Physical features of the Des Plaines Valley . to northeastern Iowa, and is widely exposed about the Great Lakes,forming the surface rock of the northeastern corner of Illinois. It takesits name from the falls of Niagara, for which the hard limestone ischiefly responsible. Since this is the only rock exposed in the DesPlaines valley, it deserves more than brief mention here. Where it has been penetrated by artesian wells in the vicinity of Chi-cago, the Niagara limestone has a thickness of from 250 to 400 feet. Itsoriginal thickness was probably still greater, for we cannot say how muchwas worn
Physical features of the Des Plaines Valley . to northeastern Iowa, and is widely exposed about the Great Lakes,forming the surface rock of the northeastern corner of Illinois. It takesits name from the falls of Niagara, for which the hard limestone ischiefly responsible. Since this is the only rock exposed in the DesPlaines valley, it deserves more than brief mention here. Where it has been penetrated by artesian wells in the vicinity of Chi-cago, the Niagara limestone has a thickness of from 250 to 400 feet. Itsoriginal thickness was probably still greater, for we cannot say how muchwas worn away from the top of it in the long interval of exposure anderosion between the Silurian and the glacial periods, when the rocksurface was finally buried by glacial drift. Like most limestones, theNiagara limestones was originally an organic deposit—that it, an ac-cumulation of calcareous skeletons and shells of marine animals, workedover by the waves and currents, and ground to a fine calcareous mud. GOLDTHWAIT] STRUCTUKE OF BED ROCK. 15.
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