NA TURE 



[June 2, 1904 



of the sea or of a lake. Nevertheless, on a review of the 

 whole evidence, at least as it is presented in this country. 

 Sir Archibald felt very confident that there is no risk of 

 confusion in this matter. The marine terraces maintain 

 their distinctive features up to the very foot of the slopes 

 where the lake terraces begin, while those in turn are 

 marked by other special peculiarities. 



Let any observer who has followed the great 50-foot 

 raised beach along the western coast of Scotland and up 

 the Linnhe Loch to the mouth of the Great Glen, look away 

 to the right hand where the wide Strath of Spean leads into 

 the interior. While yet standing on the platform of the 

 raised beach, if the air be clear, his eye may detect the 

 beginning of a line, drawn as with a ruler, at the same 

 height along the slopes on either side of the valley. This 

 is the lowest of the three great Parallel Roads of Glen Roy, 

 and runs at a height of 850 feet above the level of the sea. 

 If he will now ascend into Glen Roy, where the three 

 terraces are best seen, he will soon be struck by the dis- 

 tinctive differences between these old lake-margins and the 

 raised beaches with which he has already made himself 

 familiar. In the first place, he will remark their faintness 

 as compared with the marine platforms of the coast. 

 Though readily traceable from a distance in their horizontal 

 continuity, they are in manv places hardly discernible when 

 one is actually standing upon them. A little examination 

 soon reveals that each of them has been produced mainly 

 by the arrest of sediment washed from the slopes above into 

 the water of the vanished lake. Instructive illustrations of 

 this process may often be observed along the sides of 

 reservoirs which have been constructed in steep-sided valleys : 

 there each prolonged halt of the water at a particular level 

 is marked by a shelf of detritus which, blown in by wind 

 and washed down the declivities by rain, is stopped when it 

 enters the water, where it accumulates as a miniature beach. 



Here and there, especially on more exposed projections of 

 the hillsides, there has been a little cutting-back by the 

 shore-waves or drifting ice-floes of the old lake in Glen 

 Roy. Occasionally also, where a streamlet has entered the 

 water, its arrested detritus has accumulated as a broad, flat 

 delta or terrace. But it is manifest that, in such limited 

 expanses of water, wind-waves could have had comparatively 

 little erosive power. Nor can we imagine that, even if the 

 water froze, its floe-ice could have had any potent influence 

 in sawing into the rocks of the declivities, and producing 

 seter or rock-shelves. Certainly throughout this wonderful 

 assemblage of lake-shores, there is nothing for a moment 

 to be compared to the incised platforms of rock so abundant 

 as part of the raised beaches of the western coast of Scot- 

 land. We must remember also that the production of such 

 ice-dammed lakes took place as a mere episode in the retreat 

 of the ice. No means are available to determine what may 

 have been the length of time during which the water stood 

 at the level of any one of these Parallel Roads. We may 

 probably infer, from the absence of well marked and con- 

 tinuous intervening shore-lines, that the shrinkage of the 

 ice and the consequent lowering of the level of the water 

 were somewhat rapid. 



The Parallel Roads of Lochaber, although the most im- 

 posing, are not the only examples of the shore-lines of 

 ancient glacier-lakes in this country. Another striking case 

 is that of Strath Bran in Ross-shire, where the glaciers 

 descending from the mountains on each side ponded back 

 the drainage of the valley, and sent it across the present 

 watershed of the country at a height of about 600 feet above 

 the sea. The conspicuous gravel-terraces at Achnashean 

 are a memorial of this vanished sheet of water. 



Now, with these undoubted records of ancient lakes, let 

 us compare the structure and distribution of our Raised 

 Beaches. These shore-lines are found, on both sides of 

 Scotland, at approximately the same heights above the level 

 of the sea. They are partly terraces of deposit, and partly 

 true seter or platforms cut out of the solid rock, the same 

 beach presenting frequent alternations of both structures. 

 In general, it may be said that the detrital terraces are found 

 chiefly in bays, sea-lochs, or other sheltered places, while 

 the rock-terraces are conspicuous in more open sounds and 

 exposed parts of the coast, where the tidal currents and 

 wind-waves are most powerful. 



