COLLAPSE OF RECENT BEDS AT STAUNTON' 37 



titious reopening of the outlet. In such a case the limestone sink 

 resumes its original function of catch-basin for surface waters. 



It happens that a small portion of the central part of the town of 

 Staunton is located on a tract of land which has passed through 

 the geologic cycle outlined above. The reopening and enlarge- 

 ment of the original outlets of the limestone sink which was the fore- 

 runner of the marl bed underlying this part of Staunton, led to the 

 caving of the soft marl and gave to the residents of the city their 

 first knowledge of the presence of this subterranean stream. The 

 caved area lies near the lower end of a small valley traversed by one 

 of the minor tributaries of Lewis Creek. This stream has been con- 

 fined to a conduit of masonry throughout most of the lower part of 

 its course in the city. At, and above the caved pits (PL I), the 

 valley has a very low grade for half a mile or more and is nearly 

 flat in cross-section for a width of three to five hundred feet. On 

 both sides of this comparatively flat area the land rises one hundred 

 feet or more in gentle hill slopes, showing frequent outcrops of the 

 underlying limestone. In the bottom of the valley the limestone 

 is buried nearly everywhere by several feet of superficial deposits 

 of clay and marl. A short distance below the caved holes the grade 

 of the valley is very much steeper than above. In fact the abrupt 

 change of grade here would probably justify the use of the term 

 "hanging valley" for the small valley near the lower end of which the 

 superficial beds collapsed. Previous to the accumulation of the 

 marl beds it had evidently been a closed valley. The abrupt change 

 in the grade of the valley is undoubtedly due to the diversion of the 

 water of the valley stream to a subterranean stream prior to the 

 accumulation of the twenty to sixty feet of clay and marl which 

 now forms the bottom above the abrupt grade. This diversion 

 must have been through limestone sinks which developed in the 

 vicinity of the holes recently caved in the marl beds. At some 

 remote period outlets to the limestone sinks were closed and a large 

 pond occupied the portion of the valley above Frederick street. 

 The great numbers of freshwater shells in the marl beds afford 

 indisputable evidence of the pond conditions under which the marls 

 accumulated. It was during this stage in the history of the valley 

 that the soft marl and clay beds were deposited above the tempora- 

 rily closed subterranean channel, a superposition which brought 



