THE WORK OF STREAMS 



117 



River. In this way the Tennessee originated, made up of parts of 

 three rivers which formerly had different courses. 



After the first elevation, the region remained at approximately the 

 same level for a long time, as is shown by the accordant altitudes of 



Fig. 105. — The Water Gap near Harrisburg, Pennsylvania. The horizontal sky 

 line shows the surface of the ancient peneplain. (Maryland Geol. Surv.) 



the plainlike valleys between the mountains, but long before the 

 ridges could be reduced, another uplift (Fig. 104) occurred which 

 caused the streams to deepen their beds to the present level. The 

 last uplift must have been relatively recent, since the new valleys 

 are as yet comparatively narrow. 



(3) The Laurentian Peneplain. — The great hunting and fishing 

 region of North America is that vast area almost surrounding Hud- 

 son Bay, which stretches from Lake Superior and the St. Lawrence 

 River on the south 

 to the Arctic Ocean 

 on the north, and 

 from the shores of 

 Labrador on the 

 east to Lake Win- 

 nipeg on the west 

 (Fig. 106). This is 

 known as the " Lau- 

 rentian shield " (p. 

 389). When one 

 stands on almost 

 any eminence in this 

 region, he finds that 

 he is on a great 



Fig. 106. — The horizontal sky line of the Laurentian 

 peneplain with an incised valley. (Photo. T. C. Brown.) 



