HISTORICAL GEOLOGY 



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Fig. 27 

 Geologic section through northeastern Iowa, shoeing how character, thick- 

 ness, and distribution of deeply buried rock formations can be determined 

 by a comparison of well records. St. Lawrence and Jordan formations are 

 Cambrian; Oneota to Maquoketa inclusive are Ordovician; and above 

 these are Silurian and Devonian strata as indicated. (After W. H. Norton, 

 U. S. Geological Survey.) 



