DIFFERENTIAL EROSION 



341 



surface waters, on account of their purity, have practically no chemical 

 effect on siliceous and argillaceous sediments; but when containing even 

 small amounts of carbonic acid gas or certain other impurities, dissolve 

 limestones quite perceptibly. It is, however, to ground waters that the 

 effectiveness of chemical action as an eroding agent is mainly due; these 

 prevailingly contain various impurities that cause them to become very 

 active solvents. The great amount of dissolved mineral matter which is 

 annually carried to the ocean, either to be deposited there, or to remain 

 in solution, is attributable largely to the ground waters; in addition, 

 much of the mineral matter taken into solution is deposited either close 

 to the point of its extraction or along the streams carrying it to the sea. 

 The ground waters have no doubt played an important though largely 

 unknown part in the subtraction of materials from the particular area 

 under consideration, and in the limestone-dolomite belt, solution — both 

 by the ground and surface waters — ^lias apparently constituted one of the 

 principal agents of destruction, as is indicated by the highly calcareous 

 water in the area and by the disintegrated and partly leached character, 

 in places, 'of the rock exposures. 



The dominant weathering force throughout the area, however, ap- 

 pears to be frost action. In addition, expansion and contraction, due to 

 atmospheric temperature changes, running water, nivation, and probably 

 solifluction, are important subaerial destructive processes actively en- 

 gaged in this district. 



In this area, where throughout the greater part of the year the daily 

 temperature changes are considerable, rock breaking and splitting pro- 

 ceed with great rapidity, especially in connection with the finely cleav- 

 able slates, phyllites, etcetera. In these rocks, water fills the multitude 

 of spaces, including those along the cleavage planes; and when frozen, 

 causes ilie rocks to break up readily into slabs, which in turn split into 

 smaller flaky pieces. The more massive quartzites, limestones, dolomites, 

 etcetera, are not so susceptible to this process, but are, nevertheless, con- 

 siderably affected. 



All the rocks are to a considerable extent fractured and broken by 

 expansion and contraction dne to temperature changes, and from this 

 cause as well, the thinly cleavable beds suffer most. 



Xivation,*' or snow-drift action, has also played a somewhat important 



«The term "nivation" was originaUy employed by Matthes and has later been applied 

 •by Hobbs and numerous other writers : see 



F. E. Matthes : "Glacial sculpture of the Bighorn Mountains, Wyoming." U. S. Geol. 

 Surv., 21st Ann. Kept., pt. ii. 1800. pp. IT.'MOO. 



W. M. Hobbs : "Cycle of mountain glaciatlon." Geog. Jour., February. 1910. pp. 

 147-163. 



