188 GLACIAL HISTORY. 



its surface ; and it is not unusual to see a portion of a glacier 

 quite covered with mud and stories. 



Most glaciers also possess in the middle of their surface central 

 moraines, which sometimes consist only of a single row of blocks, 

 and at other times form a thick line of debris running down the 

 whole length of the glacier, and following all its curves and 

 bendings. Looked at from great elevations the central moraines 

 appear like regular dark lines, which may often be traced for 

 miles over the bluish-white ice. The central moraines are pro- 

 duced by the union of two or more glaciers. When two glaciers 

 issuing from separate valleys unite, a central moraine, formed 

 by two of the lateral moraines, is produced ; and therefore, as a 

 rule, every glacier will have as many central moraines as it has 

 received tributary glaciers. On the Aar glacier a great central 

 moraine commences where the Lauteraar glacier and the Fin- 

 steraar glacier unite ; it attains a height of 42 metres (or nearly 

 46 yards), and increases in width to 200 metres (or 2187 yards) at 

 its extremity. To the left of this there are seven other moraines, 

 and to the right of the central moraine there are eight others, 

 each of which possesses its peculiar kinds of rock. The Gorner, 

 the Rosetsch, the lower Bernina, and many other glaciers ex- 

 hibit very fine central moraines. As the stones of the moraines 

 protect the ice beneath them from the influence of the sun and 

 of warm air, the ice under the moraines is not melted to so 

 great an extent as that which is exposed to the open air. Thus 

 valleys and ranges of heights are produced on the glacier ; and 

 as some of the stones lying on the elevated portions slide down 

 into the hollows between them, the materials of different mo- 

 raines in very long glaciers become gradually intermingled. 



Great heaps of stones accumulate at the termination of the 

 glacier, the stones having been carried down the valley in the 

 lateral and central moraines. By the melting-away of the ex- 

 tremity of the glacier these masses of stones fall to the bottom, 

 and form the terminal moraine, which usually surrounds the 

 melting mass of ice in a crescent-shaped curve at the lower part 

 of the valley. If the glacier remains unchanged for many years, 

 all the debris that it has picked up in its course are carried to 

 this point and heaped up into a vast moraine ; if it diminishes, a 

 second crescent-shaped curve will be formed near the glacier ; 



