COLLAPSE OF RECENT BEDS AT STAUNTON 43 



stream coincides with the direction of the strike and is determined 

 by it. 



While the primary factors which lead up to the caving, or made it 

 possible, are those which have been outlined it is less easy to state 

 definitely what was the immediate cause of the caving. Various 

 causes may lead to the reopening of the outlets of a silted-up lime- 

 stone sink. Among these may be mentioned the burrowing of 

 rodents or crawfish. A very small opening through the clay plug 

 at the bottom of a closed sink hole might enlarge rapidly by the 

 downward rush of ponded water. The gradual enlargement of the 

 underground channel by solution of its walls and roof and the con- 

 sequent breaking down of parts of its roof is in continuous though 

 exceedingly slow operation in all caverns occupied by streams. 

 The influence of a flooded condition of the underground channel, 

 causing it to soften and erode rapidly any clay fillings in its roof 

 might, in some cases, be effective in causing collapse of superficial 

 beds. Another factor known to be effective in producing collapse 

 in some regions is the general lowering of the ground water level. 

 In the zinc district of southwestern Missouri, where limestone sinks, 

 both filled and unfilled, are a characteristic feature of the topography, 

 Mr. C. E. Siebenthal informs me that extensive pumpingin connec- 

 tion with mining operations is a generally recognized cause of the 

 collapsing of caves as well as caving in of silted-up sink holes. 



Having in mind some of the factors which may produce collapse 

 as outlined above we may consider which, if any, has probably 

 been the cause of caving at Staunton. One of the causes which has 

 been suggested as the effective agency in producing the caving is the 

 very heavy rain which occurred at Staunton shortly before the 

 development of the caved holes. A flooded condition of the under- 

 ground stream would tend undoubtedly to soften the clay rilling of 

 the limestone crevice as well as increase its weight. But this factor 

 must have been active at intervals for centuries at least without 

 producing caving, and probably can hardly be regarded as more than 

 a minor contributory cause at most. As an accessory to whatever 

 may have been the primary cause of the initiation of caving there 

 has been also the constantly present factor of vibration resulting 

 from street traffic and the operation of a street car line across the 

 line of caving. 



