344 P-\LEOZOIC EOCKS or THE HFDSOX BAY REGIOX 



to the latter part of the Cincinnati an epoch, about the age of the Eich- 

 mond or Maquoketa of the npper Mississippi Valley. 



The Ordovician rocks in this region that belong to the Mohawkian 

 series are here designated the Xelson Eiver limestone; from Xelson Eiver, 

 ia the banks of which they are well exposed. The strata considered 

 equivalent to some part of the Eichmond of the United States will be 

 referred to as the Shammattawa limestone, from the river of that name 

 along the banks of which the best exposures were f onnd. 



As elsewhere in this region, the strata exposed along Xelson Eiver have 

 a general dip toward the east, or downstream, a little steeper than the 

 gradient of the stream. Gentle nndnlations in the strata are also common 

 and low arches 10 or 50 or more feet high are not rare. 



ORDOTICIAy DEPOSITS EXPOSED OV XELSOX PITER 



Selson River limestone. — The lowest layers of Nelson Eiver limestone 

 consist of sandy, bluish gray dolomite, and contain Bec^pt^cuUies oweni 

 and other fossils similar to those occnrring at a somewhat higher level ia 

 the limestones exposed at the TTpper Limestone Eapids, abont 10 miles 

 farther down the river. 



At the Upper Liniesijne Eapids, which are about one mile long, there 

 is exposed a thickness of nearly 25 feet of gray limestone mottled with 

 brownish dolomite, ia layers 6 to 30 inches thick. These contain such 

 characteristic fossils as Receptaculites oweni, Hormoton\a winnipegensis, 

 Maclurina manitohensis, associated in the same layers with numerous 

 corals belonging to the genera Halysites. Favosites. and Columnaria. and 

 cephalopods belonging to the genera Orthoceras. Cyrtoceras. and Poterio- 

 ceras. 



For a distance of 4 or 5 miles above these rapids a low ledge of gray 

 limestone, mottled with patches of yeUowish brown dolomite 3 to 16 feet 

 high, outcrops in the north bank of the river at frequent intervals. The 

 layers, which lie nearly horizontal, are 3 or 4 to 12 inches thick and fur- 

 nished fossils similar to those above mentioned. 



Between one and two miles below the Upper Limestone Eapids a ledge 

 of gray, non-dolomitized limestone outcrops in the south bank to a height 

 of 6 to 10 feet. These strata dip gently toward the east, so that suc- 

 cessively higher layers appear at the top in an eastward direction. This 

 limestone, which aggregates 15 or more feet in thickness, weathers into 

 thin, irregular layers which in appearance very closely resemble weathered 

 faces of exposures of undolomitized Galena limestone in Fayette County 

 and at a few other places in northeastern Iowa. 



The fossils from this ledsre include Bec^pfacuUtes oweni and the greater 



