For several years, it had been known that Stigmaria found in the underclay
of coals was in fact the rhizophore of Sigillaria (Logan 1841; Brown 1846), and to this evidence Dawson was able to add many other convincing proofs (Fig.
An uncontrollable impact of mining was the potential to puncture the underclay
of the coal seam (bottom of upper zone).
At the base of the Pleasanton Group, between the Desmoinesian and the Missourian Series, a major unconformity exists where underclay
would seem to correlate with a drop in sea level approximately 305 million years ago (Ross and Ross, 1988).
A predictive model for water intrusions was then calibrated against actual observations, leading to a useful tool for anticipating water in the typical thick underclay
These strata are underlain by 5' 6" (1.7 m) of underclay
and red shale and sandstone, the base of which marks the base of Division 4.
After that, the spoil from each cut is placed on the underclay
exposed by the previous cut as mining progresses toward the center of the hilltop.