Pennsylvanian plants are most commonly found in shales directly
overlying coal beds. The shales are believed to be of nonmarine origin
like the coals and may contain fossils either in ironstone concretions
or on the bedding planes. The shale layers as well as the concretions
should be examined. Where the bed directly overlying the coal consists
of black slaty shale or limestone containing marine fossils, plant
remains are rarely abundant or well preserved.
Beneath the coals there generally is an underclay that is interpreted as
the material in which the coal forest grew. The underclay is in turn
underlain by a sandstone, and both are believed to be mostly nonmarine.
Stigmarian axes and “roots” are common in many of the underclays. Plant
fossils are common in the sandstone but generally are poorly preserved,
except in the local shaly lenses.
The best place to look for plant fossils in northern Illinois, except
for the strip mines of the Mazon Creek area, is probably in the spoil
heaps from shaft mines. The Colchester (No. 2) Coal has been extensively
mined by the longwall method. This technique causes the mine roof to
settle when the coal is removed, and the haulage ways are kept open by
removing the roof shale. Inasmuch as the roof shale is the Francis Creek
Formation of the Mazon Creek area, it may contain abundant plant-bearing
concretions. The shale is not everywhere fossiliferous, however, and in
many spoil heaps fossils are rare.
[Illustration: Tools for Collecting]