370 The American Geologist. June, L896 
tribute in any way to its freedom of motion, that factor would 
be wanting in the ice of these frozen streams. The narrow, 
deep, rocky channels, with frequent rectangular turns, would 
also operate against such motion. The river ice would show 
a surface approximately level, and so give minimum resist- 
ance to the glacier's motion over it. And if there is a differ- 
ential motion in the layers of ice in the glacier, as Chamber- 
lin's studies* seem to indicate, there might easily be motion 
of a glacier over a land-locked mass of river ice, without the 
latter participating in the motion. 
It woidd also appear from these same studies. \ that in lower 
latitudes the termination of the ice lobes w T ould be less abrupt 
than in Greenland, on account of the sun's rays being more 
nearly vertical ; so that any particular portion of the river ice 
might not be subjected to a heavy load until the region about 
it would be subjected to the equalizing pressure of the glacier. 
The melting of these frozen streams was probably the clos- 
ing event of the drainage of lake Maquoketa, ]; as the sands 
that mark the shore of one stage of this lake have about the 
altitude of the erosion towers in the river bluff's. 
This, then, seems to be an explanation of the numerous ero- 
sion forms that have escaped the ice action because of their 
lower altitude, while the exposed summits were smoothed and 
rounded by the glacial tools. It is not contended, however, 
that this embedding ice would be preservative of all such 
forms in the wider and more exposed portions of the river 
valley, nor in any region where the glacial ice attained a great 
thickness. 
When once a stream is frozen to its bed an overflow must 
result if the water supply is constant, and this overflow is 
frozen more easily each time as it spreads in wider and thin- 
ner sheets over the congealed mass below. The temperature 
of the older parts of the ice would perhaps remain considera- 
bly below the freezing point, so that at night the liquid layer 
would be frozen from both its upper and lower surfaces. 
*See Journal of Geology, Sept. Oct., 1895, p. 675, et seq. 
|See Journal of Geology, July-Aug.. 1895, p. 50(3. 
JSee again McGee's memoir, U. S. Geol. Survey, Eleventh Annual 
Report, plate 61, and page 573. 
