. Underground water resources of Iowa . form a low dome the Silurian and Devonian strataare markedly thinner. For example, between Burlington and Keo-kuk (PL XII) the Devonian and Silurian strata barely exceed 150 feetin thickness; farther north, at Letts, they are more than 300 feetthick; still farther north, at Tipton, the Silurian alone is 325 feetthick; and to the west, at Peila, these formations include more than400 feet of rocks. (See PI. X, p. 374; PI. XIV, p. 548.) The Maquo-keta shares in the thinning. At Davenport it is 240 feet thick, atPella 190 feet, at Burlington 100 feet, and at
. Underground water resources of Iowa . form a low dome the Silurian and Devonian strataare markedly thinner. For example, between Burlington and Keo-kuk (PL XII) the Devonian and Silurian strata barely exceed 150 feetin thickness; farther north, at Letts, they are more than 300 feetthick; still farther north, at Tipton, the Silurian alone is 325 feetthick; and to the west, at Peila, these formations include more than400 feet of rocks. (See PI. X, p. 374; PI. XIV, p. 548.) The Maquo-keta shares in the thinning. At Davenport it is 240 feet thick, atPella 190 feet, at Burlington 100 feet, and at Fort Madison and Keokukless than 50 feet. (See PI. XII.) The Galena and PlatteviUe lime-stones likewise form a wedge that tapers toward the southwest. AtDavenport their combined thickness is 340 feet and at Keokuk only140 feet. The upwarp of the Cambrian and Lower Ordovician and the thin-ning of the higher terranes up to the Mississippian combine to bring514 *? ^•?ffto Mount Clara. SOUTHEAST DISTRICT. 515 artesian water from the St. Peter and deeper aquifers within easydrilling distance of the surface. (See PL I, in pocket.) At Keokuk,for example, the St. Peter is reached only about 900 feet below thevalley level. The Silurian and the Galena and Platteville limestonesin the southeastern district also furnish exceptionally large quanti-ties of water. At Burlington 6 deep wells obtain flows from theSilurian within about 500 feet of the surface, and the same formation,or possibly the Galena, is tapped by some of the deep wells at wells at Fort Madison obtain their supplies largely from theGalena. The dome of southeastern Iowa is only the northward extension ofthe upwarp of northeastern Missouri which brings the St. Peter sand-stone to the surface about 50 miles south of Keokuk, in Ralls County,Mo. This upwarp appears somewhat narrower in Iowa than in Mis-souri. Thus, though the St. Peter sandstone lies 613 feet below sealevel at Bloomfield it is found at
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Keywords: ., bookcentury1900, bookdecade, booksubjectwatersupply, bookyear1912
