38 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 500 



more detailed descriptions and reconstructions of the environment. At 

 the present, the Spencer Farm Flora is the oldest Pennsylvanian flora 

 known in the Illinois Basin, and certain differences from other floras 

 are due to age differences. 



During late Mississippian and early Pennsylvanian time a karst 

 topography developed in northern Illinois with sinkholes or ravines which 

 had very steep sides. Plants grew on the less eroded limestone hills. 

 Rain storms would then wash "broken-off plant fragments into the ravines 

 and sinkholes. The floods would carry away the coarser elastics and re- 

 work the clays into clay pebbles. During times without rain a fine mud 

 was deposited; it does not contain any plant megafossils. This recon- 

 struction can be developed from the geologic position and the sedimen- 

 tology at the site. The setting explains the unusual composition of 

 the flora. The plants were growing on calcareous soil that might have 

 been rather dry during part of the year. In contrast, the plants we 

 find in roof-shale floras were growing on alluvial soils that were not 

 limy but were often marshy and wet the year round. 



Some taxa that occur only rarely in other beds in the Illinois 

 Basin are common in the Spencer Farm Flora. These are Megalopteris , 

 Lesleya, Palaeopteridium reussii , and Alloiopteris gracillima. These 

 are the taxa that probably belong exclusively to an upland flora. 



A count of the occurrence of genera is presented in table 7. 

 This count was done during two collecting trips and does not include all 

 specimens at our disposal. The pteridosperms are clearly dominant, and 

 the Woeggerathiales are the second most common group. Cordaitales, 

 Sphenopsids, and Ferns are much less common, but are well represented. 

 Remarkable is the scarcity of Lepidodendron, which is represented by 

 only three specimens. In roof-shale floras Lycopsids are usually more 

 common (generally around 10 percent), and Noeggerathiales are not pres- 

 ent at all. The predominance of Pteridosperms, however, can be observed 

 in the majority of roof-shale floras (Peppers and Pfefferkorn, 1970; 

 Pfefferkorn, Mustafa, and Hass , 1975). 



Comparable Floras 



There are a few floras known which contain some of the taxa 

 found in the Spencer Farm Flora. Table 8 summarized the occurrence of 

 the more important taxa in these floras. 



White (1908) gave a list of a flora from northwestern Illinois 

 without citing a precise locality. This floral list is very similar to 

 the list of the Spencer Farm Flora if a few names are translated into 

 their modern equivalents. White's flora certainly came from a similar 

 setting and might be comparable in age. White mentioned Danaeites, 

 which does not occur in this stratigraphic interval. 



Lesquereux (l88*+, Coal Flora III, p. 852) lists a flora found 

 in a sinkhole near Port Byron, Illinois. Megalopteris is present, but 

 several of the other taxa are not. 



Another flora that has similarities to the Spencer Farm Flora 

 occurs in Perry County near Rushville, Ohio (Rushville itself is situ- 

 ated in Fairfield County). Reports on the flora were given by Andrews 

 (1875), Lesquereux (l88U), and Cross (1962). Even though this flora 



