FORMATIONS IN SOUTHEASTERN IOWA. 307 
here seen resting upon the granular limestones which show vari- 
ous stages of disintegration and decomposition. The clays rest- 
ing upon the limestones and filling the interstices between the 
oxidized limestone fragments is evidently the residual product 
of the decay of the limestones. 
About one-half mile west of Rock Creek quarry, the shales 
of the Coal Measures rest upon the magnesian limestone of the 
arenaceo-magnesian beds, apparently indicating a channel of ero- 
sion at this point (Fig. 5). 
(fe VME yy 
oe by, 
MGs Me PE fs 
ek Z 
ase ERIS Fy PEPE IFES DO 2 
i ROEDERER ISDE LAR EE LOT EXPOS X PRs = 
KET EG 
Eee ae vA 
Fic. 5.—Section near the mouth of Rock Creek (Sec. 21, T. 69, N., R. 9, W.) 
showing eroded surface of the Saint Louis limestone. 
One of the best exposures is at the Sonora quarries where 
the brecciated limestone is entirely cut out by the descent of the 
Coal Measure rocks (Fig. 6). 
ass | ( 
Ry i ee sel Me 
temd_ ad al 
Fic. 6.—Section along face of Sonora (IIl.) quarry showing unconformity of Coal 
Measures upon Saint Louis limestone. 
ORIGIN OF THE BRECCIATED CHARACTER OF THE SAINT LOUIS 
LIMESTONE. 
The origin of the brecciated character often prevalent in 
limestone formations has never been satisfactorily explained. 
The explanations that have been offered are: 
(1) Wave action upon a rock-bound coast. 
(2) The systematic alternation of vigorous and quiet action 
