Great Lake Basins of the St. Lawrence. 267 
engineers of the United States War Department, do not in- 
dicate a greater depth than 870 feet, which is 292 feet be- 
low ocean level. This deepest point lies in the latitude of 
44° 30’ and rather nearer the Michigan than the Wisconsin 
shore. Buta relatively limited portion of the lake has a 
depth exceeding 600 feet, and all of this portion is located 
in its northern half. The most northern parts of the lake 
are comparatively shallow, but there is clear evidence of a 
broad river channel cut through the rocky bed of the lake 
and running along the north side of the Beaver Island group 
to the Straits of Mackinac. Whilst the depth of the lake 
waters everywhere on either side is under 100 feet, this 
ancient river channel registers from 100 to 302 feet, the 
deepest points being in the narrowest parts of the Straits. 
From the Lake Huron side, another river channel entering 
the Strait, and with depths of from 154 feet to 210 feet, 
almost completes a circle around the Island of Mackinac, 
but is presently disconnected from the Michigan river 
channel by a narrow ridge or anticlinal, about two miles in 
width—the result of more recent warpings in the strata 
there—running from Point St. Ignace south-eastward, and 
over which there are now from 17 to 70 feet of water. 
These two subaqueous river channels were, without doubt, 
at one time connected, and at a previous period of these 
lakes’ history, formed the outlet for the waters of Lakes 
Superior and Michigan. Both of these channels are 
flanked by the rocks of the Onondaga, Helderburg and 
probably Niagara groups, and have no doubt been enlarged 
by water action. It is at the same time a coincidence that 
in Lake Michigan the channel runs almost parallel with the 
northern coast of the lower peninsula of Michigan west of 
Mackinac and of the subaqueous ridge which connects the 
Helderberg rocks here with those of the Beaver Island 
group. Whilst this course is nearly due east and west, it 
will be noticed in this connection that the line of direction 
of the jutting headlands and islands immediately near them 
on the north shore, at and east of Mackinac Straits, is almost 
due south-east, and must be attributed to other causes. 
