INTRODUCTORY. 5 



strata, and when the slope is diminished the pressure re- 

 unites the faces of the fissure, and the surface becomes 

 again comparatively smooth. Where there are extensive 

 areas of tension, the surface of the ice sometimes becomes 

 exceedingly broken, presenting a tangled mass of towers, 

 domes, and pinnacles of ice called seracs. 



Like running water, moving ice is a powerful agent in 

 transporting rocks and earthy debris of all grades of 

 fineness ; but, owing to the different consistencies of ice 

 and water, there are great differences in the mode and 

 result of transportation by them. While water can hold 

 in suspension only the very finest material, ice can bear 

 upon its surface 



rocks of the great- fc^ aa ^ > ~"~" ~\ 



est magnitude, and ^Sk \ A 



can roll or shove ''%k £-*•>-'''' "- >^1bF 



along under it \ ^^ *- £r 



boulders and peb- • w - ' ** -* ^ ^ 



bles which would ^ jjf^ 



be Unaffected ex- f ig . 6.— Section across Glacial Valley, showing old 

 Cept by torrential Lateral Moraines. 



currents of water. We find, therefore, a great amount of 

 earthy material of all sizes upon the top of a glacier, which 

 has reached it very much as debris reaches the bed of a 

 river, namely, by falling down upon it from overhanging 

 cliffs, or by land-slides of greater or less extent. Such 

 material coming into a river would either disappear be- 

 neath its surface, or would form a line of debris along the 

 banks; in both cases awaiting the gradual erosion and 

 transportation which running water is able to effect. But, 

 in case of a glacier, the material rests upon the surface of 

 the ice, and at once begins to partake of its motion, while 

 successive accessions of material keep up the supply at any 

 one point, so as to form a train of boulders and other 

 debris, extending below the point as far as the glacial 

 motion continues. 



