Review and Comment. 
347 
Tyndall on the other hand says: 17 
“ I say nothing of lake basins; but it is a physical certainty 
that, given time enough, glaciers must scoop out valleys. A 
glacier with a thickness of a thousand feet will press on every 
square yard of its bed with a weight of 486,000 pounds. Such 
a weight with a motion derived from a pressure from behind 
must excavate. ” 
Dr. Von Haast 1? describes a valley on the west coast of New 
Zealand in which the ice advanced over beds of gravel without 
destroying them to any appreciable extent. 
McGee 19 observed many cases in Iowa in which deposits of till 
of moderate thickness have been made above residuary clays. 
In some cases the clay was disturbed somewhat, in others it was 
not, and the clays were not compacted more than is the lower 
till below the latest glacial drift. 
Two generations of Hugh Millers have recorded the finding of 
beds of boulders over which the ice has passed, striating and 
grinding their upper surfaces without moving them. 
The moving of ice over beds of till, of gravel, and even over a 
meadow without destroying the turf is recorded by Neumayr in 
his “Erdgeschichte. ” 
In most of the cases like those above referred to the observa¬ 
tions were made near or at the ice front. Many other similar 
cases have been noted by various observers. 
A few instances have come under my own observation in re¬ 
gions far remote from the front of the ice in its most advanced 
position. 
The first of these was seen near Big Stone City, South Dakota, 
soon after the grading of the railroad at that place. Several 
long and tolerably deep cuts were made which exposed the struc¬ 
ture of the till beautifully. 
At the base of the section was sixteen feet of older till, red, 
quite compact and with few boulders apparently. The upper 
surface of this till was a nearly level plain. The ice of a later 
advance had overrun the earlier till and had cut off a portion, 
how much I have no means of knowing, and had “ glaciated ” the 
17 Hours of Exercise in the Alps, pp. 233-247. 
18 Geol. of Canterbury and Westland. 
19 Am. Jour. Sci., vol. 18, pp. 301-303. 1879. 
