100 



Trans. Acad. Sci. of St. Louis. 



times rising to a height of two or three hundred feet 

 above the flood plains. 



Where the course of the stream has led through the 

 usual limestone formations of the region portions of the 

 valley are from half a mile to a mile in width. However 

 when the level of the Grand Falls Chert was reached the 

 process of erosion was sharply checked, and a series of 

 shallows and rapids were formed, as the water, etching- 

 its way slowly through the hard strata, leaped from ledge 

 to ledge. This change in the topography of the valley 

 is well shown on the map, where the contour lines are 

 crowded close to the stream in the vicinity of Reding 's 

 Mill. Three miles below the rapids culminate in the 

 Grand Falls, where the stream makes a drop of twenty- 

 four feet. For most of the intervening distance the 

 creek has cut through the upper layers of the chert, which 

 is exposed on one or both sides in perpendicular or over- 

 hanging cliffs, twenty to forty feet in height. Extending 

 for some distance back from the tops of these cliffs the 

 rock has been washed bare or is covered only with a 

 sparse mantle of soil, thus forming the barrens. 



So hard and dense is the rock that the ordinary forces 

 of surface erosion : rain, wind and frost, have little effect 

 upon it. Looking at the gnarled and rugged faces of the 

 cliffs or at the smoothly polished promontories of the sur- 

 face, one might well believe that the storms of ages would 

 beat upon them to little purpose. Indeed few works .of 

 nature impress the mind more forcibly with a sense of 

 their strength and indestructibility than these massive 

 beds of the Grand Falls Chert. 



The stream, however, aided by the sharp gravel and 

 boulders derived from the beds themselves, is slowly but 

 incessantly cutting its way through them, and in places 

 undermining the cliffs. The process of disintegration is 

 facilitated by the fact that the rock is deeply fractured at 

 intervals by fissures, either vertical or at various angles, 

 that eventually allow undermined portions of the cliffs to 

 shear off into the stream. Here and there great castle- 



