IOWA ACADEMY OF SCIENCE 89 



The Structure of the Coalfield. 



The structure of the rocks of the coal fieid is, in general, monoclinal, 

 the most prevalent strike being northeast; the dips are usually steep to 

 the northwest. The main topographic features of the region have a 

 general northeast-southwest trend and are related, in a broad way, to 

 the structure. But detailed study of small areas within the field em- 

 phasizes very clearly that the structure is much more complex than is 

 indicated by a general study of the field. This complexity is due to 

 folding, to faulting, and, in the northeastern part of the field, to asso- 

 ciated igneous rocks. Apart from a few well defined lines of faulting 

 with northeasterly and southwesterly trend, there are many small faults 

 running in vairous directions. ^Moreover, within small areas in the 

 field the srtikes and dips are aften irregular. The rocks are frequently 

 very much broken and jointed, and slickensided surfaces, especially in 

 the coal, are common. 



The complex structure was produced liy the intense crustal move- 

 ments to which the rocks were subjected during late Tertiary or post 

 Tertiary time. Recent subsidence of the region is indicated by the 

 presence of alluvial deposits, in places several hundreds of feet in thick- 

 ness, in the broad valleys now occupied by comparatively small streams. 



The Coal. 



The coal beds are restricted to the Kushtaka formation, which has a 

 known surface area of about fifty square miles. The evidence is fairly clear 

 that an additional area of more than twenty square miles underlies the 

 Tokun formation at varying depths. The coal beds are distributed through- 

 out the thickness of the Kushtaka formation. Where sections of the forma- 

 tion are the best exposed more than a dozen seams of coal may be seen 

 but several of these are thin and unimportant. In places the coal beds 

 have thicknesses of more than twenty-five feet of good coal; at many 

 places beds exceeding ten feet may be seen. Owing to the complexity 

 of structure, and the small amount of development work, it is impos- 

 sible to correlate the coal beds in one part of the field with those of 

 other parts, even when the outcrops are not widely separated. The 

 thicknesses of the beds often vary greatly within short distances along 

 both the strike and the dip. In some places the evidence suggests that 

 the irregularities in thickness are due to movements, the coal having 

 been scpieezed into great pockets of irregular shape. IMoreover, in some 

 places there is a somewhat abrupt change from a bed of good coal of 

 considerable thickness into coal of a much lower grade, or into carbon- 



