National Geographic : 1892 Mar 21
The Birth of Bergs. below water more rapidly than it does in the air, is shown by the fact that icebergs roll over, which is due to this alone. It is quite possible that the icebergs darkened by mud and rock may not have come from the bottom, but may be merely expos ing the side of some old crevasse into which debris from a sur face moraine has fallen. The bergs which we saw rise from below the water usually came up after a very heavy fall from above, as though some crack had been started by the shock of the falling ice; only a few of them were discolored by debris; most were pure blue ice* Moreover, they did not rise very high out of the water. All this makes me think that they did not originate at any very great depth. Just as a stick thrown obliquely into the water may rise again at an angle, so a berg, on account of its shape, may rise so obliquely as only to reach the surface some distance from the ice-front, thus suggesting that the glacier sends out a foot along the bottom of the inlet, from the end of which the ice breaks off; but the considerations I have mentioned make it evident, I think, that this is not the case. A series of observations on the temperature and density of the water of Muir inlet at different depths and at different distances from the ice would undoubtedly afford information that would enable us to reason very accurately about the form of the ice-front below the water. The ice at the bottom of the glacier- in contact with its bed moves very slowly, and it is not improbable that the melting, where it meets the salt water, quite equals the advance. The slope from that point up is determined by the strength of the ice. If the progression of the bottom is greater than the rate of melting, the glacier will advance until it comes to a broader part of the fiord, and thus presents a broader front to the water. If the fiord were of uniform depth and breadth, the ice could only find a position of equilibrium at one end or the other. The effect of the depth of the water in determining the posi tion of the glacier's end is not apparent. As the depth is greater the pressure against the ice is greater, but at the same time the water produces a greater upward pressure on the ice, diminish ing its pressure against its bed and thus reducing the friction. Although these effects cannot balance at all depths, I am unable to indicate which one is in general the stronger. *The discolored bergs seen by Mr Russell in Disenchantment bay are probably from the debris-covered parts of the neighboring glaciers.
1892 May 15
1892 Feb 19