THE AGE OF ICE. 



645 



The percentage of stones present is variable. They are most 

 common in the hilly districts, while in the lowland region the clay 

 may predominate. Most of them show markings all over. They vary 

 in size from grains to blocks several feet or even yards in diameter. 

 Their shape is peculiar. They are neither round nor oval like the 

 pebbles in river-gravel, or the shingle of the sea-shore ; nor are they 

 sharply angular like newly- fallen debris at the base of a cliff; but 

 seem to be like the latter in general shape with the sharp corners and 

 edges smoothed away. They are to the geologist what hieroglyphics 

 are to the Egyptologist — the silent but impressive records of an age 

 long passed away. 



In narrow valleys the till often accumulates in such amount as to 

 cover the solid floor many yards in depth. In such cases, the surface 

 may be level, and, in the subsequent periods, the streams have made 

 excavations in the mass, leaving the till in the terrace-form. Its un- 

 stratified character will be determined by examining the earth along 



Fig. 4. 



Greskin Burn, Dumfriesshire.— Stream cutting through Terrace of Till. 



the sides of the escarpment, as, superficially, it is difficult to distin- 

 guish the material from the terraces of later age. 



When the till is removed from the underlying rocks, their upper 

 surface almost invariably shows a smoothed and often highly-polished 

 appearance, and the whole pavement is marked with those peculiar 

 scratches or striae that form so characteristic a feature of the embedded 

 stones. The extent to which the polishing is carried depends very 

 much upon the nature of the rock. As the best-preserved stones of 

 the bowlder-clay consist of close-grained limestone and clay iron-stone, 

 so the same materials in the ledge-cOndition preserve most perfectly 





