THE LA URENTIAN PENEPLAIN 649 



The edge of the .plain along the north shores of Lake Huron 

 and Lake Superior stands at a considerable elevation above these 

 water bodies, and there are many south-flowing streams in this 

 locality occupying steep-sided valleys incised well beneath the 

 peneplain. In the northwest part of the area similar gorge-like 

 valleys occur, but they are generallv smaller, shorter, and are 

 less prominent features. 



So far as known, the valleys of this class are never found to 

 be occupied by pre-Cambrian sediments and always have a well- 

 defined "shoulder,"' where the gradient curve of the valley side 

 intersects the flatter curve of the upland surface, and in their 

 general relations suggest that they date their origin from a time 

 subsequent to the period of planation. 



6. Mo?iad?iocks. — In the various descriptions which have been 

 quoted above it will be noted that frequent reference is 

 made to the occurrence of residual domes and ridges which 

 stand prominently above the generally even surface of the 

 peneplain. In northwestern Labrador there is a narrow range 

 of mountains described in part by Bell (5) and by Daly (9), 

 reaching in places an elevation of about six thousand feet 

 above sea-level. The range is narrow, having only a width of 

 about fifty miles. The somewhat meager descriptions would 

 lead one to infer that this range, like that of the less rugged 

 Adirondacks in New York state, represents residual mountain 

 peaks which have not been reduced to the peneplain condition. 

 With the exception of these two small and widely separated 

 areas, the residual elevations rising above the peneplained surface 

 are generally more or less isolated domes and ridges, and they 

 are found in all parts of the region. Their elevation and develop- 

 ment seem to depend largely on the character of the material 

 of which they are composed, and of the material by which they 

 are surrounded. The most prominent monadnocks known within 

 the area seem to be those mentioned by Barlow and McOuat as 

 occurring in the region west and north of Lake Temiscaming, 

 where hard quartzite ridges stand in places as much as seven 

 hundred feet above the plain. The Baraboo ridge in Wisconsin, 



'See 41, p. 155. 



