464 FRANK BURSLEY TAYLOR 
Gulf Stream by the submergence of the Isthmus of Panama or 
by the elevation of a supposed Antillean continent, or those that 
rely upon mere epeirogenic uplift in the north, they have no 
sort of intelligible relation to the characters revealed by the 
drift. In fact they appear to fall so far short of explaining 
these characters that there seems to be no longer a hope of 
gaining any real advantage from their consideration. 
SUMMARY. 
1. Between Cincinnati and Mackinac the Wisconsin drift 
formation has fifteen terminal moraines which form a consecu- 
tive series marking the retreat of the last ice-sheet; and there 
are three more farther north probably belonging to the same 
set. The series seems to be complete and is believed to con- 
stitute the simplest and most perfect known. 
2. Making due allowance for the influence of topography, it 
appears that the intervals between the members of the series are 
remarkably regular, suggesting periodic halts or oscillations of 
the retreating ice-front, which appear to be attributable only to 
a periodic change of climate. But, excepting the annual period 
and a thirty-five-year period deduced by Forel and others 
from glacial and meteorological observations, the only periodic 
change of climate known is that due to the precession of the 
equinoxes with a period averaging 21,000 years and a minimum 
of 10,500. 
3. A study of the forms of the moraines where they were 
made under the simplest conditions shows that they were always 
made at the climax of a readvance. In one instance at least 
the readvance appears to have covered more than half of the 
space of the previous retreat. Theoretically, the influence of 
precession on climate would cause a to and fro oscillation of the 
ice-front after the manner of a simple harmonic, and this super- 
posed upon a greater and slower change, would produce an 
oscillating retreat with minor periodic readvances. This appears 
in fact to have been the manner of retreat during the formation 
of the moraine series, for those formed under the simplest con- 
