STREAM PIRACY AND NATURAL BRIDGES IN THE 

 LOESS OF SOUTHEAST MISSOURI 



C. L. DAKE 



Missou?;! School of Mines 



In Perry and Cape Girardeau counties, in southeastern Mis- 

 souri, are loess deposits locally deeply gullied into a sort of rather 

 mild badland topography. During the field season of 19 13, the 

 writer observed in this gullied loess an unusual combination of 

 erosion phenomena, which illustrates in miniature one process of 

 formation of natural bridges.^ 



Along a roadside a gully had cut into the loess to a depth of 

 over six feet, and, owing to a filling of bowlders and debris at its 

 lower end, had established a temporary base level. The tributary 

 gullies were also near this base level, and during a recent storm 

 had actively widened their valleys. 



At a distance of perhaps ten feet from the junction of the main 

 gully and a tributary, undercutting of the banks in opposing direc- 

 tions had reduced the divide at that point to a wall of loess not more 

 than fifteen inches thick, the upper grassy portion of which had 

 crumbled away, lowering the divide a foot or two, at the narrow- 

 est place, and forming a sort of saddle. 



The bed of the larger ditch was ten or twelve inches lower than 

 that of its neighbor, and, in continually wearing against the loess 

 wall, had completely undercut the divide, opening a gap about a 

 foot high by eighteen inches long, leaving a natural bridge of loess, 

 with an open arch between the two gullies. 



The further undercutting of the larger channel had so encroached 

 upon its smaller and higher neighbor as completely to divert the 

 drainage of the latter through the arch, over a low rapids, leaving 

 the lower course of the beheaded gully several inches higher than 

 its former upper course, with a distinct step between them. The 



'H. F. Cleland, "North American Natural Bridges, with a Discussion of Their 

 Origin," Bull. Geol. Soc. America, XXI (1910), 313-38. 



