THE " GREAT ICE AGE." 169 



glaciers, inasmuch as in these it is removed by the glacier tor- 

 rent when it has attained a certain degree of fineness, while in 

 the greater glaciers of the glacial epoch it would be carried 

 much further in association with the solid ice, and be subjected 

 to more grinding and regrinding against the bottom. Hence a 

 larger proportion of slimy mud would be formed, capable of 

 finally indurating into stiff clay such as forms the matrix of the 

 till and boulder clay. 



The long journey of the bottom debris stratum of the glacier, 

 and its final deposition when in a state of neutral equilibrium 

 between its own tendency to repose and the forward thrust of 

 the glacier, would obviously tend to arrange the larger frag- 

 ments of rock in the manner in which they are found imbed- 

 ded in the till i.e. the oblong fragments lying with their longer 

 axes and their best marked striae in the direction of the motion 

 of the glacier. The " striated pavements" of the till are thus 

 easily explained ; they are the surface upon which the ice ad- 

 vanced when its deposits had reached the critical or neutral 

 height. Such a pavement would continually extend outward. 



The only sorting of the material likely to occur under these 

 conditions would be that due to the earlier deposition and 

 entanglement of the larger fragments, thus producing a more 

 stony deposit nearer inland, just as Mr. Geikie describes the 

 actual deposits of till where, " generally speaking, the stones 

 are most numerous in the till of hilly districts ; while at the 

 lower levels of the country the clayey character of the mass is 

 upon the whole more pronounced." These " hilly districts," 

 upon the supposition of greater submergence, would be the 

 near shore regions, and the lower levels the deeper sea where 

 the glacier floated freely. 



The following is Mr. Geikie's description of the distribution 

 of the till (page 13) : "It is in the lower-lying districts of the 

 country where till appears in greatest force. Wide areas of the 

 central counties are covered up with it continuously, to a depth 

 varying from two or three feet up to one hundred feet and 

 more. But as we follow it toward the mountain regions it be- 

 comes thinner and more interrupted the naked rock ever and 

 anon peering through, until at last we find only a few shreds 

 and patches lying here and there in sheltered hollows of the 

 hills. Throughout the Northern Highlands it occurs but 

 rarely, and only in little isolated patches. It is not until we 

 get away from the steep rocky declivities and narrow glens and 

 gorges, and enter upon the broader valleys that open out from 



