122 foley. SEDIMENTATION AND GROUND WATER [Ch. 6 



The Delta Environment 



Delta deposits are extremely heterogeneous, with great ranges in 

 composition within short distances, both vertically and horizontally. 

 Sediments are generally rather fine-grained and consist of sand, silt, 

 clay, and vegetable matter. Where sand has been deposited, deltas 

 are important sources of ground water. Delta deposits of existing 

 streams are likely to have good recharge from the stream itself. Delta 

 plains constitute large areas of the earth's surface at present, and 

 many of them are densely populated. The deltas of the Ganges, 

 Bramaputra, Nile, Hoang Ho, Rhine, and Mississippi rivers are ex- 

 amples of large, heavily populated deltas where ground water is ob- 

 tained in large quantities and has been one of the factors in the de- 

 velopment of the areas. 



MARINE ENVIRONMENTS 



The widespread, relatively uniform marine sediments are the most 

 extensive and continuous of all aquifers in areas where they have not 

 been disturbed by faulting and folding. The great area of Paleozoic 

 marine sediments of the Central States contains many very important 

 aquifers. The Cambrian sandstones of eastern and southern Wiscon- 

 sin, Illinois, and Iowa provide water to wells in large quantity. A 

 well in Madison, Wisconsin, with 240 feet of screen in Cambrian sand- 

 stone, produced more than 3,000 gallons a minute of water of excel- 

 lent quality. Most of the municipalities in the area underlain by this 

 water-bearing bed depend entirely on it for water supply. 



The St. Peter sandstone of Ordovician age has long been famous 

 as an aquifer in the North Central States, but ground water attributed 

 to it in some places probably actually comes from other formations. 

 The St. Peter is a poorer aquifer in Illinois than in Wisconsin, ap- 

 parently owing to an increase in fine material in the sand at greater 

 distances from the Wisconsin arch, in the core of which pre-Cambrian 

 rocks are now exposed. 



The permeability of the Cambrian sandstones in Wisconsin varies 

 from area to area but has proved to be surprisingly uniform over dis- 

 tances of 20 miles, as shown in the results of pumping tests conducted 

 in the Milwaukee-Waukesha area, Wisconsin (Drescher, 1948). 



Marine limestones, where they have become cavernous as a result of 

 solution, are among the world's most productive aquifers. The Eocene 

 Ocala limestone of Florida and Georgia yields many millions of gal- 

 lons of water daily to wells and springs (Stringfield, 1936) . Lime- 





