GEOLOGY OF SOUTHWESTERN IOWA 



527 



upper strata in the brick yard shale pit one-fourth mile south of 

 the Missouri river bridge at Nebraska City, Nebraska. This 

 ledge is not a constant horizon; in passing south it ^ades into 

 sandstone and can not be recognized a few miles south of the 

 state line .in the state of Missouri. In recent years, Wabonsie 

 creek has been diverted into Wabonsie Lake. At the present 

 time the lake is silted up, no standing water is visible and the 

 old lake bed is grown up with marsh grass. At one time the 



Fig. 169a. — ExpOiSure of the "blue ledge" and associated strata southeast cf 

 Essex. (Calvin.) 



main channel of Missouri river evidently was at the foot of the 

 rock ledges in the bluffs east of the lake. At present a road grade 

 twelve or fifteen feet in height above the marsh has been con- 

 structed at the foot of the bluffs, concealing strata seen by White 

 and described in his "Section at Wilsons," Volume 1, Iowa Ge- 

 ological Survey, 1870. For a distance of nearly one-half mile the 

 lake is bordered' by a ledge of li^'ht gray, irregularly bedded, 

 shattered limestone, tw^elve to fifteen feet thick. In one place 

 the base of the limestone was seen resting on a black shale. South 



