247 



SO.ME EVIDENCE IXDICATIXG THE IMPORTANCE OF I'KOST 

 ACTION IN WIDENING VALLEYS. 



Glknn Cui.bkktson. 



At several pciiits along the lower reaches of Clifty gorge in Jefferson 

 county, Clifty Creek in its first attempts at valley widening has eroded the 

 sides of the valley to such an extent as to cause more or less extensive 

 landslide areas. In the case of several of these, the naked soil and rock are 

 exposed for a horizontal distance of several hundred feet, and vertically 

 s(>veral scores of feet. About one-half mile from the Madison and Hanover 

 road across the mouth of the valley, there is such an erosion area on the 

 slope facing west. Near the south end of this exposure a large mass of the 

 original rock extends out into the vaUey. as may be seen by referring to 

 the figure. 



The formation into which the stream is cutting at this point is that of the 

 Lorraine shales of the Ordovician. At this horizon the Lorraine is com- 

 ixised of alternating layers of limestone from one to three inches in thickness 

 and comparatively soft blue shale comprising at least 75 per cent of the 

 whole. At the point "A" in the figure, the stream at every flood stage imder- 

 cuts (he steep slope and removes all debris consisting of broken lime stone 

 and shale resulting from the weathering of the exposure. The projection 

 at "B" is composed of the same material as that at "A", and is from its 

 position especially exposed to the action of the stream at flood stage and 

 thruout its course in the two mile gorge the stream gradient is high. 



"Why has this projection "B" been able to endure while a similar material 

 both upstream and down has been carried away? In the judgment of tlic 

 writer, it is due to the fact that a spring emerges from the hillside above, 

 the waters of which flow over the projecting rocks at "B", and not only pre- 

 vents freezing, but keeps them uniformly at the same temperature and 

 moisture. The part '"B" which stands out some twelve or fourteen feet be- 

 yond the part "A" and at least six feet beyond the part "C which is com- 

 pletely protected from the force of the flood waters by the projection, is 

 kept well covered by the spring waters at all times. The difl:erence in the 

 rate of valley widening here which is quite marked, is probably due to 

 two, if not three processes. First and foremost is frost action. The 

 alternate freezing and thawing which occurs many times during the winter. 

 loosens annually large quantities of material from the steep sloi>e at "A" 

 which is entirely unprotected by vegetation. The Spring floods i)eriodically 

 remove this. Some material of this sort, however, is loosened and accumu- 

 lated during the summer and autumn. Hence another cause of weathering 

 must be sought, that does not take place at "B". Alternate expansion and 

 contraction due to change in temperature is responsible for part of this 

 work, but in the writers judgment, alternate wetting and drying, which 

 takes place many times during the summer, may be equally as efficacious. 



At two other points in Jefferson Coimty, a similar protective influence of 

 the waters of springs has been noticed. In these cases the formations 

 protected were resistant Devonian limestones, and the mass of the pro- 

 jecting area was not nearly so great as in the case of the less resistant 



