Great Ice-Dams. — Taylor. 7 
though it was constantly advancing slowly forward, it fitted 
itself the while so closely to the ground over which it passed 
that it constituted a dam or barrier as impervious and impas- 
sable for water as the solid, rocky range of the Laurentides. 
In the valleys and basins so obstructed, the water from melt- 
ing ice and snow and from rain gathered and formed into lakes. 
Each lake stood at the level of the lowest point on its rim and 
this became its outlet or place of overflow. With continued re- 
treat, the first small lakes expanded their boundaries over the 
newly uncovered ground and with the opening of new outlets 
their surfaces dropped to lower and lower levels. Lakes in 
descending series began in this way in several basins probably 
almost simultaneously. One of the earliest began with a com- 
paratively small lake at the head of the Maumee valley 
in Ohio and Indiana and spread from that over the whole of 
the Erie and parts of the Huron and Ontario basins. It is of 
the history of this series, with special reference to its great 
ice-dams, that this paper is designed to treat. 
Peculiarities which characterize causes are generally more 
or less manifest in results and are often the main reliance in 
analysis. Thus, certain peculiarities which characterized the 
glacial retreat and exercised a marked influence upon the gla- 
cial lakes may now be used as keys to the lake history. A 
clear perception of them at the start will simplify the story 
and be a material aid in the following presentation. 
Retreat of the Ice-sheet. 
The retreat of the ice-sheet was not a simple drawing back 
at a steady, uniform rate, but was an oscillating movement and 
apparently periodic, as though alternating two steps back and 
one forward, two back and one forward, continually. Wher- 
ever the ice-front stood for a time at one line, it dumped its 
detrital load and built a more or less defined ridge or terminal 
moraine. In the general retreat of the ice-sheet the moraines 
built at the climaxes of rcadvance were left standing, while 
those built at the alternating climaxes of retreat were always 
overridden and destroyed. Thus, the "moraines of recession," 
as those now found in consecutive series are called, show by 
the rhythm of their intervals the periodic nature of the oscilla- 
tions of the receding ice-front. Fifteen moraines in series oc- 
