THE LE CLAIRE LIMESTONE. 



185 



Hale in Jones county. They are again seen at numerous 

 points west of the Jones county line in Linn. Indeed they are 

 somewhat generally, though by no means universally, distri- 

 buted in the east central part of Scott, southwestern parts of 

 Clinton, western Cedar, the southern parts of Jones, and the 

 southeastern parts of Linn. They seem to be limited to the 

 southwestern corner of the Niagara area. A line drawn from 

 the mouth of the Wapsipinicon through Anamosa would mark 

 approximately their northeastern limit. 



The Le Claire limestone is in some respects unique among 

 the geological formations of Iowa. In the first place it varies 

 locally in thickness, so much so that its upper surface is exceed- 

 ingly undulating, the curves in some places being very sharp 

 and abrupt. In the second place it differs from every other 

 limestone of Iowa in frequently exhibiting the peculiarity of 

 being obliquely bedded on a large scale, the oblique bedding 

 often affecting a thickness of fifteen or twenty feet. The 

 phenomena suggest that during the deposition of the Le 

 Claire limetone the sea covered only the southwestern part 

 of the Niagara area, that at times the waters were compara- 

 tivel}' shallow, and that strong currents, acting sometimes in 

 one direction and t;ometimes in another, swept the calcareous 

 mud back and forth, piling it up in the eddies in lenticular 

 heaps or building it up in obliquely bedded masses over areas 

 of considerable extent. The oblique beds observe no regular- 

 ity with respect to either the angle or direction of dip. Within 

 comparatively short distances they may be found inclining to 

 all points of the compass. (Plate I., Fig. i.) 



The masses of sand, heaped up by currents, and forming 

 the well known shoals and bars that are so common a feature 

 of certain bays and other bodies of water along the Atlantic 

 coast of the United States from New Jersey to Florida, illus- 

 trate the origin and structure of the ridges of calcareous 

 material piled up by the Silurian seas during the Le Claire 

 stage of the Niagara. The shoals and sand-bars of the mod- 

 ern coasts alternate with areas covered by comparatively deep 

 water. The sea bed, in regions presenting these phenomena, 



