290 The American Geologist. April, 18<j7 
CORRESPONDENCE. 
MoRAiNRs OF Recession and their Significance in (xlacial The- 
ory.* The Wisconsin stages of glaciation shows tifteen terminal mo- 
raines in consecutive series in the interval between Cincinnati and the 
straits of Mackinac, and three more farther north probably belonging- 
to the same series. The count of moraines was made in the axes of a 
connected series of open valleys and on the lake shore where the oscilla- 
tion of the ice-front wa i freest and its amplitude greatest. With due con- 
sideration of topography and its influence on ice-motion, it seems ap- 
parent that the series is completely representative of all the oscillations 
which the ice-front underwent in retreating over this interval, and 
further, that the oaciilations were distinctly of a periodic nature and 
that the periodicity affected a wide area. The oscillations of the ice- 
front is attributed to climatic oscillation and this, it is thought, can be 
ascribed only to an astronomical cause. Tlieoretical considerations and 
also the bast conclusions deducible from recent observations in Green- 
land by Chamberlin, Salisbury and others, indicate a very slow rate of 
motion for the ice-sheet. And further, the same theoretical considera- 
tions and observations indicate that nearly all the drift transported by 
the ice-sheet was carried englacially,^ but that the load of debris in 
transport in a given section of the ice was quite small. On these grounds 
the annual and thirty year periods of climatic oscillation are rejected 
as far too short. It is believed that the building of a terminal moraine 
live miles or more wide and 50 to 300 feet or more high required hun- 
dreds or probably a few thousands of years. The imbrication of the 
drift, the form of the simplest moraines, and other phenomena show 
that the moraines were generally, if not always, built after a re-advance 
of the ice. The oscillations of the ice-front were like those which would 
be produced by the precession of the equinoxes through its influence on 
climate. This, therefore, is taken as the probable, but not the certain 
cau53. The average period of precession is 21,000 years. Its extreme 
minimum is 10,509 years. The minimum is believed to accord well with 
the evidence, but the average seems rather long. 
While the oscillations were going on there was a greater and much 
slower change of climate in progress. The regularity of the moraine 
series proves the extreme slowness and regularity of this greater 
change — whether its rate was uniform or varied slowly in a uniform 
way. The latter mode is indicated by the facts, and this too, then, as 
it seems to the author, has a distinctive character which can be as- 
cribed only to an astronomical cause. The oscillation of the ice-front 
and also the greater climatic change that caused the epoch of glacia- 
tion are both astronomic in quality. Croll's hyixjtheeis, even with Bell's 
modification, is not thought to be a satisfactory explanation. 
F. B. Taylor. 
*Ahstract af paper read at Washington. December, 1896. 
