History of Linn County Iowa / From Its Earliest Settlement to the Present Time [1911]

by Luther Albertus Brewer; Barthinius L. Wick

THE GLACIAL EPOCH

The warm climate of Tertiary times changed slowly to one of arctic cold. The winters lengthened and the summers becoming ever cooler and yet cooler failed at last to melt the winter snows. Vast sheets of glacial ice, such as that which shrouds Greenland today, covered much of the continent. The geologic panorama thus presents our area as buried beneath one after another of slow-moving glaciers hundreds of feet thick. The proofs of their existence are found in almost every cutting which goes below the soil. Any quarry will show the rock deeply rotted and pitted by long preglacial decay. Here and there upon its surface will be found remnants of the deep red residual clays, the subsoils of preglacial times. Upon these clays formed from the decaying rock rest stony clays in which clay, sand, and stones faceted as only glacier ice can facet, are mingled pell-mell together, as only glacier ice can mingle. Occasionally is found the unmistakable track of the glacier left on the underlying rock scraped smooth and marked with parallel scorings, as at the north end of the cut of the Chicago, Milwaukee & St. Paul Railway at Linn Junction.

The glaciers also brought from ledges of granite and other crystalline rocks in Minnesota, Wisconsin, and Canada the boulders which form a conspicuous feature in some of our prairie landscapes. These, the "first settlers," traveled to their destinations far more leisurely than any ox carts of the immigrant pioneers; for the glaciers can not have moved faster at most than fifty feet a day, and probably at less than a tenth that rate, judging by the rates of motion of present glaciers.

The ice sheets of the glacial epoch plastered the county thick with the stony clays which they dragged along in their basal layers. The thickness of these glacial deposits probably averages from fifty to one hundred feet. Old valleys cut in rock by Tertiary rivers were buried wholly from view, as, for example, one extending north from Prairieburg; and the farmer now plows his corn in fields which lie two and three hundred feet above the channels of ancient rivers. In places the old valleys were left to be re-occupied by the rivers. Such are the reaches of wide valley of the Cedar south of Center Point. In other places the rivers were diverted wholly from their ancient beds and made to flow in new channels which they have not yet had time to widen and deepen to their ancient measures. Such are the narrow rock bound valleys of the Wapsipinicon south of Troy Mills and of the Cedar at the Palisades.

On the final retreat of the glaciers waters from the melting ice swept over the county, leaving deposits of sand on the lower lands and in the valleys. Since the glacial epoch the rivers have cut their beds a score of feet and more below the deposits of glacial floods and in many places, as near the Ivanhoe bridge, remnants of these ancient flood plains are left as terraces or "benches" or "second bottoms." At Bertram the sands deposited by glacial waters near the mouth of Big Creek stand about fifty feet above the level of the river.

THE LOESS

A large part of the county is covered with a deposit of fine yellow silt called loess. Dry, it crumbles into powder at a finger touch; wet, it is somewhat plastic and can be moulded into brick and tile. On the hill and uplands the loess is thickly spread, adding in places at least forty feet to their elevation. Over the lowlands it is thin or absent. This yellow earth has been and is to be of greater value than mines of yellow gold. It is of inexhaustible fertility. It contains abundant mineral plant foods, partly constituent, and partly brought up into it by ground water; and these foods are so finely pulverized as to be of readiest solution and absorption by the roots. In wet weather the loess mantle absorbs the rainfall like a sponge; in months of desert drouth, like those of the summer of 1910, it returns the water to the surface, like a wick, to preserve the crops from failure.

A disadvantage of the loess lies in the readiness with which it washes. The forest which once covered nearly all the uplands protected the soil from wash by means of its mattress of roots and the thick prairie sod was equally efficient where hill slopes were grassed over. But where forests have been thoughtlessly cut down, and steep slopes turned to plow land, it is but a few years until the brown top-soil is all washed away and the fields in spring when freshly plowed are as yellow as a deep cut in road or brick yard. The foot path in the pasture or the furrow of the plow becomes a gully in a single heavy rain, and unless checked soon becomes a gulch scores of feet in width.

By accenting the height of the ridges the loess also adds to the scenery of the county. Our area lies in a part of east central Iowa where the stony clays deposited by ancient glaciers accumulated in long ridges and belts of upland rising many feet above the intervening undulating plains. Because of the alternation of ridge and lowland no part of the state except the valley of the Upper Mississippi has so beautiful and wide and varied prospects. Over more or less of their course the rivers of the county have cut their channels lengthwise in the ridges, thus giving rise to the bold scenery of the Wapsipinicon above Central City, and of the Cedar near Mount Vernon. Some of these picturesque reaches of river and cliff and forest slope should surely be converted into county parks in the near future and preserved for the gratification of all coming generations. Unless this is done we may expect that the forests will be cut down and the hill slopes gashed with countless gullies; while the lichened rocks of the river cliffs fringed with fern and tamarisk will give place to unsightly quarries.