THE LA URENTIAN PENEPLAIN 66 1 



Laurentian swell by which the drainage from the main divide in 

 the interior of Labrador reaches the St. Lawrence River is much 

 more complex. One would expect the drainage of such a region 

 to find its way outward toward the southwest, somewhere along 

 the line of the present St. Maurice River. At the present time 

 almost all the streams flow nearly at right angles to this appar- 

 ently normal direction and cross the St. Lawrence swell in very 

 straight courses by deep, steep-sided gorges or canyons. 



The transverse drainage has been explained by assuming that 

 the gorges through which the streams flow were produced by 

 some cataclysm, either by the production of gigantic gaping fract- 

 ures or by the down faulting of narrow graben blocks. The only 

 evidence upon which this theory is based seems to be the exist- 

 ence of the gorges themselves, and the apparent difificulty of 

 otherwise accounting for the courses of the incisions in a direc- 

 tion which is almost at right angles to what would be considered 

 the normal one. There are, however, at least two other well- 

 known processes by which features such as these could be nor- 

 mally developed. 



In the first place, the swell may not have been in existence 

 at the time when the streams were first beginning to flow and the 

 gorges were not cut. The edge of the plateau may have been 

 arched up slowly by the same forces which produced the Appa- 

 lachian folds of the New Foundland and Acadian areas, ^ or by 

 more recent dynamic processes. Provided this gentle arching pro- 

 ceeded slowly enough, the streams might have persisted in their 

 original direction, apparently a consequent one (as suggested 

 below) sawing their way into the ridge as rapidly as it rose. 

 In brief, the streams may be antecedent rivers. While this pro- 

 cess was going on in the southeast, the rivers on the other side 

 of the main divide would, provided they had an equal capacity 

 for work — a function of their volume and grade — have been 

 competent either to have cut out longer and deeper gorges or 

 to have widened their valleys. The information at present avail- 

 able is not sufficient to decide the question. Mr. Low has drawn 



'Daly has recently shown that the post-Pleistocene beaches on the northeast coast 

 of Labrador have undergone differential elevation since they were formed (9). 



