252 A. M. PURDUE CAVE-SANDSTONE DEPOSITS OF THE OZARKS 



(at least tentatively) regarded by Mr E. 0. Ulrich, of the U. S. Geolog- 

 ical Survey, as the Lower and Upper Saint Peter, and he named the 

 limestone Everton, from the town of Everton, Boone county, Arkansas. 



■ Unconpormity at the Top of the Everton Limestone 



The Everton limestone overlaps the Lower Saint Peter to the north and 

 is (at least locally) unconformable on the rocks below. At the top of the 

 Everton limestone there is a pronounced unconformity, which is strikingly 

 visible at all points where this formation outcrops on the hillsides. A 

 typical one of these exposures is shown in the following figure : 



Scale „ 

 PIG0RB 1. — Unconformity at the Top of the Everton Limestone. 



Prom the nature of this unconformity there would seem no doubt but 

 the irregular upper surface of the limestone is due to solution that took 

 place at the base of the soil while this was the surface rock. In many 

 places the limestone was dissolved out in wide basins a hundred feet or 

 more in diameter, while in others the depressions were smaller and more 

 cistern-like. As the sea advanced, the soil, which doubtless was mainly 

 clay, was carried seaward and the Upper Saint Peter sandstone was put 

 down on the very irregular but clean surface of the Everton limestone. 



Sandstone Masses at the Horizon op the Unconformity 



The sand filling these caverns is the last to succumb to weathering 

 agents; so that in those parts where streams have cut down through the 

 limestone there are on the hillsides, at the limestone level, numerous 

 masses of sandstone, some large and bulky, others tall and slender, stand- 

 ing sentinel-like, as so many silent witnesses to geography of the past. 



The Everton limestone is absent over western and the extreme northern 

 part of Arkansas, but the unconformity that occurs at its upper part 

 covers the whole area; so that sandstone masses like those described are 

 present throughout the region. Those most convenient to be seen are in 

 the vicinity of Eureka Springs, though they are larger and more numerous 

 over the northern part of the Harrison quadrangle. Where the Everton 



