FORMATIONS IN SOUTHEASTERN IOWA. 309 
is subjected to wave action at low tide it is conceivable that the 
alternation of forcible and quiet action might constitute an 
essential factor in the formation of extensive sheets of brec- 
ciated limestone. In its advance upon the land at the close of 
the Keokuk epoch the sea would doubtless encounter irregular- 
ities of the surface, some of the elevations reaching nearly or 
quite to the surface of the water. The Bentonsport arch appar- 
ently represents an area of this character. The first deposits 
would contain more or less arenaceous material derived from 
the receding shore. Owing to the prevailing calcareous char- 
acter of the preceding formations, however, the arenaceous mate- 
rial would be insufficient to constitute a widespread sandstone 
formation. These conclusions are supported by the lithological 
character of the arenaceo-magnesian beds, the lower portion of 
which is generally more or less arenaceous and in places decid- 
edly so. The calcareous mud derived by the trituration of the 
waves upon those parts of the sea bottom within their reach 
would be deposited in the quiet lagoons or carried out into 
deeper water to form the fine-grained compact limestones. As 
the brecciated formation is made up chiefly of fragments of 
these fine-grained rocks, it is evident that solidification must 
have taken place very quickly. The presence of lenticular 
masses of breccia made up of fragments of these fine-grained 
white or gray limestones interlaid with the massive brown are- 
naceo-magnesian beds implies that their formation was going on 
contemporaneously with the sedimentation of the latter beds. 
Their isolated occurrence, however, as well as their position 
below the main mass of breccia is not readily explicable unless 
they represent the thinning out of local masses exposed by tan- 
gential erosion. It has been suggested that similar occurrences 
were due to transportation by shore-ice.t The assumption of 
this agency, however, would preclude the acceptance of coral 
growth as a factor in the formation of the brecciated limestone, 
as it implies conditions of temperature inconsistent with the 
development of reef-building corals. 
*WALCOTT, C. D.: Bull. Geol. Soc. Am., Vol. V., p. 197. 
