RECENT INVESTIGATIONS 295 



■feet below the flat-topped spurs on. either side. About 8 miles below the 

 Iron fault, near the site of the proposed town of Stockmore, the so- 

 called West fork comes in from the west after running for 25 miles along 

 the strOve in the red beds of the Permian or Trias. Below the forks, in 

 the softer Mesozoic rocks, the valley becomes still wider and contains 

 considerable bottom land, while the bounding ridges have worn down into 

 low hogbacks, except where protected by a covering of Tertiary beds, 

 lapping up on the spurs or occasionally standing out as residual mesas, 

 like Blacktail mountain in the section (figure 1). 



The magnificent section of Paleozoic beds exposed along the walls of 

 the gorge is rendered rather difficult to read, through the abundant slips 

 or faults parallel to the stream, by which narrow slices of the walls are 

 let down a few hundred feet here and there, probably by sapping, so that 

 portions of the clifl: sections are duplicated. This may account for Mr 

 Ber key's tendency to overestimate the thickness of his various divisions 

 of the Paleozoic rocks. 



Another peculiarity of topographic structure is observable at the head 

 of the side ravines tributary to the main valley when they occur in lime- 

 stone formations. In the midst of a remarkably well watered region, 

 these ravines have no rvmning water, and in their basin-like heads are 

 many minor depressions without outlet, such as are characteristic of 

 moraine ridges, but in shape rather longitudinal than round. On either 

 side of the main valley, near the mouth of the ravine where these were 

 first observed, there are large springs issuing from the base of the cliffs, 

 in streams 15 to 20 feet wide, with sufficient volume of water to supply 

 irrigation ditches which extend to the arable lands many miles down the 

 valley. Hence the explanation that suggested itself was that, in the 

 easily soluble limestones, surface waters had eaten their way along cracks 

 and small faults, finding their run-off in such springs, and had thus 

 eroded increasingly large caves that had finally collapsed, producing 

 something analogous to the sink-holes of the western Appalachian region. 

 This hypothesis was later confirmed by the finding of a typical Kentucky 

 sink-hole with circular limestone walls and a funnel-shaped bottom. 

 The structure is developed on so large a scale in this region that it de- 

 serves a special name, for which sink-hole or karst topography is sug- 

 gested. 



Geology of the Eegion 



The geology of the region is not so easy to read as at first glance would 

 appear, because the cliff sections along the valley are complicated by the 

 slips or faults mentioned above, and on the flat-topped spurs, owing to 



