234 
Frank Carney 
disposal; and the farmers find them convenient in disposing of 
excess water from their fields. The ^^Blue Hole^^ near Castalia 
is the outlet of an underground stream. 
Near the south side of the sheet several limited areas rise 
slightly above the 790-foot contour, and a few dunes reach 800 
feet. The more important relief features of the area are due to 
outcrops of limestone; the shoreline structures and dunes are of 
minor importance; however, the altitude of some of the hills 
has been increased 25 feet by dunes blown up from the fringing 
beaches. The series of hills have a general northeast-southwest 
direction in alignment with the axis of the Cincinnati anticline 
which here, as elsewhere in the state, is frequently a drainage 
divide. 
Resume of lake history. Many workers in geology have con- 
tributed to our present knowledge of the pro-glacial lakes. In 
the Erie basin, which alone is concerned in the present paper, 
the most recent and critical work is that of Mr. F. B. Taylor 
and Mr. Frank Leverett. My indebtedness to these gentlemen 
can not, in all cases, be specifically cited, because in conversation 
and by correspondence they have given me the benefit of findings 
as yet unpublished. 
It was formerly thought that the recessional movement of the 
ice-front, during the Wisconsin stage, was interrupted only by 
halts which are indicated today by the morainic loops about the 
lake basins. Later it was established that slight oscillations of 
the ice-margin varied the regressive movement. At least one such 
readvance was registered on the Thumb of Michigan® (the pen- 
insula between Saginaw Bay and Lake Huron) ; theoretically there 
must have been two other advances of the ice in this area, though 
the evidence has not been published. 
In reference to the history of several of the pro-glacial lakes 
of the Erie-Huron Basin, the Thumb of Michigan was a critical 
area, because, flowing westward across it, lay the outlet channels 
of these lakes; a readvance of the glacier might cover a given 
channel, and bury part of the correlating beach beneath outwash 
deposits; Taylor reports this condition.^ 
® F. B. Taylor^ ‘‘Relations of Lake Whittlesey to the Arkona Beaches/' Mich. 
Acad, of Science, report for 1905, p. 34. 
^ Ibid., p. 34. 
