Glacial Phenomena of the Long Island or Outer Hebrides. 147 



Boulder- clay, Erratics and perched blocks, Morainic debris and Mo- 

 raines, Freshwater lakes and Sea-lochs. Numerous bearings of striae, 

 which abound, were given; and these were held to prove that the whole 

 Outer Hebrides have been glaciated by ice that flowed outwards from 

 the mainland of Scotland. The position of abundant rocJies moutonnees 

 points to the same conclusion; and this is still further supported by 

 the " travel v of the Till. That deposit is generally absent or very 

 sparingly present on the rock-faces that look towards the mainland; 

 but it is heaped up in their rear, and spreads over the lower tracts 

 that slope gently towards the Atlantic. On the west side of the 

 islands not a few boulders occur in the Till which have been derived 

 from the east ; and the same is true of certain erratics lying loose 

 at the surface of the ground. The islands are well glaciated up to 

 a height of 1600 feet above the sea; and the line of demarcation 

 between the glaciated and non-glaciated areas is extremely pro- 

 nounced. Above 1600 feet the hills show rugged, splintered, jagged, 

 and sometimes serrate tops. The author regarded the Till or boulder- 

 clay as the morainic material that gathered underneath the ice ; and 

 proof of this is given. Erratics and perched blocks are very numerous ; 

 and most of these, as well as much of the morainic debris, are believed 

 to have been dropped where we now find them during the final melting 

 of the ice-sheet. It was shown, however, that certain erratics and 

 perched blocks and some well-marked moraines are due to local 

 glaciers, as are also some of the striations in a few of the mountain- 

 valleys. The origin of the rock-basins which are now lakes was dis- 

 cussed, and attributed to the erosive action of ice. To the same 

 cause were assigned the rock-basins which occur in certain of the 

 sea-lochs. 



In concluding, the author pointed out that we may now arrive at 

 a true estimate of the thickness attained by the ice-sheet in the 

 north-west of Scotland. If a line be drawn from the upper limits 

 of the glaciations in Hosshire (3000 feet) to a height of 1600 feet in 

 the Long Island, we have an incline of only 1 in 210 for the upper 

 surface of the ice-sheet ; and of course we are able to say what 

 thickness the ice reached in the Minch. Between the mainland and 

 the Outer Hebrides it was as much as 3800 feet. jSTo boulders de- 

 rived from Skye or the mainland occur in the Till of the Outer 

 Hebrides ; and this was explained by the deflection of the lower portion 

 of the ice-sheet against the steep wall of rock that faces the Minch. 

 The underpart of the ice that flowed across the Minch would be 

 deflected to right and left against the inner margin of tho Long 

 Island; and the deep rock-basins that exist all along that margin 

 are believed to have been scooped out by the grinding action of the 

 deflected ice. Towards the north of Lewis, where the land shelves 

 off gently into the sea, the under strata of the ice-sheet were enabled 

 to creep up and over the district of Ness, and thus gave rise to the 

 lower shelly boulder-clay of that neighbourhood, which contains 

 boulders derived from the mainland. The presence of the overlying 

 interglacial shell-beds proves a subsequent melting of the ice-sheet, 



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