862 J. GEIKIE ON THE GLACIAL PHENOMENA 



due to obstruction would exert excessive erosive action upon the bed 

 of the sea, and this would eventually result in the formation of deep 

 rock-basins having the same trend as the present coast-line of the 

 Outer Hebrides. Now, as I have pointed out elsewhere*, such a 

 set of rock-basins does actually exist in the very position which, 

 from theoretical considerations, they ought to occupy. Such being 

 the course followed by the under portions of the mer de glace in the 

 Minch, it is evident that the debris dragged on underneath the ice 

 would be rolled north-east and south-west along the bottom of the 

 Minch, and would thus never invade the Outer Hebrides at all. 

 Now and then a few boulders might be pushed beyond the reach of 

 the under-currents, and these would be dragged over the Long 

 Island ; but this would be quite exceptional, and hence we need not 

 be surprised at finding only an occasional stranger from Skye or the 

 mainland in the till of the Outer Hebrides. 



Again, the very general absence of large angular erratics and 

 perched blocks, derived from the lands beyond the Minch, is quite 

 what we should expect. When the ice-sheet reached its greatest 

 development, the Long Island was all but completely buried, only a 

 few insignificant points in Harris and South Uist projecting above 

 the surface of the mer de glace. So that the chances that any 

 superficial erratics, travelling outwards from Skye or the mainland, 

 should be caught on the tips of the Cliseam, or Beinn-mhor, or 

 Hecla, were exceedingly small. The absence of such large angular 

 erratics in the Outer Hebrides we may therefore look upon as an 

 additional proof of the great depth attained by the ice that over- 

 flowed these islands. 



By far the larger number of erratics which are found lying loose at 

 the surface must belong to the later stages of the glacial period, most 

 of them having been stranded during the gradual dissolution of the 

 ice-sheet, while others are the relics of local snowfields and glaciers. 

 Consequently they need not have travelled any distance. Each cliff 

 and mountain-top, as it appeared, would begin to break up under the 

 influence of frosts much more intense than are now experienced in 

 our islands ; and thus numbers of large angular blocks and heaps of 

 debris would be supplied to the surface of the dissolving ice. And 

 this morainic rubbish would eventually be scattered over the 

 islands, most abundantly in the hilly districts, and less plentifully 

 over the low grounds at a distance from the mountains. 



2. Shelly Boulder -clays and Interglacial Beds. — It is very remark- 

 able that shelly boulder-clays occur nowhere in the Long Island, 

 save only upon the low grounds in the extreme north of Lewis. I 

 searched for them everywhere, but without finding a single trace in 

 any of the islands further south. For this peculiar restriction of 

 their range there must be some reason ; and the explanation, as it 

 seems to me, is not far to seek. I have shown how the general 

 absence from the unfossiliferous till of erratics from Skye and the 

 mainland is to be accounted for by the deflection of the lower strata 

 of the ice-sheet at the bottom of the Minch. Two under-currents of 

 * ' Great Ice Age,' second edition, p. 286. 



