426 IOWA ACADEMY OF SCIENCE Vol. XXVI, 1919 



TERRANAL EQUIVALENCY OP UNCONFORMITY 



Iowa's coal measures, as is now generally known, rest with 

 marked discordance of strata upon the rough, tilted and beveled 

 edges of all the older geological formations of the region. These 

 unconformable relations are very widespread. This break in con- 

 tinuity of succession at the base of our. coal-bearing series clearly 

 represents an old land surface that was subjected to the forces of 

 erosion for a period long enough for sloping strata to be planed off 

 from the Carbonic limestones down to Cambric sandstones. In the 

 interval between the deposition of the last of the early Carbonic 

 formations and the coal measures of the upper Mississippi basin 

 enormous regional denudation really took place. The magnitude 

 and stratigraphic significance of this erosion was long little appre- 

 ciated. 



Commonly the phenomenon under consideration was regarded as 

 local in its nature. Unconformities occurred at many different hori- 

 zons in the coal measures. That this basal discordance was really 

 a great hiatus was never fully considered. That the space repre- 

 sented an epoch much longer in duration than that in which was 

 formed all the coal measures above it was a most startling phase of 

 the problem presented. 



The base of the Des Moines series, or lowest horizon of the coal 

 measures of Missouri, was believed to extend southward beyond the 

 Arkansas river, where it appeared to coincide with the Grady coal, 

 or base of the Cavanoil formation. With the base of the Des 

 Moines series of Missouri thus approximately located in the Arkan- 

 sas section, and the top of the early Carbonic horizon well defined, 

 it left in the south an immense thickness of nearly 19,000 feet of 

 coal measures sediments that were entirely unrepresented in the 

 north. 



1 he magnitude of the hiatus at the base of the coal measures in 

 Iowa, Missouri and Kansas, is the more readily comprehended when 

 v;e find a place where uninterrupted sedimentation attained such 

 vast proportions as 19,000 feet in vertical measurement. The epoch 

 of which there is no measurable record in one part of the region 

 finds in the adjoining district sediments of greater stratigraphic sig- 

 nificance than all the coal measures above the break.-" It is a case 

 in which on one side of an old shore-line is the land area that suf- 

 fered profound denudation, and" on the other the water area in 

 which sedimentation was carried on to a prodigious extent. In 

 point of time the one is the exact equivalent of the other. 



"Bun. Geol. Soc. America, Vol. XII, pp. 173-196, 1901. 



