864 J. GEIKIE ON THE GLACIAL PHENOMENA 



that they must at one time have occupied very considerable areas, 

 not only in Lewis, but in the low-lying districts of the other islands 

 to the south ; for it is hardly conceivable that the interglacial sub- 

 mergence of 175 feet was confined to Northern and Eastern Lewis. 

 The only portions, however, that now remain are the small patches 

 at Garrabost and near the Butt. In no other part of the Long 

 Island did I find any trace of them, although I was continually on 

 the look-out. Yet it is by no means improbable that, could we 

 obtain good sections across the low flats of North and South Uist, 

 patches of them might be obtained here and there underlying and 

 probably overlying till. The interrupted and patchy nature of the 

 interglacial beds, and their remarkable absence from so much of the 

 low grounds, where they cannot but have at one time existed, are 

 undoubtedly due to the erosive action of the latest ice-sheet, which, 

 like the ice-sheets of earlier glacial periods, made a nearly clean 

 sweep of the Hebridean ridge, leaving patches of the earlier deposits 

 in sheltered nooks, and sparing them to some extent in areas where 

 the ice had freedom to flow in one broad uninterrupted stream, and 

 where consequently its erosive action was not so great. 



Before I had examined the glacial phenomena of the southern 

 islands of the Outer Hebrides I was hardly prepared to believe that 

 the latest ice-sheet attained so great a development. It seems, 

 however, impossible now to resist the conclusion that the last mer 

 de glace — that, namely, which cut out the interglacial deposits — 

 actually overwhelmed the whole Outer Hebrides. Whether it 

 reached to as great a height as the earlier ice-sheet, whose moraine 

 jorofonde is represented by the Lower shelly Boulder-clay, I cannot 

 say ; but it is possible that a minute examination of the evidence, 

 and careful mapping-out of all the details, may eventually enable us 

 to settle this point. It is possible also that we may yet obtain 

 evidence to show that there were more than two incursions of the 

 mer de glace. We know, indeed, from the facts supplied in other 

 parts of the country, that there really were more than two cold 

 periods. But unfortunately the records of the earlier phases of the 

 glacial epoch are always more or less fragmentary ; and in a region 

 like that under review we cannot expect to meet with early glacial 

 and interglacial deposits in such abundance as in lower latitudes, 

 and in places where there was less obstruction to the flow of the ice. 

 As it is, we must look upon all the unfossiliferous till and morainic 

 debris of the Long Island as pertaining to the incursion of the latest 

 ice-sheet, although much of it may be merely that of earlier ice- 

 periods reworked and redistributed. 



3. Local Glaciers. — While the latest ice-sheet slowly melted away, 

 local ice was enabled to flow down the valleys in all directions. At 

 first these local glaciers coalesced with the general mer de glace ; but 

 eventually this connexion was severed, and many of them continued 

 to exist probably to late glacial times, to that period, namely, which 

 is represented on the mainland of Scotland by those shelly clays 

 which are found resting upon the youngest accumulations of boulder- 

 clay. None of these local glaciers, in their independent condition, 



