36 Cincinnati Society of Natural History. 



weak horizon, a fold occurring whenever this horizon is 

 properly situated topographically. Their direction, more 

 or less parallel to stream valleys is favorable to the hypothesis 

 that they have resulted from buckling due to removal of 

 load in the valley. 



The second group, at the top of the Eden, seems to be 

 adequately explained by the hypothesis of buckling. Here, 

 the axes are parallel to the stream valleys. In all cases 

 where folding exists at this horizon, the stream bed is sud- 

 denly cut deeper just beneath the Eden-Fairview contact. 

 This horizon lies just beneath a formation which contains a 

 rather large proportion of limestone and is therefore stronger 

 than the shale beneath. When this support is removed 

 buckling may occur, as the shales cannot withstand the 

 downward force on either side. 



Joint cracks. — In the Cincinnati region the rocks are 

 affected by two very persistent systems of vertical joints, 

 having general east-west and north-south directions. These 

 joints are not uniformly spaced, varying usually from about 

 one to three feet apart. Both systems extend far beyond 

 the Cincinnati region, as indicated by the following statement 

 from Matson: "In the Blue Grass region there are two well- 

 developed systems of vertical joints at approximately right 

 angles to each other. These joints have general north-south 

 and east-west directions and are apparently'' very per- 

 sistent." It is probable that these two systems of joints 

 are coextensive with the Cincinnati anticline. They affect 

 both shales and limestones, but are more prominent in the 

 limestones. These joints are taken advantage of in quarry- 

 ing the limestone strata, the joints forming easy fracture 

 planes often doing away with the necessity of blasting. 

 Weathering first takes place along these joint planes, mani- 

 festing itself in the yellow oxidized borders of the joint 

 blocks. The joint cracks greatly facilitate the circulation 

 of ground- water, and become much enlarged by solution in 

 the purer limestone strata. 



♦Matson: U. S. Geol. Surv. Water Supply Paper 233, 1909, p. 28. 



