DIFFERENTIAL EROSION 343 



In the limestone-dolomite belt the streams have, as yet, only succeeded 

 in trenching a few gorgelike valleys, leaving extensive interstream por- 

 tions of the upland still undisturbed; but to the south, where the domi- 

 nant rock formation consists of the Mesozoic slates, phyllites, quartzites, 

 etcetera, the streams have produced wide flaring valleys, and throughout 

 considerable tracts have left no traces of the original plateau surface. 

 The almost flat valley bottom of Black River, in the vicinity of the inter- 

 national boundary^, is at least 5 miles wide, while those of some of its 

 tributaries are over 2 miles in width. 



The great relative difference in the amount of destructive work the 

 streams have been able to accomplish in the different areas or portions 

 of this one area is entirely due to the differences in the geological forma- 

 tions. The slates and phyllites, on account of their readily cleavable 

 nature, are very susceptible to the mechanical activities of running 

 water, and the intercalated quartzite beds being left exposed and often 

 unsupported also readily fall a prey to stream action ; also the numer- 

 ous other disintegrating and eroding forces, previously mentioned, con- 

 tribute a vast quantity of already comminuted material to the streams. 

 Moreover, erosion is ever3^where rapidly progressing except where the 

 slopes have become sufficiently gentle to allow of the growth of a pro- 

 tecting mantle of vegetation. 



In the limestone-dolomite belt the upland is prevailingly covered by 

 debris, leaving only the valley walls exposed to disintegrating and erod- 

 ing activities; and even there, although freezing, and thawing and ex*- 

 pansion and contraction have played a notable part, still, on account of 

 the massive though not generally hard character of the rocks, these de- 

 structive processes ha^■e been so slow that solution appears to have been 

 a comparatively imjiortant eroding force. 



The arctic climate has also been an important factor in causing the 

 topography in the different portions of this area to be so strikingly con- 

 trasted. A cold climate retards rock decay, rock solution, and practically 

 all chemical activities, which are among the main forces by which the 

 limestones and dolomites are affected; but rock breaking and splitting, 

 to which even the hardest and most indurated slates and phyllites are so 

 susceptible, are favored in regions where the daily temperature changes 

 are great, which is the case in this area during the greater part of the 

 year. 



SUMMARY AND DEDUCTIONS 



The area under consideration forms a part of what is believed to have 

 been formerly a peneplanated and subsequently uplifted surface. The 



