CONDITIONS AFFECTING DISTRIBUTION, FORM AND SIZE. 341 



corrasion graded soft sliale and liard cores alike to even plains in the 

 valleys. More recently the main streams have sunk their channels to 

 lower levels and their valleys have been broadened by the minor drain- 



FiQURE 6.— View of Tepee Butte. 

 Drawn from photograph. 



age, laying bare the shale. The soft rock thus exposed is being degraded 

 so rapidly that the degradation of the cores can keep pace with it only 



FitJi'KE 7.— Ideal Section of Butte represented in Figure 6. 



by the aid of high declivity. Tlie eiiuilibrium of attack and resistance 

 thus maintains high declivity at and near the cores. 



COXDITIOXS AFFECT I XO FORM AND SIZE. 



Examination of the butte summits shows that the limestone is dis- 

 integrated primarily by fracture, and the cause is doubtless found in 

 changes of temperature, including frost. The exposed top of the core is 

 usually shattered and large fragments lie on adjacent slopes. Lower 

 down are progressively smaller fragments, and the external part of the 

 conical mass is evidently a talus of limestone and shale debris. Beneath 

 and protected by this is a conical annulus of undisturl)ed shale surround- 

 ing the core (see Hgure 7), but it is prolxible that the forn) of the ])utte 

 depends almost exclusively on factors affecting the disintegration and 

 transportation of the limestone debris. Among these must be reckoned 

 frost, heating and cooling, wetting an<l drying, rain-beating, wind erosion, 

 solution, burrowing and the mechanical and chetnical action of roots. 

 Unk^ss some one factor can be shown to dominate the rest, it will l)e a 

 matter of great difliculty to analyze the process by which the concave 



XLVIII-Bti.i.. Grot. Hoc. Am.. Vol. r,. 1«94. 



