22 The Amcricaii Geologist. J«iy. i«h' 
land through tlie deep fiords move al a relatively rapid rate 
and break ofif as icebergs as fast as they advance. No doubt 
the ice-lobes in our lake region would have done the same 
r.nder the ?an:e conditions, but the conditions affecting them 
were very different. Our lobes were not narrow ice-tongues 
squeezing out through canyon-like fiords, but w£re broad 
lobate masses and moved forward with extreme slowness. 
Fiord tongues move so fast that great cracks and crevasses are 
produced in them dividing them into blocks which readily 
float away as soon as they enter deep water. Our lake lobes, 
on the other hand, moved so slowly that they must have re- 
mained essentially solid to their front edges, and so did not 
easily break up.* No doubt their fronts were undercut to 
som.e extent by the waves and pieces were sometimes broken 
off, but such breaks must have been rare and did not count 
as an important factor in ablation. In no case so far discovered 
was it effective enough to make the lobe front concave, as has 
been assumed by some authors for comparatively shallow wa- 
ters in lake Agassiz and in the basin of lake Champlain.t In 
depths of 200 feet the solidity of the ice-lobes appears to be be- 
yond question. For the continuous tracing of the Defiance, 
Saginaw, Port Huron and Detroit moraines, and the long 
northern limb of the Toledo moraine leaves no room for doubt. 
These moraines all show that the lobes which made them fitted 
themselves to the relief of the submerged land just as perfectly 
as if no water had been present. The facts show three things 
with respect to the condition of the ice in the lobes: it must have 
been hundreds of feet thick near the edge; it must have been 
essentially solid and without crevasses, and it must have moved 
at a relatively slow- rate — very much slower than the motion 
of Alpine glaciers and Greenland fiord tongues. J 
*Recent photographs by Chamberlin, Salisbury and others showing 
the fronts and edges of tongues of the Greenland ice-sheet, where 
they terminate on hind, may be taken as good illustrations of this 
condition. (See recent volumes of Jour, of Geol., and Bull. G. S. A.) 
fWarren Upham in "Glacial Lake Agassiz." Monograph XXV, 
U. S. Geol. Survey, i8g6; plates XVII. XIX and XX. S. P. Bald- 
win in the '"Pleistocene History of the Champlain Valley." Am. Geol., 
vol. XIII, March 1894, p. 181. T. C. Chamberlin in James Geikie's 
"Great Ice Age." 1894. Plate XV, facing page 727. 
^Remembering that the specific gravity of ice in fresh water is 
.92, it is plain that a solid mass of ice having a superficial area of say 
one square mile or more and a thickness of iioo feet would not quite 
