Interglacial Chronometer. — Winchell. 75 
5. Tlie recession of this waterfall backward till it cut a gorge 
through the obstruction which it had been compelled to surmount 
rather than flow round. 
6. The greater depth, width and different direction of the old 
gorge from the gorge w^hich the river excavated in the later stage 
of its history. 
7. The existence of another older valley from which the river 
might be presumed to have been expelled. 
8. The movement of the ice, in general, in that direction, 
about the falls of St. Anthony, which would tend to throw the 
river from its supposed older channel toward its supposed newer. 
These conditions all do exist, in connection with the second 
right angle, as plainly as they do in connection with the first. 
The map on the accompanying plate (vi) illustrates them. The 
difficulty which we encounter in attempting to handle the facts 
consists in the consciousness that the}' relate to a very old history. 
We have not often attempted to place time limits on geological 
data, and when we have ventured to do it we have confined our 
assays within post-glacial time. Our next step must be beyond 
the border that sets off' post-glacial history as a unit of geological 
time amenable to our scrutijiy, and leads into interglacial time. 
We reach the results inductively, just as we have for post-glacial 
time. Here are a set of facts. They need reasonable interpreta- 
tion. That hypothesis which suited the other set of identical 
facts is naturally the first one invoked for this. Does it apply as 
well? 
The movement of the ice of the first glacial epoch has been 
stated to have been from the northeast in the vicinit}' of the falls 
of St. Anthony, that of the second from the northwest. This is 
based on the prevalent direction of transport shown by the nature 
of the drift materials when they are referred to their native places. 
The lower till — the copper-colored or red till — has a preponder- 
ance of rock debris from the region of lake Superior, and north- 
ern Wisconsin. The upper or gray till, which has a marked con- 
trast with the red, has a preponderance of rock debris from the 
direction of the Red river valley. These tills come into contact 
about Minneapolis and St. Paul, and in all places where the two 
are seen in place, the gray overlies ths red, l)ut they are frequently 
separated b}' a layer of sand and gravel referaljle to the epoch of 
the red. When the glacier which deposited the red drift ap- 
