236 C. R. KEYES — CRUSTAL ADJUSTMENT IN MISSISSIPPI VALLEY. 



clusively that most if not all of the '' faults " in question have no exist- 

 ence. A notable instance is the " great fault " at Elk Cliff, in Marion 

 county, which Owen * reported as having a throw of more than 150 feet. 

 Lately careful examination of the locality has shown without the slightest 

 doubt that the abrupt change in lithologic features at this place in the 

 short distance of a few rods is due largely to erosion during Carboniferous 

 times. At Redrock bluff, two miles above Elk Cliff, the old steep-sided 

 channels of erosion are upward of 100 feet in depth. Faults of 50 or 75 

 feet have been reported from other places in the Iowa coal-field, but in 

 every ca^e investigated no satisfactory evidence has been obtained to 

 substantiate the claims. 



Farther south in Missouri, on the Mississippi river above Saint Louis, 

 the Cap-au-Gres " fault " has been estimated to have a development of 

 more than four hundred feet. Others of minor note have also had atten- 

 tion called to them. 



Dislocations in Iowa. — In all the Iowa coal-field less than half a dozen 

 true faults are known to be clearly defined in surface exposures, for, as 

 in most cases of faulting, the exact line of displacement is commonly 

 more or less completely obscured through the weathering of the strata or 

 by extensive superficial deposits. Ordinarily it would be impossible to. 

 recognize these faults, except as fortunate artificial excavations reveal 

 them. The development of the coal-mining industry in the state, how- 

 ever, has been the means of disclosing series of small faults'which would 

 otherwise have remained forever unknown. Owing to the important role 

 stratigraphic displacements play in the economy of mining, slips of only 

 two or three feet are brought quite prominently into account. The 

 large number and proximity of these small faults have been in some 

 cases rather surprising, and the chief object of the present paper is to call 

 attention to some of the dislocations as presented in the Iowa coal-field. 



Normal Faults. — All the faults observed are of the normal kind, with 

 a hade varying from a few to sixty or seventy degrees and a throw of 

 from a few inches to several feet. The ruptures and slippings in the 

 workable beds of the Iowa Coal Measures rarely interfere seriously with 

 mining operations as they often do in other regions, yet their geologic 

 import is significant. It has been only through he extensive removal of 

 comparatively thin beds that they have been brought to light. The 

 line of displacement as displayed in a mine entry is usually as sharply 

 defined as if drawn with a pencil, though often theedges of the two parts 

 are sometimes broken. In most cases the planes of sedimentation for a 

 few inches on each side of the fault line are bent more or less sharply 

 upward on the down-throw side and downward on the up-throw side. 



*Geol. Survey Wisconsin, Iowa and Minnesota, Philadelphia, 1852, p. 117. 



