190 ILLINOIS STATE ACADEMY OF SCIENCE 



distinctive texture make this formation an excellent hori- 

 zon marker. A slight unconformity appears to exist at 

 its base. A few miles east of Oregon the typical Glen- 

 wood is supplanted laterally by a series of beds, transi- 

 tional between the typical St. Peter sandstone and the 

 typical Platteville limestone, which consists of alternat- 

 ing thin limestones, ordinary sandstones, vermicular 

 sandstone, clay, and shale. One section of these transi- 

 tional beds is 40 feet thick. 



The Platteville limestone crops out generally in the vi- 

 cinity of Oregon, Byron, and Leaf River, where in the 

 main low anticlines have been eroded. It is composed 

 principally of limestone with a variable content of mag- 

 nesium carbonate. The lower part of the formation can 

 be identified readily by its buff color in natural exposures 

 and its stratigraphic relations to the Glenwood and St. 

 Peter sandstones. It is difficult in many places to dis- 

 tinguish the upper portion from the lower beds of the 

 overlying Galena dolomite, especially if the outcrop is a 

 small isolated one of beds not far from the contact of the 

 two formations. There are, however, certain dependable 

 contrasting characteristics by which most of the exposed 

 beds can be confidently identified. A diagnostic feature 

 is its content of fossils, which are more or less common 

 throughout the formation, but are so abundant and of 

 such variety at some horizons as to constitute a "fossil 

 sea floor." Gastropods, pelecypods, and brachiopods 

 are most common, with corals, cephalopods, ostracods, 

 and trilobites somewhat less abundant to rare. Evidence 

 has been discovered that the formation lies unconforma- 

 bly in places upon the St. Peter sandstone. 



The Galena dolomite is the uppermost bed rock over 

 the greater part of this quadrangle. Partial sections only 

 are exposed in any of the outcrops, most of which are 

 roadside or gully ledges a few feet thick, although con- 

 tinuous sections of considerable thickness occur in some 

 ravines, gorges, and quarries. A diagnostic feature is 

 the fossil Receptaciditcs, which ranges from a short dis- 

 tance above the base of the formation to well past its 

 middle portion, with some horizons marked by its abun- 

 dance. Other fossils are scarce, but locally the beds con- 



