Water Supply and Irrigation Papers of the United States Geological Survey . ^s-talline rocks is a belt of sandstone of varying width bordering LakeSuperior. On the south the same formation extends in a crescent around thecrystalline area, with its northeast prong joining the Lake Superiorbelt near Marquette, Mich., and its northwestern prong passing intoMinnesota near St. Croix Falls, Wis. The sandstone adjacent to Lake Superior in the western half of the WISCONSIN DISTRICT. 235 urea is of a red and brown color, well cemented and firm. Eastwardthe color becomes lighter and the eastern portion
Water Supply and Irrigation Papers of the United States Geological Survey . ^s-talline rocks is a belt of sandstone of varying width bordering LakeSuperior. On the south the same formation extends in a crescent around thecrystalline area, with its northeast prong joining the Lake Superiorbelt near Marquette, Mich., and its northwestern prong passing intoMinnesota near St. Croix Falls, Wis. The sandstone adjacent to Lake Superior in the western half of the WISCONSIN DISTRICT. 235 urea is of a red and brown color, well cemented and firm. Eastwardthe color becomes lighter and the eastern portion as well as that on thesouth is white or yellow, poorly cemented and unconsolidated, oftencrumbling in the hand, while in some cases it is so hard that it mightpass for a quartzite. The formation consists of line calcareous sand,coarse siliceous sand, shales, marls, and layers of sand highly impreg-nated with iron oxide, thereby causing the water from some of the. Fig. 37.—Wisconsin outcrop of Potsdam and St. Peter sandstones. horizons to flow forth a blood red. The rock is very porous. Itsgrains are angular, and in some of the lower layers as coarse as No. 4shot. The sandstone underlies the entire area to the east and south,dipping east, south, and west on the sides of the central thickness varies from nothing along the crystalline margin to 1,488f eet at Eockford, 111. At Streator, 111., it has a total thicknessof morethan 1,737 feet, showing a rapid increase toward the south. 236 UNDERGROUND WATERS, EASTERN UNITED STATES. [no. 114. Lower Magnesian limestone.—Fringing the Potsdam sandstone on1 he south from the eastern part of Michigan to St. (Jroix River, in PolkCounty, Wis., is a narrow belt of Lower Magnesian limestone, varyingin thickness from 50 to 200 feet. Along St. Croix and Mississippirivers it caps many of the sandstone ridges. It is a dolomite, withconsiderable quantities of chert, crystalline quartz, and quartzos
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