icycalmca

I was perusing some old physiographic texts, and came upon
a footnote re: a re-working of Winchell's estimate of
the time elapsed since the last time glacial ice started to
retreat in Minnesota, which was 8000 years.
That estimate was the basis for his estimate of
the maximum age of the glaciated backside of the KRS;
applying estimates of relative degrees of weathering
of different types of surfaces on the KRS
gave relative ages for those surfaces.
As the inscription appeared to be 1/16 as weathered
as the glaciated backside, Winchell assigned
an approximate age of 500 years to the inscription.
Winchell had earlier investigated the recession of
the gorge below the St. Anthony Falls
(on the Mississippi River in Minneapolis), and had
estimated that it had taken 8000 years for the falls to
recede to their current position, which he took to be
the length of time that had elapsed since the last
ice sheet had covered the area of the falls.
(Actually, about 16,000 or 17,000 years had elapsed
since the Minneapolis area was deglaciated.)

But, in

Physiography of Eastern United States
Nevin F. Fenneman
1938

on page 586 is this footnote:

1 F. W. Sardeson, attaching different values to certain factors,
makes the time
12,000 years since the falls originated, and
8000 years since Glacial Lake Agassiz found its northern outlet.

(Actually, that was more like
16,000 years and
10,500 years.)

I have not found a Sardeson reference that says that
(it might be somewhere in
Pan-American Geologist, which is defunct and inaccessible to me),
but in

Frederick W. Sardeson
1908
Beginning and recession of St. Anthony Falls
Geological Society of America Bulletin
vol. 19
pp. 29-52

he gives (p.52) rough figures, based on a more extensive examination
of teh area and its rocks, etc., of
30,000 years since the falls area was deglaciated and
10,000 years since Glacial Lake Agassiz found its northern outlet.

In any case, the accuracy of Winchell's weathering ages
would have been in dispute soon after he presented them.

Daryl Krupa