SPECHTS FERRY BRYOZOAN FAUNA 3 



8. Roadcut, County Hwy. U, 3.05 miles north of the junction of County 



Hwy. U and Wis. Hwy. 133, Grant County, Wisconsin. 



9. Natural exposure, SE£ SE\ NW{ sec. 34, T. 29 N., R. IE., on right 



bank of Galena River, approximately 4.0 miles north of Galena, Illinois. 

 10. Natural exposure, NE£ SE j SW^ sec. 34, T. 29 N., R. 1 E., about 



50 feet north of abandoned railroad bed, approximately 3.7 miles north 



of Galena, Illinois. 

 The stratigraphic position of the specimens within the Spechts Ferry, which 

 is invariably less than 10 feet thick in the collecting area, was not recorded. 

 Many of the weathered-out specimens obviously were somewhat out of place but 

 still resting on Spechts Ferry strata. However, all specimens described and stud- 

 ied for this project came without question from the Spechts Ferry. 



Tangential and longitudinal sections of 245 specimens, representing some- 

 what over half of the specimens amenable to sectioning in the collection, were 

 prepared by the author during the course of this study. Several sections were made 

 from some of the zoaria . All sections and remnants of the zoaria from which they 

 were prepared, as well as all nonsectioned specimens, have been deposited in the 

 paleontologic collections of the Illinois State Geological Survey, Urbana, Illinois. 



Age and Distribution of Spechts Ferry Bryozoans 



Although originally defined as the basal member of the Decorah Shale (Kay, 

 1928, p. 16), the Spechts Ferry Shale later was made the uppermost member of the 

 Platteville Formation (Kay, 1935, p. 287-289), and was generally considered to 

 be equivalent to Black River strata in New York. Herbert (1949) returned the Spechts 

 Ferry to the Decorah on the basis of lithologic evidence and the presence of a wide- 

 spread unconformity at the base of the Spechts Ferry. Templeton and Willman 

 (1952) correlated the Spechts Ferry with the Selby Shale, the basal member of the 

 Rockland Formation, at the base of the Trenton Group in the New York type region. 

 The Spechts Ferry was assigned to the Trentonian Stage on the Ordovician corre- 

 lation chart (Twenhofel etal., 1954). Agnewetal. (1956, p. 286-289) also re- 

 turned the Spechts Ferry to the Decorah Shale. 



The age of the Spechts Ferry cannot be established definitely as either 

 Blackriveran or Trentonian on the basis of its bryozoan fauna. The use of bryozoans 

 for making regional correlations and age determinations is hampered greatly by lack 

 of knowledge concerning bryozoan faunas of Middle Ordovician age. Only three 

 faunas of this age from widely separated regions in North America have been de- 

 scribed with any degree of completeness (Coryell, 1921; Loeblich, 1942; Fritz, 

 1957), notwithstanding that many formations of Middle Ordovician age have rich 

 and diversified bryozoan faunas (Bassler, 1933, p. 282). Unfortunately, the 

 bryozoan faunas of the type sections of the Black River and Trenton Groups are 

 virtually unknown. Much of our knowledge concerning the geographic and strati- 

 graphic distribution of Middle Ordovician bryozoan species is based on faunal 

 lists included in papers that are primarily stratigraphic in nature, as in Bassler 

 (1932, p. 59-115) and Wilson (1949, p. 24-170). Such lists are not entirely 

 satisfactory for use in correlation studies because the relative abundances of the 

 species are not usually stated nor are they figured or described so that identifi- 

 cations may be verified. For these reasons, namely, the small number of adequate- 

 ly described faunas of Middle Ordovician age and faunas known very incompletely 

 through faunal lists, bryozoans presently are not as practical for correlation 

 studies and age determination as are better known groups of invertebrate fossils. 



