Great Ice- Dams, — Taylor. 19 
found continuing toward the northeast and north around the 
cast side of the same highlands, at the north side of which the 
two undoubtedly become again united. Thus, in central On- 
tario, between lake Huron and lake Ontario, a large area of 
the highlands was left as an insular tract surrounded on all 
sides by the ice-sheet and having its only possible way of 
drainage toward the southwest along the line of contact be- 
tween the two lobes. The general slope of this insular area is 
in a southerly direction and it is probable that while the higher 
northern part of it was dry land, a lake of some size covered 
its southern portion. The existence of this lake has not been 
established by observation and is only suggested here as a 
logical probability following from the observed position of the 
moraines and the certainty that the ice in the confluent lobes 
was at least several hundred feet thick. At its apex the Detroit 
dam stood in nearly 2CO feet of w^ater, while the Euclid in the 
bed of lake Erie stood in hardly less than 240 or 250 feet. 
The Port Hnroii-Goivanda Dam. At its next position in 
Michigan the ice-front stood at the Port Huron moraine and in 
Leverett's series at the Gowanda. The two ice-lobes had now 
separated entirely, leaving a distance of about 50 miles be- 
tween their adjacent limbs in southern Ontario. At this and 
the two succeeding stages the lake in the Erie basin was held 
in by two great dams. The Port Huron dam was nearly 
200 miles long and stood in about 150 feet of water at its apex. 
The Gowanda dam was probably much longer and its point 
probably stood in something like 400 feet of water. The line 
of the latter dam is less clearly indicated than any other of the 
series and the place of its apex can only be conjectured. As 
indicated by the positions of the dams preceding and following 
it, however, it seems probable that its apex was about in the 
center of lake Erie. 40 or 50 miles west of the meridian of the 
Ohio-Pennsylvania line. At this time there was also a lake in 
the Saginaw valley and its retaining dam stood on the Saginaw 
moraine in nearly 150 feet of water and was about 125 miles 
long. At this stage of the retreat the insular area in Ontario 
had grown somewhat larger. For the ice had drawn awav 
from the southern half of its boundary and left that side facing 
the open lake, although the ice still walled in its northern 
sides. 
