1 HE LA URENTIA N PENEPLA IN 659 



may show that the Paleozoic sediments which they contain, and 

 which are in every case, except that of Lake Nipigon, signifi- 

 cantly below the level of the peneplain, are preserved because 

 thrown into their present protected positions by the downthrow 

 of graben blocks. If so, the probability of this plain being of 

 Cretaceous age will be strengthened. What seems to be a com- 

 parable case is known in Scandinavia. The earlier sedimentary 

 rocks were dislocated by a series of faults ; later planation left 

 only a few small patches at base-level upon the downthrown 

 parts of the tilted blocks. Subsequent elevation of the whole 

 area, and erosion of these softer remnants, produced a series of 

 depressions in some of which are still found isolated patches of 

 the soft rocks. The lower portions of these depressions 

 frequently form lake basins, the most important of which are 

 Boren, Roxen, Glau, and Braviken.^ 



So far as the geological record is known, it appears that the 

 Labrador portion of the peneplain has never been submerged 

 since Paleozoic time. To this long exposure to subaerial erosion 

 may be attributed the extremely low average gradients of 

 approximately one foot per mile, the lowest found anywhere 

 upon the plain. 



3. The origin of the basins, valleys afid gorges. — The minor 

 basins upon the upland surface, now forming the basins of small 

 lakes, undoubtedly owe their origin to processes of differential 

 disintegration and subsequent erosion and denudation by some 

 agency. The last denuding agent in operation was, of course, 

 the Pleistocene ice-sheets. There is some question, however, as 

 to whether the denudation of the surface is due to these ice- 

 sheets, or to some earlier ice-sheet, or to some other cause. 

 Reference will be made to this in discussing the fourth problem. 



The origin of the valleys and gorges incised beneath the 

 plain to a significant degree presents some features of particular 

 interest. Those in which the Cambrian sediments lie may be 

 older than the sediments which they contain, and in that case 

 would be of much earlier date than the main peneplain surface. 



'See also Eighteenth Annual Report State Geolocrist of New York. Part V, 

 pp. 143 ff. 



