Handbook of Paleontology 253 



is in the southern Appalachian area. With the great 

 retreat of the seas at the end of Canadian (Beekman- 

 town) time the greater part of the continent became 

 dry land which was gradually covered with sands that 

 were moved about and built into dunes by the winds. 

 These deposits seem to be confined to the Mississippi 

 valley. Submergence during the Lower Ordovician 

 (pre-Chazyan) permitted the sea to rework these 

 sands into a basal bed. Such a basal sandstone (the 

 St Peter sandstone, top member of Buffalo series) oc- 

 curs at the surface or underground over nearly the 

 whole area of the Upper Mississippi valley (Illinois, 

 Iowa and Missouri, much of Wisconsin, Minnesota 

 and Michigan, Indiana and smaller parts of other 

 states) marking the horizon of the unconformity be- 

 tween the Ordovician and older formations (either 

 Upper Ozarkian or, more generally, Upper Canadian) 

 and always succeeded by early Mohawkian (usually 

 beds of Lowville age). This is a pure quartz sand- 

 stone ranging in thickness up to 200 feet, with well- 

 rounded grains assorted according to size. In the 

 Lower Ordovician in several places (as in the Hudson 

 valley in the vicinity of Albany) only deposits of muds 

 and sands that contain characteristic graptolite faunas 

 were laid down. As in the case of similar Beekman- 

 town (Canadian) shales, the fossils of these Ordovi- 

 cian shales indicate a connection with the Atlantic. 

 Elsewhere limestones commonly characterize the ear- 

 lier deposits of the Ordovician. The Middle Ordovi- 

 cian was also a time of limestone-making on a wide 

 scale. During this epoch (Mohawkian) the dominant 

 rocks were thin-bedded limestones and shales. The 

 Middle Ordovician limestones and especially the Tren- 