As the highest terraces are the oldest, they have been 



NO. 1805, VOL. 70] 



longest exposed to the influences of denudation, and are thus 

 the faintest and most fragmentary. But the dimensions and 

 perfection of a raised beach do not depend merely on age, 

 but in large measure on the length of time that the water 

 stood at that level, and the varying local conditions that 

 favoured or retarded the planing-down of solid rock or the 

 deposition of littoral sediment. 



That these beaches unquestionably mark shore-lines of 

 the sea may be inferred on three grounds: — (i) Their posi- 

 tion on both sides of the island at corresponding heights. 

 No possible arrangement of ice-dams in the Atlantic and 

 in the basin of the North Sea can be conceived that would 

 have everywhere ponded back the land-drainage to similar 

 levels. (2) Their independence of local conditions. The 

 same terrace may be traced down both sides of a sea-loch 

 and round the coast into the next loch, retaining all the 

 while its horizontal continuity. Not only on the mainland, 

 but on the chain of islands outside, the same parallel bar 

 has been incised, both on the inner or sheltered side and 

 also on the outer flank looking to the open .Atlantic. (3) 

 Their organic remains. From the youngest of the beaches 

 up to the highest, the terraces of deposit contain marine 

 organisms which have not been scooped out of some earlier 

 formation, but lie in the positions in which the animals 

 died, or into which they were washed by shore-waves and 

 currents. The fossils of the latest beaches are entirely 

 identical, or almost so, with forms still living in the adjacent 

 seas, while those of the higher beaches are boreal or .\rctic. 



In some sheltered places, such as the Dornoch Firth, 

 especially near Tain, and some inlets on the west side of 

 the island of Jura, a number of successive bars or terraces 

 of deposit may be observed up to heights of 100 feet or more 

 above the sea. But there are in Scotland three strand-lines 

 so conspicuous and so persistent that attention may be con- 

 fined to them. From what has been taken to be their 

 average height above mean sea-level or Ordnance-datum, 

 they are known respectively as the loo-foot, the so-foot, and 

 the 25-foot beaches. 



The author here adverted to what he had long regarded 

 as a reproach to the geologists of this country. No- 

 systematic effort has ever yet been made to determine 

 accurately, by a series of careful levellings, the precise 

 heights of these old shore-lines. We only know that, 

 roughly speaking, a raised beach retains its level for long 

 distances, and appears to lie at the same height on both 

 sides of the country. But we are still ignorant whether 

 or not an appreciable difference of level might not be de- 

 tected between the western and the eastern development of 

 the same beach, nor do we know whether it would not 

 betray some variation in its height between its northern 

 and southern limits. There seems to be a tendency for the 

 levels of the beaches to rise slightly towards the head of 

 an estuary or sea-loch. But whether this difference is more 

 than can be accounted for by the ordinary elevation of the 

 tidal wave as it ascends a narrowing inlet remains to be 

 determined. 



Obviously, until accurate information is obtained on alf 

 ascertainable differences of level in the system of our raised 

 beaches, we must remain unprovided with some of the most 

 important material for a discussion of the history of these 

 beaches. It is surely not too much to hope that one or 

 more observers, endowed with the requisite geologica! 

 knowledge and geodetic skill, may before long be found 

 who will undertake the investigation of this interesting 

 subject, and thus aid in the solution of a problem which 

 does not merely concern the evolution of our own islands, 

 but is of high importance as a question in geological theory. 



The 100-foot, 50-foot and 25-foot beaches of Scotland 

 were briefly described, and it was pointed out that in the 

 structure of these old sea-margins a feature of special 

 interest is presented by the platforms which have been 

 eroded out of the solid rock, and which afford not a little 

 light as to the origin of the Norwegian seter. The surface 

 of these rock-terraces is flat, and usually covered with a 

 thin coating of grass-grown soil through which harder 

 knobs and stacks of the underlying rock here and there 

 protrude. At the inner margin of the terrace, the rocks 

 rise into a cliff or steep bank, the base of which is fre- 

 quently pierced with caves. That these caves were mainly- 

 due to erosion by moving water is abundantly evident in 



