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.