THE TOMAH AND SPARTA QUADRANGLES 623 



Except for worm tubes, of which there appear to be at least two 

 varieties, fossils have not been observed in the Dresbach sandstones 

 of the two quadrangles. The most common worm tubes are of the 

 Scolithus type. They are extremely abundant in the upper beds. 



Conditions of origin. — The sediments of the Dresbach formation 

 were accumulated in shallow water above wave-base or above 

 water-level. It does not seem probable that at any time the waters 

 attained sufficient depth to remove the sands from the wash of the 

 waves. The possible eolian cross-lamination and certainly the 

 mud cracks prove that at times the sites of deposition were above 

 water-level. The writers' conception of the conditions of deposi- 

 tion is that of an extensive sand flat, of which parts may have been 

 continuously underneath the water, other parts were exposed at 

 low tide, while other areas — sand islands, bars, etc., thrown up by 

 the waves and piled higher by the winds, the different environments 

 shifting more or less from time to time — were above water-level. 

 These conditions granted, the clean washing of the sands, the local 

 thin laminae of shale, the presence of mud cracking, the abundance 

 of both wave and current ripple mark, the excellent sorting of the 

 sands in most of the beds and their quite perfect rounding in some 

 beds, the great development of cross-lamination of which some 

 appears to be eolian, the variable and lenticular character of Dres- 

 bach bedding, and the general absence of fossils follow as natural 

 consequences, and it is difficult to conceive how all these characters 

 could have developed on an extensive scale under any other 

 conditions. 



THE FRANCONIA FORMATION 



The Franconia formation was defined by Berkey^ without its 

 upper and lower limits being stated. In some of the publications 

 of the United States Geological Survey the strata which constitute 

 the Franconia formation appear to have been included in the 

 St. Lawrence,^ which as thus defined includes two stratigraphic 



'C. p. Berkey, "Geology of the St. Croix Dalles," Amer. Geol., XX (1897), 

 373, 377- 



2 G. W. Hall, O. E. Meinzer, and M. L. Fuller, "Geology and Underground 

 Waters of Southern Minnesota," Water Supply Paper No. 256, U.S. Geol. Surv. (191 1), 

 pp. 63-68; W. H. Morton and others, " Underground Water Resources of Iowa," Water 

 Supply Paper No. 2Q3 (1912), PL II, p. 65. 



