Physical features of the Des Plaines Valley . rily be places, small tunnels or openings in the limestone may be seen in thequarries about Joliet, though none of them are of striking appears in the northeast corner of the quarry near Grinton andJackson streets. It is a thin horizontal opening worked out along abedding plane in the limestone, close to the present quarry floor. Thesteady flow of water through these openings renders them most unwel-come to the quarrymen, necessitating the constant use of pum23s. It is 2<) THE DES PLAINES VALLEY. Lbuli,. no. U said th
Physical features of the Des Plaines Valley . rily be places, small tunnels or openings in the limestone may be seen in thequarries about Joliet, though none of them are of striking appears in the northeast corner of the quarry near Grinton andJackson streets. It is a thin horizontal opening worked out along abedding plane in the limestone, close to the present quarry floor. Thesteady flow of water through these openings renders them most unwel-come to the quarrymen, necessitating the constant use of pum23s. It is 2<) THE DES PLAINES VALLEY. Lbuli,. no. U said that a Pi)riiig issuing from tlic limestone near here, was formerlya regular stopping place for the stage, as it passed down the Des Plaines valkV through Jolict, GLACIATION. The Glacial Period.—The rolling upland with its maturely developedvalley systems and its residual soil was destined not to remain. Anextraordinary change of climate led to the development of a great icefield over much of the northern part of North America. The extent of. Fig. 5. Map of area covered by the North American ice sheet of the glacialepoch at its maximum extension, showing the approximate soutliern limit of glaci-ation, the three main centers of Ice accumulation, and the driftless area within theborder of the glaciated region. {Courtesy of U. 8. Geological Survey.) the ice sheet (shown in Fig. 5) was some 4,000,000 square miles, fullythe size of the ice sheet which now covers the Antartic continent. Whysuch an enormous ice sheet grew up in North America, with its centersof accumulation nearly thirty-five degrees away from the pole, hasnever been satisfactorily explained. It has been ascribed, in turn, to GOLDTHWAIT.] SURFACE OF BED ROCK. 27 a general uplifting of the continent to an altitude above the snow line;to a shifting of ocean currents by up-warpings of the sea floor; to certainchanges in the earths planetary relations (eccentricity of the orbit andprecession of the equinoxes) which might
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