In the space of one hundred and seventy-six years the Mississippi has shortened itself two hundred and forty-two miles.

In the space of one hundred and seventy-six years the Mississippi has
shortened itself two hundred and forty-two miles. Therefore ... in the
Old Silurian Period the Mississippi River was upward of one million
three hundred thousand miles long ... seven hundred and forty-two years
from now the Mississippi will be only a mile and three-quarters long.
... There is something fascinating about science. One gets such
wholesome returns of conjecture out of such a trifling investment of
fact.
-- Mark Twain