~@. K. Warren— Valley of the Minnesota and Mississippi. 425 
sion, as they are near Dubuque. This is in accordance with 
usually received ideas, that where the stream is confined by 
hard banks its increased velocity, due to such contractions, 
may have caused the streams to abrade deeper. It is improb- 
able that the ancient river, where it cut its way either as a 
cataract or in any other manner of river erosion, made the val- 
ley as wide as we now see it. It most probably underwent 
subsequent widening from the impinging of the currents against 
the foot of the high banks, thus removing the débris falling from 
the cliffs above, as well as scouring away the unbroken strata 
against which it washed. Even now, although the great river 
has disappeared, we see that the valley is still widening in some 
places where the river flows at the foot of the high bluffs, 
although, in the great majority of cases, the atmospheric ero- 
sions have covered the steep, rocky scarps with detritus, which, 
clothed with vegetation, preserves them from the influence of 
the air. Where the river now impinges against the banks com- 
posed of soft strata, we sometimes see its effect in the fresh-cut 
appearance of the cliff, and are led to give greater weight to 
similar operations in the past, when like forces were probably 
more intense, 
I have selected one (Diagram D) such cliff in the wide part 
of the valley below La Crosse, which part indicates much wid- 
ening since the first cutting out of the river's course. It is 
apparent that no stream of water could have cut down from 
to AB while the opening D was available. But if we allow 
that cataract on the right of the diagram represents the condi- 
tions when the stream began to flow which cut the valley, then- 
its present course is natural, the subsequent widening bringing 
it to the state we see on the left of the diagram. 
e 
recent geological times, although antedating the historical era. 
In a similar way the time required by the Mississippi to cut the 
gorge from Fort Snelling to the Falls of St. Anthony has been 
caleulated by Professor N. H. Wincbell,* on data that makes it 
vary from 6,000 to 12,000 years, by assuming that the forces in 
producing this result have remained uniform during this period. 
_ That any date in geological time, capable of being expressed 
in definite numbers of years, can be deduced from existing ob- 
Servations, seems highly improbable; but whenever a condition 
18 observed which may be referred properly to the causes now 
at work, whether of greater intensity or less, it is reasonable to 
regard the work done as of recent geological origin. We cannot 
heglect this uncertain method of drawing inferences, since it is 
the best we have, and we should endeavor, by continued inves- 
* Report Geological Survey Minnesota, 1876. 
though the result is uncertain, it indicates the origin to b 
