628 W. H. TWENHOFEL AND F. T. THWAITES 



sandstones are generally thin-laminated and quite regularly bedded. 

 In some of the layers, particularly those in the upper member, 

 cross-lamination is present to a high degree. The angles of inclina- 

 tion are generally low and of variable direction; but inclinations 

 of southerly direction appear to be the more common. The thick- 

 ness varies from 78 to no feet. 



Fossils are extremely abundant in some layers, most of them 

 being trilobites. One of the locally developed dolomitic layers of 

 the lower member contains numerous brachiopods which are 

 identical with, or closely related to, Billingsella coloradoensis 

 (Shumard). The broken condition of the trilobites indicates that 

 they were washed about by waves and currents before their final 

 deposition. 



Conditions of origin. — ^The St. Lawrence strata are of marine 

 origin, and the sediments composing them were deposited in shallow 

 water. Some beds were deposited under conditions which per- 

 mitted the development of mud cracks, such having been found 

 near the base of the formation on the northeastern edge of the 

 Tomah quadrangle and near the middle of the lower member on 

 the Wisconsin River. The numerous bands of conglomerate prove 

 waters sufficiently shallow for the transportation of good-sized 

 pebbles over extensive areas; presumably these were derived from 

 the shores and carried out to deeper waters. They may possibly 

 indicate backward and forward migration of the shore line. It 

 is believed that the broad sand-flat conditions postulated for the 

 times of the preceding formations were still existent, and that the 

 St. Lawrence sediments indicate somewhat greater depths of water 

 and greater distances from land conditions than obtained during 

 the deposition of the Dresbach sediments of the two quadrangles. 



THE JORDAN FORMATION 



Succeeding the fine-grained sandstones of the upper St. Law- 

 rence are sandstones which are correlated with the Jordan forma- 

 tion of Minnesota, the writers using that term in the sense defined 

 by Ulrich in the paper by Walcott. Throughout the two quad- 

 rangles the sandstones referred to this formation in large part make 

 the upper tier of cliffs. 



