Malott: The "American Bottoms" Region 9 



limestone does not appear above the valley alluvium, on ac- 

 count of the presence of a disconformity and the consequent 

 deposition of the succeeding sandstone in its place. 



The Mitchell limestone to the east of the region under 

 discussion forms a surface rock of high importance from a 

 physiographic standpoint, being responsible for a structural 

 peneplain characterized by magnificent subterranean drain- 

 age systems and an attendant highly developed karst topog- 

 raphy. Since its outcrop is limited to a very small part of 

 the area under discussion, and its position is for the most 

 part at or below drainage level, it is of little physiographic 

 influence here. 



The Sample Sandstone and its Horizon. Overlying the 

 Mitchell limestone are gray and blue shales containing lenses 

 of sandstone, sometimes quite massive and attaining a thick- 

 ness of from 10 to 30 feet. This shale and sandstone 

 horizon fills the interval between the Mitchell and another 

 limestone from 5 to 20 feet thick, and occupies the position 

 of the Sample sandstone. The Sample sandstone receive its 

 name from its excellent exposures near Sample, Breckin- 

 ridge County, Ky., where it occurs as a division in the Gasper 

 Oolite.' The limited thickness of 20 feet, and the small areal 

 outcrop of the shales and the associated lenses of sandstone 

 render it of little importance physiographically in the region 

 under discussion. 



The Beaver Bend Limestone. The limestones mentioned 

 in the preceding paragraph as succeeding the Sample sand- 

 stone horizon is a bedded limestone, often having massive 

 beds, and is highly oolitic. It is conspicuously jointed, and, 

 on this account, gathers waters into concentrated streams 

 from the porous sandstone usually found above it. These 

 waters come out as springs at the base of the limestone on 

 the underlying impervious shale. Field work has demon- 

 strated that this limestone is found consistently practically 

 from the Ohio River northward as far as the Chester Series 

 extend, into Putnam County. Locally, however, it appears 

 to be separated by only a slight interval from the top of the 

 Mitchell limestone ; and in a number of places it has been 

 removed by the erosion which in certain places extended down 



^ Charles Butts, Mississippi Series in Western Kentucky. K(ntuckii Geological Siirvry. 

 1917. 



2—16903 



