GEOLOGICAL EFFECTS 365 



the last case mentioned a later stage, following the flat top butte, is 

 where erosion has displaced the concretions and left them strewn over 

 the summits and sides of a rounded knob. Such may result therefore 

 where the concretions are largely restricted to one stratum. 



More commonly, however, they are distributed through several strata 

 and with more or less uniform prominence. For example, they may 

 occur as lenticular concretions in a bed of shale or in the limestone in 

 the form of flint nodules. In the former case the result follows the 

 w^ashing away of the embedding shale, in the latter case where the lime- 

 stone becomes dissolved and leaves the concretions resting in a stratum 

 of residuary clay. The form of the concretion has much to do with its 

 efiiciency in this respect. Those which are globular are quite apt to roll 

 down the slope and leave the summit unprotected, but in the case of 

 lenticular forms they are apt to keep their position and form a very 

 effective covering. If the concretions crack into angular fragments they 

 become especialh^ efficient in protecting the surface. This is apt to be 

 the case of chert nodules, w^hich have produced the knobs of central 

 Tennessee and southeastern Missouri. Similarly, the large calcareous 

 concretions of the Pierre in Dakota break into angular fragments, so that 

 sometimes a single large concretion may furnish capping for a quite con- 

 spicuous knob. 



In the case of globular concretions there may be a tendency to form 

 knobs in a secondarj^ stage as follow^s : In an early stage of erosion they 

 may roll down the sides of a ravine and accumulate in the bends of the 

 watercourse at the bottom. As erosion continues, these bends resist the 

 erosion of the water, which is therefore turned aside, and in the process 

 of ages the general surface of the country is lowered below the level of 

 the accumulation first formed, and it may stand conspicuous as a knob. 

 Action simifar to this has been frequently noticed on the sides of the 

 deep valleys carved out of easily eroded material, with stone deposited 

 either in the clays, as in the case of boulder-clay, or where the stony 

 material rests on the top, as the capping of a terrace. Examples of this 

 sort have been noticed in the hillsides of the Missouri valley where it 

 cuts through the Pierre shales overlaid by the terraces and till of the 

 glacial period. No cases can be quoted where concretions have been 

 the sole cause of such action. 



.Y^ T UEA L RE VE TME^^ TS 



Under this head we may include cases where the concretions have 

 accumulated at the foot of cut banks and steeper slopes bounding rivers 

 and lakes, as in figure 26, plate 49, and figure 2, plate 50. From what has 

 been said in the last section, it may be readih^ seen that a stream cutting a 



