54; AQUEOUS AGENCIES. 



require special notice. These are : 1. Moraines ; 2. Glaciers as a geo- 

 logical agent ; 3. Glacier -motion ; and, 4. Glacier -structure. 



Fig. 43.— Zermatt Glacier (Agassiz). 



Moraines. 



There are four kinds of moraines described by writers, viz., lateral 

 moraines, medial moraines, terminal moraines, and ground moraines. 

 Lateral moraines are continuous lines of earth and stones, arranged 

 on either margin of the glacier and evidently formed from the ruins of 

 the crumbling cliffs of the inclosing valley. This debris does not fall 

 from every part of the valley-sides, but generally only from certain 

 bold, projecting cliffs. It is converted into a continuous line by 

 ,the motion of the glacier, just as light materials thrown constantly 

 into a river at one point would appear as a continuous line on the 

 stream. 



Medial moraines are similar lines of debris, occupying the central 

 portions of the glacier. Sometimes there is but one ; sometimes two, or 

 more ; sometimes the whole surface of the glacier is almost covered with 

 them. The true explanation was first pointed out by Agassiz. They 

 are formed by the coalescence of the interior lateral moraines of tribu- 

 tary glaciers, carried down the main trunk by the motion of the ice- 

 current. The accompanying map (Fig. 44) of the Mer de Glace and 



