Water Supply and Irrigation Papers of the United States Geological Survey . of the streams. Sometimesthis deposit attains a thickness of 100 feet, but generally it is verymuch thinner. In some cases it may be entirely wanting, so that thewell-borer passes from the bowlder clay into the Cretaceous bedswithout noticing the transition. This formation plays an importantpart in the water supply of the region and will be further describedunder that head. TILL OR BOWLDER CLAY. This formation presents the same features that are found in corre-sponding regions elsewhere, as in central Minnesota, Iowa,
Water Supply and Irrigation Papers of the United States Geological Survey . of the streams. Sometimesthis deposit attains a thickness of 100 feet, but generally it is verymuch thinner. In some cases it may be entirely wanting, so that thewell-borer passes from the bowlder clay into the Cretaceous bedswithout noticing the transition. This formation plays an importantpart in the water supply of the region and will be further describedunder that head. TILL OR BOWLDER CLAY. This formation presents the same features that are found in corre-sponding regions elsewhere, as in central Minnesota, Iowa, and Illi-nois. It is an unstratified mixture of clay, sand, and worn pebblesand bowlders, the last mentioned sometimes attaining a diameter ofseveral feet. In this formation are found local developments ofstratified sand, commonly spoken of as pockets, though they are GEOLOGICAL SURVEY WATER-SUPPLY PAPER NO. 34 PL MELLTOWN. Sec. 35 S\V.,T. 100 N. 59 W. ~.° . •„ 1,200 A. T. :#: Blue till. 58. ! I V i - i-T; - .-f,.:- - Shale with pyrites. 3 = 227. * «_- LOGS OF WELLS IN HUTCHINSON COUNTY, SOUTH DAKOTA todd] GENERAL GEOLOGY. 19 sometimes known to be portions of channels of considerable length,and also of sheets that may locally separate the bowlder clay into twoor more members. The till of this region is much more clayey thanat points farther east because for some distance the ice had movedover and deeply eroded the dark-colored clays of the this reason the erratics are perhaps less frequently striated andplaned. The bowlders most widely distributed are gray and reddishgranites, and peculiarly compact and fine-grained limestones of astraw color or clear white. The latter contain Favosites and cup-corals, with occasional Brachiopods, indicating their Paleozoic in prominence are bowlders of a fine-grained trap or green-stone. Besides these, in some portions of the area, a large percen-tage of the erratics, usually those of smaller size,
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