72 AQUEOUS AGENCIES. 



500 feet thick. Under the influence of the melting power of the sun un- 

 equally affecting different parts, they assume various and often strange 

 forms. The accompanying figure (Fig. 66) gives the usual appearance 

 in the northern Atlantic. Those separated from the antarctic barrier 

 present, before they have been much acted upon by the sun, a much 

 more regularly prismatic appearance. Fig. 67 gives the appearance of 

 one of these prismatic blocks or tables, 180 feet high, seen by Sir James 

 Ross in the antarctic seas. 



Icebergs as a Geological Agent — Erosion. — The polishing and scor- 

 ing effects of the ice-sheets and of their discharging glaciers must, of 

 course, extend over the sea-bottoms about polar coasts as far as the 

 glaciers touch bottom, which, considering their immense thickness, 

 must be for considerable distances (Fig. 65, s' to g). This, however, is 

 glacier agency rather than iceberg agency. On being separated they 

 float away, and are carried by currents with their immense loads of 

 earth and bowlders, amounting often to 100,000 tons or more, as far 

 as 40° or even 36° latitude, where, being gradually melted, they drop 

 their burden. If the water be not sufficiently deep, they ground, and 

 being swayed by waves and tides they chafe and score the bottom in a 

 somewhat irregular manner ; or, packing together in large fields, and 

 urged onward by powerful currents, they may possibly score the bot- 

 tom over considerable areas somewhat in the manner of glaciers. A 

 large iceberg will ground in water 2,000 and 2,500 feet deep ; they 

 have been found by James Ross actually aground in 1,560 feet of water 

 off Victoria Land. A true glaciated surface, however, can not be pro- 

 duced by icebergs. 



Deposits. — The bottom of the sea about polar shores is found deep- 

 ly covered with the materials brought down by glaciers and dropped 

 by icebergs (Fig. 65). Again, similar materials are carried by icebergs 

 as far as these are drifted by currents, and spread on the bottom of the 

 sea everywhere in the course of these currents. Where stranded ice- 

 bergs accumulate, as on the banks of Newfoundland, large quantities of 

 such materials are deposited. These banks are in fact supposed to 

 have been formed, in part at least, in this way. Such deposits have 

 not been sufficiently examined ; they are probably somewhat similar to 

 those of glaciers, exhibiting, however, some signs of the sorting power of 

 water. Balanced stones or bowlders in insecure positions can hardly 

 be left by icebergs. 



Shore-Ice. 



In cold climates the freezing of the surface of the water forms 

 sheets of ice many inches or even feet thick, and of great extent, about 

 the shores of rivers, bays, and seas. They often inclose stones and 

 bowlders of considerable size, and when loosened in spring from the 

 shore they bear these away, and again drop them at considerable dis- 



