412 The American Geologist. Juno, i897 
on each continent is mainly attenuated. In some parts it is 
represented only by scattered boulders. The outermost till 
deposits often occur in patches, with intervening tracts which 
were doubtless covered by the border of the ice-sheet, but re- 
ceived little or no drift. 
At varying distances of a few miles to two or three hundred 
miles back from the limits of glaciation, the drift is amassed 
in prominent knoUy and hilly belts which are traced hundreds 
of miles along the boundaries held by the North American 
and European ice-sheets after they had thus considerably 
waned from their maximum areas. Once inaugurated b}' the 
formation of the first and outermost marginal moraine, this 
stage or epoch of the Ice age was thenceforward characterized 
on both continents, by a rhythmic accumulation of morainal 
belts of ridges and hillocky drift, with far more abundant 
boulders than are found on the intervening tracts where the 
drift is spread with a smoother surface. 
The Minnesota lobe of the North American ice-sheet, in its 
recession 450 miles from Des Moines, Iowa, to Vermilion lake 
in northeastern Minnesota, formed twelve moraine belts, which 
I have mapped for the Minnesota Geological Survey, averag- 
ing about forty miles apart along the axis of this lobe. Upon 
its sides the series of moraines is crowded closer together, and 
many irregularities of the glacial retreat with re-advances, 
are shown by blended and overlapped moraines. The correla- 
tive series of moraines in Illinois, Indiana, Ohio, and Michi- 
gan, traced by Leverett, Taylor, and others, comprises a 
somewhat larger number of separate marginal belts, with less 
average distance between them. 
What recurring conditions caused these moraines to be 
amassed successively, one after another, to the number of 
twelve to fifteen or more in the northern part of the United 
States, with others, partly known but less fully traced and 
mapped, farther north in Canada? It is evident that the most 
southern and outer moraine is the oldest, and the others suc- 
cessively newer in their order from south to north; for if any 
member of the series had been covered by a re-advance of the 
ice-sheet, its uneven contour would have been much smoothed 
down. The moraines therefore record a somewhat regularly 
i-hythmic action of the waning ice-sheet. Was this depend- 
