THE ERA OF THE FERNS 
Near Morris and Wilmington in northern Illinois, such 
a river delta probably existed in early Pennsylvanian 
times. In that region we find several of the oldest known 
fossil plant localities of North America. The fossils can 
be dug out of the banks of the Mazon Creek and from 
the mines southwest of Wilmington; in fact, we can 
restore a Pennsylvanian landscape from the many 
thousands of plant impressions that are found there. 
Specimens have been collected for many years in this 
stretch of land, and probably almost all of the fossil 
plant species that ever were preserved in this locality have 
been identified. There is no other place in North 
America that has been more thoroughly explored for 
fossils. Nevertheless, it has not been possible to assemble 
more than one hundred plant species from Mazon Creek. 
There must have been a great monotony in that swamp 
forest. The predominant types were fern-like plants, 
many of them bearing seeds (pteridosperms), while some 
were true ferns. These fern-like types were either tree- 
ferns or climbing ferns. Some were shrubs, and others 
even herbaceous plants. There was a dainty little fern 
which later got the generic name Sphenopteris , which 
means wedge-shaped fern, because the leaves were cut 
up into small wedge-shaped sections. Another fern was 
the comb-fern (Pecopteris). There was the broad-leaved 
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