A curious deposit of an impalpably fine and unstratified silt, known by the German name bess, lies on the older drift sheets near the larger river courses of the upper Mississippi basin.
It is of inexhaustible fertility, being in this as well as in other respects closely like the bess in China and other parts of Asia, as well as in Germany.
The best explanation suggested for bess is that, during certain phases of the glacial period, it was carried as dust by the winds from the flood plains of aggrading rivers, and slowly deposited on the neighboring grass-covered plains.
Most of the bess is now generally believed to have been deposited by the wind.
The larger part of it seems to date from the closing stages of the Iowan epoch, but bess appears to have come into existence after other glacial epochs as well.