NATURAL BRIDGES BY STREAM EROSION 



325 



through a joint or other crack in the harder overlying rock, thus forming 

 a natural bridge. What may be considered a type of this mode of for- 

 mation is a bridge over the Thur (described by Friih^^) near Krum- 

 menau, in Switzerland. In this region the underlying rock consists of 

 a series of alternating beds of southerly dipping marls, sandstones and 



PLMEAU 



\300 M. 



PLATEAU 



Figure 7. 



-Map indicating Formation of Natural Bridge in Potvell County, Kentucky, hy 

 the headward Cutting of two Streams in a Plateau Region 



resistant, 



fissured conglomerates several meters thick. 



Keeping these 



conditions in mind, the formation of the bridge is clear. The river loses 

 itself here and there through fissures in the conglomerate to a layer of 

 marl. Instead of working chemically, the water carries the marl away in 

 suspension, thus leaving a natural bridge or tunnel. Since the stream in 



" J. Friih : "Naturbriicken," etc., p, 369, 



