Documents d'approvisionnement en eau et d'Irrigation de l'United States Geological Survey . gamme maximum de 600 pieds. Le niveau des eaux souterraines est faible dans les crêtes et étroit. Les calcaires qui en partie employés. le surface de ces hautes terres sont piquées avec dolines par whichthe pluie se déverse dans les passages caverneux et conclut rapidement façon tounderlying et massif de grès poreux de grande capacité de stockage,qui fournissent de nombreuses springs où qu'ils sont coupés par les vallées. Eaux de la dérive. L'extérieur de la région non glaciée l'état est recouverte de feuilles d'ofvarious dérive l'âge. La plus récente

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Water Supply and Irrigation Papers of the United States Geological Survey . amaximum range of 600 feet. The ground water level lies low withinthe steep-sided and narrow ridges. The limestones which in part formthe surface of these uplands are pitted with sinkholes through whichthe rainfall drains into cavernous passages and rapidly finds way tounderlying massive and porous sandstones of great storage capacity, which supply copious springs wherever they are cut by the valleys. WATERS OF THE DRIFT. Outside the driftless area the State is covered with sheets of drift ofvarious age. The most recent is the Wisconsin drift sheet, occupyingthe north-central portion of the State. It presents a surface withcountless undrained depressions and streams which have hardly begunthe work of erosion. Ground water here stands high, and exuding inthe sags gives rise to the many lakelets and the extensive sloughs of theregion.220 NORTON.) IOWA. 221 The Iowan drift sheet, which forms the surface in the northeasternportion of the State, has a topography that is somewhat older than. Fig. 34.—General geologic section of Iowa. that of the Wisconsin, the lakelets and sloughs having been largelydrained by the natural erosion of the streams. 222 UNDERGROUND WATERS, EASTERN UNITED STATES. [no. 114. Except a narrow strip along Mississippi River, extending from ScottCounty southward, which was invaded from the east by the Illinoianice, the remainder of the State is covered with the Kansan drift sheet, an old drift deeply weathered, and for the most part much cut bystreams before the deposition of the surface silt known as1 loess.Beneath the Kansan Iowa geologists have recognized still earlier drift, without surface outcrop, known as the Upre-Kansan. Overlapping one another like the shingles on a roof, these broadsheets of stony clay, with their attendant beds of sand and gravel, largely control the distribution and the quality of the ground water ofcommon wells. The stiff, stony clays of which the

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