3G8 The American Geologist. June. u#6 
Briefly stated, the explanation proposed is this: that these 
spared monuments of erosion along the Maquoketa river were 
surrounded by compact masses of ice formed from the waters 
of the springs and streams native to the region before the [ce 
age; and that, thus embedded, they were firmly held, while 
the blade of the huge glacial microtome cut only thin sections 
from their summits ; and that the truncated columns were left 
undisturbed by the slow melting of the river ice. after the 
glacial ice had disappeared from above. 
The objection may be made that the recent studies of exist- 
ing glaciers in Alaska and Greenland do not show such frozen 
streams in advance of the glacial ice. The majority of the 
glaciers, however, that are now seen in operation, are not ad- 
vancing but retreating; and this in spite of a heavier snow- 
fall than we can assume for northeastern Iowa in the Glacial 
period. This is evident^ true of the Alaskan icefields, and 
while the Greenland ice may have again assumed the aggres- 
sive, its advance is evidently slow and accompanied by rapid 
marginal waste; so that the supply of water is spasmodic and 
largest in the warm season, having volume enough to remove 
masses that might congeal in their channels during the cold 
season. 
A glacier whose front is stationary, as the result of an equi- 
librium between forward motion and marginal waste, as well 
as one that is retreating, as a result of the preponderance of 
the latter, will manifestly have its front beyond the line of 
perpetual snow or frost; but it does not appear that this 
would necessarily be true of an extensive field of ice, where 
showing a decided advance of front over a comparatively level 
country. That is, during the early portions of the Kansan 
and Iowan stages of the Ice age, in northeastern Iowa, the 
snow line may have been but little behind, or even in advance 
of, the front of the ice-sheet. 
The conditions favoring the formation of such frozen 
streams would seem to be diminished drainage, constant sup- 
ply of water at high levels, large snowfall, seclusion from the 
sun's rays and a sufficiently cold climate. These conditions 
were present in a marked degree in the region under consid- 
eration. 
