218 University of Kansas Geological Survey. 
major part of the Benton area. The soil so formed on account of 
the great quantity of Ostrea shells is very productive. The shales 
as a rule, however, are closely textured, so that there is a tendency 
to the formation of a subsoil which bears among agriculturists the 
name of “hard-pan.” The hard-pan not only prevents the disinte- 
eration of the shales below, but becomes almost impervious to water, 
so that when water falls upon the surface of the ground and sinks 
through the stratum of soil to the surface of the hard-pan it flows 
along this surface until it reaches some neighboring outlet. Hence, 
instead of being stored up in the soil it is drained away and in a 
short time, the soil not being very deep, that part which remains 
has evaporated and the crops suffer as a result. 
The Blue Hill Horizon.—The Blue Hill shales, unlike the Ostrea 
shales, are very loosely coherent, and easily removed by erosion. 
Wherever they have covered the surface they have formed a very 
unproductive soil which is called iocally “gumbo.” The unpro- 
ductiveness of the soil is due to the unfossiliferous character of the 
shales. The Upper Benton shales are preserved from erosion in 
the central portion of the Benton area by the massive Fort Hayes 
limestone of the Niobrara. Shales not protected in this way have 
been eroded away leaving the protected shale beds with nearly per- 
pendicular walls. 
The maximum thickness of the Blue Hill shales is 100 feet. They 
are dark blue in color and nonfossiliferous. Williams Butte, a 
single isolated point among the Blue Hills, forms an excellent ex- 
ample of the erosion of the upper shale beds. It is 120 feet in 
hight, and is protected by the Fort Hayes limestone. 
The Sali Creek Gravel Beds —The Salt Creek gravel bed, or con- 
glomerate gravel stones, were at first difficult to account for. 
Chiefly owing to their resemblance to the grit rock of the Tertiary, 
they were thought to belong to that formation. They belong more 
probably, however.to a later period. This gravel horizon is about 
15 feet above the saliferous shaies of the Upper Dakota, which 
form the bottom of the creek bed. The beds dip under the soil 
towards the bluifs, which are protected by the Benton limestone. 
An examination of the rock showed it to be composed of gravel, 
ferruginous nodules, ete., from the shales and limestone beds of the 
