the whole of what are now the prairies. Before the advent of the
ice the whole section consisted of a broadly banded coastal plain
much older than that of the Atlantic coast. When the ice with its
burden of material scraped from the hills of the north passed
over the coastal plain, it filled the hollows with rich new soil.
The icy streams that flowed out from the glaciers were full of
fine sediment, which they deposited over enormous flood plains.
During dry seasons the winds picked up this dust and spread it
out still more widely, forming the great banks of yellow loess
whose fertile soil mantles the sides of many a valley in the
Mississippi basin. Thus glaciers, streams, and winds laid down
ten, twenty, fifty, or even one hundred feet of the finest, most
fertile soil. We have already seen how much the soil was improved
by glaciation in Wisconsin and Ohio. It was in the prairie States
that this improvement reached a maximum. The soil there is not
only fine grained and free from rocks, but it consists of
particles brought from widely different sources and is therefore
full of all kinds of plant foods. In most parts of the world a
fine-grained soil is formed only after a prolonged period of
weathering which leaches out many valuable chemical elements. In
the prairies, however, the soil consists largely of materials
that were mechanically ground to dust by the ice without being
exposed to the action of weathering. Thus they have reached their
present resting-places without the loss of any of their original
plant foods. When such a soil is found with a climate which is
good for crops and which is also highly stimulating to man, the
combination is almost ideal. There is some justification for
those who say that the north central portion of the United States
is more fortunate than any other part of the earth. Nowhere else,
unless in western Europe, is there such a combination of fertile