Page:History of Iowa From the Earliest Times to the Beginning of the Twentieth Century Volume 1.djvu/47



and the next year and the next, the process is repeated; and so it has been for all the years making up the centuries since the close of the glacial epoch. Organic agents in the form of plants and burrowing animals have worked unremittingly through many centuries and accomplished a work of incalculable value in pulverizing, mellowing and enriching the superficial stratum, and bringing it to the ideal condition to which it was found by the explorers and pioneers from whose extent dates the historical period of our matchless Iowa.”

It is estimated that the last invasion of Iowa by the glaciers was from 100,000 to 170,000 years ago. For many years scientists have been investigating the causes which have produced the great treeless plains of the Mississippi Valley. East of Ohio prairies are unknown, but as we go westward they increase in number and size. In western Indiana, and from there to the Rocky Mountains west and north the vast prairies *  prevail, although groves are often found, and the margins of lakes,  rivers and creeks are generally bordered with the same extent as trees. From 98° of longitude west and treeless plains become almost a desert.

The soil of prairies varies in formation and quality to almost as great an extent as in the timbered regions. In Michigan, Indiana and Illinois the prairies are inclined to be quite level, the surface soil is a black vegetable formation to six inches to five feet or more in depth. In Iowa the prairies are more rolling, affording better surface drainage.

In southern Wisconsin, Minnesota, the Dakotas, western Nebraska, northern Missouri, and western Kansas, the vegetable formation is lighter, sand and gravel being quite common on the surface.

Along the first river bottoms, the soil is generally a deep rich alluvium. The second bench often presents a mixture of sand and gravel while the bluffs show soil of a lighter color, with clay near the surface. Large bodies of

* Prairie is a French word signifying meadow. It was first applied to the great treeless plains of North America by the French missionaries who were the discoverers of the prairie regions of the west.