840 J. GEIKIE ON THE GLACIAL PHENOMENA 



tion in which they were " shot." The mounds are generally of small 

 size, and often pave the whole "bottom of a mountain-valley, at the 

 mouth of which they not infrequently spread out so as to cover a 

 fan-shaded area. As already indicated, they are very often crowned 

 with large angular erratics. 



Again and again we see morainic matter resting upon till in such 

 a way as to prove beyond question its more recent origin, as may be 

 seen in the accompanying section (Pig. 10). On the other hand, simi- 



Fig. 10. — Till and Morainic debris, Fincastle Road. 



N.W. s.e. 



a. Till ; m. Morainic debris, showing rude bedding. 



lar debris often passes insensibly into till, so as to form part and parcel 

 of the same deposit. Thus it is evident that the moraine matter does 

 not belong exclusively to one and the same period. Some of it is as 

 old as the till, while some, again, is certainly more recent. And 

 the same may be said of the loose erratics and perched blocks. 



7. Origin of Erratics and Moraines. — The origin of the deposits 

 in question seems sufficiently obvious. When the great sheet of ice 

 that overflowed Harris began to melt away, it is evident that a time 

 would arrive when it would be no longer able to override the higher 

 elevations of the island. These, therefore, would begin to rise, as it 

 were, more and more prominently above the level of the mer de glace. 

 The margins of the ice-flow would now be dotted with big blocks 

 and debris showered upon it from slowly emerging crags and cliffs ; 

 while, at the same time, much moraine matter would doubtless be 

 carried down to it by small local glaciers, streaming outwards from 

 such glens as those that drain the Langa, the Cliseam, and the lofty 

 mountain-tracts of North Harris. Then a time would at length 

 arrive when these local glaciers would cease to coalesce with the 

 great mer de glace, and when the latter itself would be unable to 

 overflow even the lower parts of Harris. Under such circumstances, 

 all the mountain-tracts that were capable of sustaining glaciers 

 would have their small local glaciers, whose moraines would be de- 

 posited at higher and higher elevations as the climate improved, 

 and the snow-fields melted away. Some of these small snow-fields, 

 moreover, even although they gave rise to no true glaciers, might yet 

 have succeeded in scattering erratics locally in large numbers, either 

 suddenly by avalanches, or gradually and continuously by the 

 downward movement of the snow and neve. 



The morainic debris that passes into and forms part and parcel of 

 the till belongs, I believe, to the time when the ice-sheet was de- 



