Page:History of Iowa From the Earliest Times to the Beginning of the Twentieth Century Volume 1.djvu/40

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was balmy and laden with the odor of flowers and fruits. The bright summer days seemed never ending. A listless languor sent the birds and beasts into the shade at midday. Tropical vegetation grew spontaneously; brilliant foliage and flowers, luxuriant ferns and clinging vines mingled with the forests and open vistas in landscapes of surpassing beauty.

But in the course of time a change was perceptible. The intense heat of the long summer days was tempered by refreshing breezes, and the nights became delightfully cool. The winters were slowly growing colder. Snow storms came and piercing winds swept over plain and forest. Tropical plants were stricken with early frosts; ice formed in lakes and streams where it had never had before appeared. The more hardy animals sought the shelter of wooded ravines and deep gorges. Snow fell to unusual depths; year after year it came earlier, and winter continued later. The earth became frozen to great depths; fruits and trees disappeared. As the snow piled higher each succeeding year, and the summers were too short and cold to melt it, all animal life perished. The pressure of mountains of snow and the percolating rains converted the mass into a solid sheet of glacier ice that not only covered nearly all of Iowa, but reached out over the northern half of North America.

The ice sheet of this period had its southern margin south of the latitude of St. Louis. The ice was slowly moving outward from the center of accumulation, grinding over the underlying rocks, crushing them into the finest powder. Fragments of enormous size were frequently caught in the lower portion of the flowing ice and carried forward bodily, grinding the rock strata into rock flour, and being themselves planed and grooved on the lower surface. All bowlders of crystalline rock which we find strewn over our State were carried from their native ledges in British America by these ice sheets of what geologists call the Quaternary period.