256 A. M. PUEDUE CAVE-SANDSTONE DEPOSITS OF THE OZARKS 



In cases where the underground water passages leading from the cav- 

 erns were short, open, and straight, the loose material that had accumu- 

 lated on the floor of the caverns would, at least in large part, be removed 

 by the ebb and flow of the tide, as the shoreline advanced on the land; 

 but in those cases where such passages were long, small, and crooked 

 the effect of the tidal surge would largely be destroyed and the loose 

 material would remain in the floor of the cavern. 



The arches so common at the base of the sandstone masses are in part 

 explained by the weathering out of this loose material, and in part by 

 the uneven floor of the caverns, as a result of differential solution. 



Age of the Deposits 



The very close similarity between the sandstone masses occurring at 

 the horizon of the Everton limestone and those occurring below, in the 

 Ordovician dolomites, points to a common age of the two. It is the 

 writer's belief that the caverns were filled at the same time the Upper 

 Saint Peter sandstone was put down, and from the same material. 



Age OF THE Caverns 



The caverns probably were formed in part, at least, during the time of 

 elevation that immediately preceded the deposition of the Upper Saint 

 Peter sandstone, though their beginning may antedate this time; but 

 while solution was going on over the Everton limestone, producing the 

 very uneven surface and probably removing a portion of its outer border, 

 the caverns were being formed in the manganesian limestone below. 



When the conclusion here presented of the origin of the sand masses 

 was forced on the writer by his own observations and in the face of pre- 

 conceived ideas, it was difficult to account for an underground circulation 

 of water sufficiently active to form the caverns at the time named; for 

 while the surface of the Ordovician limestone in which the deposits 

 occur is one of unconformity, it is surprisingly even, indicating that the 

 region stood only a short distance above sealevel; but an even surface is 

 the one, other things being equal, that is most conducive to a large 

 imderground water circulation. The amount of underground water may 

 be sufficient to force a rather rapid circulation, even though the exit 

 should be below sealevel; also the even surface is itself an evidence of 

 such circulation, for without it surface drainage must have existed, 

 which would have resulted in numerous valleys. Such valleys are only 

 sparingly in evidence along the contact between the Ordovician lime- 

 stone of the area and younger rocks laid down on them. 



