Date of the Glacial and the Upper Miocene Period. 367 



little harm to the rocky sea-bottom over which it floats. It is 

 true that an iceberg moving with a velocity of a few miles an 

 hour, if it came in contact with the sea-bottom, would, by the 

 mere force of concussion, tear up loose and disjointed rocks, and 

 hurl some of the loose materials to a distance; but it would do 

 but little in the way of grinding down the rock against which it 

 struck. But even supposing the bottom of the iceberg were 

 properly shoed with the necessary grinding materials, still it 

 would be but a very inefficient grinding agent ; for & floating ice- 

 berg would not be in contact with the sea-bottom. And if it 

 were in contact with the sea-bottom, it would soon become 

 stranded and, of course, motionless, and under such conditions 

 could produce no effect. 



It is perfectly true that although the bottom of the berg may 

 be devoid of grinding materials, yet these may be found lying on 

 the surface of the submarine rock over which the ice moves. But 

 it must be borne in mind that the same current which will move 

 the icebergs over the surface of the rock will move the sand, 

 mud, and other materials over it also; so that the markings 

 effected by the ice will in all probability be erased by the current. 

 In the deep recesses of the ocean the water has been found to 

 have but little or no motion. But icebergs always follow the 

 path of currents ; and it is very evident that at the comparatively 

 small depth of a thousand feet or so reached by icebergs the mo- 

 tion of the water will be considerable ; and the continual shift- 

 ing of the small particles of the mud and sand will in all proba- 

 bility efface the markings which may be made now and again by 

 a passing berg. 



Much has been said regarding the superiority of icebergs as 

 grinding and striating agents in consequence of the great velo- 

 city of their motion in comparison with that of land-ice. But 

 it must be remembered that it is while the iceberg is floating, 

 and before it touches the rock, that it possesses high velocity. 

 When the iceberg runs aground, its motion is suddenly arrested 

 or greatly reduced. But if the iceberg advancing upon a sloping 

 sea-bottom is raised up so as to exert great pressure, it will on 

 this account be the more suddenly arrested, the motion will be 

 slow, and the distance passed over short, before the berg becomes 

 stranded. If it exerts but little pressure on the sea-bottom, 

 it may retain a considerable amount of motion and advance to a 

 considerable distance before it is brought to a stand ; but, exert- 

 ing little pressure, it can perform but little work. Land-ice 

 moves slowly, but then it exerts enormous pressure. A glacier 

 1000 feet in thickness has a pressure on its rocky bed equal to 

 about 25 tons on the square foot ; but an iceberg a mile in 

 thickness, forced up on a sloping sea-bottom to an elevation of 



