Pioneer work on the St. Croixian series of Minnesota and Wisconsin was done by Owen. In Minnesota this was followed by the work of N. H. Winchell and his associates. The classification thus established has been used with but slight variations for more than 40 years. The studies of Ulrich and the more recent work of Twenhofel, Raasch, and others in Wisconsin suggest various modifications of merit which are being used more widely and which may prove very helpful in the interpretation of the St. Croixian series. It is suggested by the authors that the delimitations of the Minnesota formations be brought into approximate accord with those now being used in adjacent regions.

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