FIELD STUDIES OF CONCRETIONS IN SHALE 677 



slipping off the steep sides of the concretion and accumulating around it 

 (see figure 1). As burial proceeded, the normal bedding planes passed 

 over the concretions and eventually all evidence of the concretion died out. 

 A feature of considerable import noticed on these concretions was their 

 marked slickensides. These were on both the under and upper sides, but 

 were far less perfectly developed below. In the entire absence of evidence 



Figure 1. — Sketch of Concretion in Pennsijlranian Shale in Boone County, Missouri 

 Shows lenses of clay between the bedding planes. 



of lateral growth, such as crumpling of the beds, which would be neces- 

 sary in these beds, because the concretion is not due to local cementation, 

 as shown by the stratification planes passing over and not through them, 

 it must be concluded that the slickensides are due to upward growth sub- 

 sequent to deposition, or that the beds slipped down around the concretion 

 during consolidation. 



CONCRETIONS IN THE CRETACEOUS (GRANEROS) SHALES IN SOUTH DAKOTA 



AND WTOMING 



The lower part of the Graneros shale (Cretaceous) in the northern 

 Black Hills contains many concretions. Some of the best of these are in 



cone-in-cone. 



Figure 2. — Sketch of Concretion in Mowry Shale near Belle Fourche, South Dakota 



Note horizontal stratification in the concretion and the bedding planes curving 



around it. 



the Mowry shale member. These concretions attain a length of 6 or 7 

 feet and a thickness of 15 to 18 inches. They are circular, elliptical, and 

 irregular in plan and distinctly lenticular in cross-section, becoming 



