,Great la -Dams. — Taylor. 23 
77ie Ice-daJiimcd Lakes. 
Lake Mawnee. (First stage.) The lake which was held in 
front of the Defiance ice-dam has been called lake Maumee * 
Its beaches were known to the earliest inhabitants, by whom 
they were chosen as roadways through the forests and were 
called "ridge roads." The one on the south side of the lake is 
known as the Van Wert ridge and runs westward from Findlay 
float free of the bottom in 1000 feet of water, and every foot added to 
its thickness would increase the pressure on the bottom and reduce 
the tendency to tioat. It can hardly be doubted that where the ice 
filled deep lake basins like that of lake Ontario, its thickness -^o or 40 
miles back from the edge might have equaled or exceeded iioo feet. 
Indeed, we can not avoid the conclusion that when the ice-front stood 
at Lockport on the moraine of that name, the thickness of the ice in 
the deepest part of the Ontario basin directly north of Lockport must 
have been at least iioo to 1200 feet, else the glacial surface could not 
have sloped toward Lockport, which it certainly did. That point 
was then probably not over 30 or 40 miles back from the apex of the 
lobe in the west end of the basin as the lobe would have been with 
the water absent. As it was, the thinner edge of the ice probably broke 
ofif back to a point where its thickness was iioo feet or more, though 
actual evidence of such a condition has not yet been reported. In this 
connection it is interesting to note the conditions of the great '"icy 
barriers" of the Antarctic regions. The Antarctic ice-sheet appears 
to be a noble example of its kind and in most respects truly com- 
parable to the great Pleistocene sheet that overspread so much of 
North Am.erica, except that its wastage is mainly in the sea by ice- 
bergs, while that of the older sheet was mainly on land by melting. 
The great Victoria barrier is a continuous ice-wall rising vertically 
from the water 100 to 200 feet, with few cracks or crevices and with 
flat and even top — the whole extending about 450 miles in length; and 
there are several others of similar character and proportions. The 
bergs that break off from these barriers are not numerous, but they 
are of gigantic size and of a tabular form not seen in Greenlan''. Prof. 
Wyville Thomson of the "Challenger" Expedition says: "The ice- 
bergs, when they are first dispersed, float in from 200 to 250 fathoms." 
(1200 to 1500 feet.) "I conceive that the upper surface of one of these 
great tabular southern icebergs, including by far the greater part of 
its bulk, * * * -was formed by the piling up of successive layers of 
snow during the period, amounting perhaps to several centuries, dur- 
ing which the ice-cap was slowly forcing itself over the low land and 
out to sea over a long extent of gentle slope, until it reached a depth 
considerably above 200 fathoms, when the lower specific weight of 
the ice caused an upward strain which at length overcame the cohesion 
of t'le mass, and portions were rent off and floated away." (Quoted 
by Prof. Huxley in his "Discourses Biological and Geological." p. 
80.) Probably this process became effective in the deepest parts of our 
glacial lakes, but the areas over 1000 feet deep were very limited, the 
deepest being about 14.30 feet in lake Superior. The conditions of 
the icy barriers of the Antarctic regions, and not of the fiord tongues 
of Greenland, furnish a true parallel to the conditions of our ice- 
lobes, where they entered the deeper parts of our glacial lakes. 
*So named by C. R. Dryer in "Geology of Allen County," in the 
sixteenth report of the State Geologist of Indiana, 1888, pp. 107-126. 
