78 PUBLIC PARKS OF IOWA 



TOPOGRAPHY OF MOREHEAD CAVE AREA. 

 By T. E. Savage, Geologist. 



Near the middle of the north half of section 6, South Fork township, 

 is an area known locally as "the caves.'-' These consist of a series of 

 natural bridges that have been developed by the waters of a small creek 

 eroding a subterranean passage, and the subsequent partial caving in of 

 the roof of the cavern. 



The upper or most northerly bridge has a length of 150 feet across the 

 gorge and a width of about sixty feet. The stream flows in a channel 

 about ninety feet below the top of the bluffs. It has carved a passage 

 fifty feet in height beneath the span of the bridge. About eight rods fur- 

 ther down the stream a second arch crosses the ravine. This latter is 

 several rods in width, but is so choked with silt and driftwood that the 

 passage can only be followed with difficulty. 



A few rods further down the creek there is a sink hole sixty feet in 

 depth, having a diameter at the top of seventy-five feet. Climbing down 

 to the bottom of this shaft the explorer can readily follow an underground 

 passage three hundred feet in length, forty to seventy feet in width and 

 eight to twenty-five feet in height. At various points along this main 

 passage there are to be seen entrances to smaller galleries which wind 

 in and out along the sides and roof of the cavern. A beautiful spring, 

 furnishing a stream of water four feet in width, issues from one of these 

 lateral canals. At the lower end of the passage the stream emerges in a 

 gorge whose bounding cliffs rise 125 feet on either side. This locality 

 is a justly popular resort for drives and picnics for the people in all of 

 the southwestern portion of the county. 



In the southern part of Richland township, near the village of Cotton- 

 ville, another series of caverns or underground passage? have been de- 

 veloped. 



Such' channels are usually formed where streams having a steep 

 gradient cut deeply into thick bedded limestones. Professor Shaler has 

 shown that their genesis also requires forest conditions. As the rain 

 water filters through leaf mould over woodland areas, it becomes charged 

 with carbonic acid gas from plant decay. As this carbonated water slowly 

 percolates along the crevices and joint planes of limestone strata, it grad- 

 ually widens the fissures by taking into solution some of the material 

 along the way. The amount of limestone thus dissolved by the water is 

 always in direct proportion to the amount of carbonic acid gas that the 

 water contains. As the passages become enlarged a larger volume of 

 water follows them, and, in turn, the larger stream of water more rapidly 

 increases the size of the channels by abrasion as well as by solution. 



In the course of time the streams of such a region desert the surface, 

 and find an outlet to their major streams through subterranean channels. 

 If not too deep beneath the surface, the roof of these passages will even- 

 tually be broken through at some points giving rise to natural bridges. 

 Gradually the underground channel may be converted into a gorge by 

 the falling down of the roof along its entire course. Geology of Jackson 

 Cormty, pp. 571-3. 



