° to 15° F., which is about equivalent to the differ- 

 ence of climate bet» een Paris and St. Petersburg. There is also 

 to be taken into account the probable increase of heat consequent 

 on the gradual diminution of those conditions, whatever they 

 were, to which was owing the cold of the glacial period. 



The effect of those changes was to produce a thaw which 

 gradually led to the destruction of the great ice-sheet, though 

 subsequently other changes brought for a time a return of cold 

 sufficient to maintain local glaciers in the higher mountain 

 ranges. The conditio»s under which the ice-sheet would be 

 placed during this period would be similar to that of glaciers 

 where they extend below the snow line. In Europe the glacier 

 gradients are usually so steep, and the ice is so fissured, that 

 even in such po^itions water rarely lodges on the surface, but in 

 the Himalayas, where glaciers descend into wide valleys with 

 small gradients, and the summer heat is considerable, the 

 glaciers often become covered with tarns and small lakes. They 



have been noticed by Sir J. D. Hooker, and more lately by Col. 

 Godwin-Austen, who describes in the midile of one of these 

 glaciers, a series of such lakes, some being 500 yards in length, 

 and 200 to 300 in breadth, and of great depth. As intervenhig 

 barriers give way, these lakes descend from lower to lower 

 levels, and finally escape. 



In the same way the old ice-sheet must have become covered 

 with pools and lakes, for owing to the irregular surface of the 

 ice, and the inevitable absence of all channels of drainage, the 

 water must everywhere have lodged, until channels were formed, 

 and a means of escape established. The extent of these bodies 

 of water would depend upon the height and permanence of the 

 obstructions. In the Lochaber district they were, owing to the 

 causes before named, of great size and permanence, such as to 

 form high barriers at the entrance to Glen Roy, Glen Spean, 

 and Glen Gluoy, behind which the ivaters accumulated and ro.-e 

 until they found a channel of escape over the cols at lower levels, 

 when a permanent water-level would be established so long as 

 the main barriers existed. 



It is w ell known that the Parallel Roads are terraces composed 

 of perfectly angular fragments of the local rocks with a few 

 rounded pebbles both local and foreign to the district. The 

 former show an entire absence of any prolonged beach wear. 

 The wear of the latter is due to other causes. The slope of the 

 hills above and below the ' ' roads " varies from 25° to 40°, and 

 the inclination with the horizon of the "roads" themselves, 

 which are from 50 to 70 feet wide, varies w ithin the limits of 

 from 5° to 30°. 



Of the internal structure of the "roads" very little is known. 

 The only published section is the one given by the Rev. Thomas 

 Brown in his paper on the "Parallel Roads," and in this there 

 is no appearance of any such structure as would result from suc- 

 cessive additions to the ledge by the tipping over of dibris re- 

 moved from the shore. 



Although, therefore, the "roads" indicate a line of watar- 

 level, there is nothing in their form or structure to show that 

 they have b en formed by the long-continued action of lake 

 waters on a shore line. To what, then, are they to be ascribed ? 

 What the cimditions were immediately antecedent to the 

 formation of the first, second, and fourth road, is not shown, 

 but in the case of the third road the conditions preceding its 

 formation are to be traced uninterruptedly from the conclu-ion 

 of No. 2 "Road." When the lake stood at the level of 

 " Road" No. 2, its waters escaped by the col leading to Glen 

 Spey, while, when they stood at the level of No. 3 "Road," 

 they escaped by the Glen Glaster Col. Now, as there is a 

 difference of 76 feet between the height of the two cols, it is 

 evident that a l)arrier must have existed on the latter col during 

 the time the lake stood at the higher level. Whether the barrier 

 was detrital or ice-formed is immaterial for the argument. 



Now, it is well known to engineers that a breach once esta- 

 blished in a detrital barrier becomes so rapidly enlarged that, if 

 not at once stopped, nothing can stay the rapid destruction of 

 the barrier, as, in the ca.-e of the Ilolmfirih, Crinan, and other 

 floods. Nor is evidence wanting of similar catastrophes in con- 

 nection with glacier lakes. In the notalile case of the Gietroz 

 Glacier barring the valley of the Drance, a lake nearly 2 miles, 

 and at one end 200 feet deep, was drained in twenty minutes. 

 The still greater flood recorded by Vi^fne in a branch of the 

 Indus drained a lake formed by a detrital barrier, and esti- 

 ma'ed by Mr. Drew to have been 35 miles long by \ mile broad 

 and 300 feet deep at its lower end, in one day. 



In the same way it is to be assumed that the Glen Glaster 

 barrier, which wa> probably formed by a remnant of the glaciers 

 descending from the mountain ranges (2,994 f^^') ^t the head of 

 the glen, gave way with great suddenness, and caused the rapid 

 fall of the waters from the level of the higher " road " in Glen 

 Roy to that of that glen's second "road," at the height of the 

 Glen Glaster Col, when the escape of the v^ater^ was stopped. 



Now, it must be borne in mind that at this time the great 

 mantle of snow and ice which had so long covered the country 

 w as passing away, leaving the surface of the hills in Glen Roy 

 covered with a thick coating of angular local debris mixed with 

 sand and clay, the result of the intense ci ild and of the decom- 

 position of the underlying schistose and granitic rocks. This 

 and the glacial afbris must have long remained bare and unpro- 

 tected by vegetation ; at all events that lielnw the water-line was 

 so. Now, the angle of repose of purely an^fular and subangular 

 dlbris varies within the limits of from 35" to 48°, but that of 

 clayey sands, which, when dry, is from 21° to 37°, becomes, 



