"Soils are everywhere the product of rock disintegration, and so the quality of the soils in a given locality must necessarily be determined in large measure by the kind of rock from which they were derived. "From this point of view, therefore, the history of Iowa's superb soils begins with first steps in rock making. The very oldest rocks of the Mississippi Valley have contributed something to making our soils what they are, and every later formation laid down over the surface of Iowa, or regions north of it, has furnished its quota of materials to the same end. The history of Iowa's soils, therefore, embraces the whole sweep of geologic times. "The chief agents concerned in modifying the surface throughout most of Iowa since the disappearance of the latest glaciers have been organic, although the physical and chemical influences of air and water have not been without marked effect. The growth and decay of a long series of generations of plants have contributed certain organic constituents to the soil. Earth worms bring up fine material from considerable depths and place it in position to be spread out upon the surface. They drag leaves and any manageable portion of plants into their burrows, and much of the material so taken down into the ground decays and enriches the ground to a depth of several inches. The pocket gopher has done much to furnish a surface layer of loose, mellow, easily cultivated and highly productive soil. Like the earth worm, the gopher for century after century has been bringing up to the surface fine material, to the amount of several tons annually to the acre, avoiding necessarily the pebbles, cobbles and coarser constituents. The burrows collapse, the undermined boulders and large fragments sink downwards, rains and winds spread out the gopher hills and worm castings, 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 in which it was found by the explorers and pioneers from whose advent dates the historical period of our matchless Iowa." The last invasion, we are informed, was from 100,000 to 170,000 years ago--somewhat prior to the recollection of the "oldest inhabitant." CHAPTER II _The First Inhabitants_