286 The Realm of Nature CHAP. 



winding course of more than 1000 miles receives on its 

 right bank the Missouri, a river of much greater length, 

 formed by the union of tributaries from 900 miles along 

 the Rocky Mountain Range. Farther south the Arkansas, 

 another long river, flows in from the Rocky Mountains. 

 The steep eastern slope of this range, unlike that of the 

 Andes, stops at an elevation of nearly 6000 feet above the 

 sea, and thence the rivers flowing to the Mississippi cross a 

 slope so gentle that the land is spoken of as the Great 

 Plains. As the elevation diminishes the slope decreases 

 also, and the lowlands of the basin become known as the 

 Prairies. The Ohio River, flowing down the slope of the 

 Appalachians, is the largest tributary reaching the Mis- 

 sissippi on its left bank. 



368. Arctic Basins. In the northern half of North 

 America several nearly level terraces, of from 200 to 300 

 miles in breadth, separated by narrow zones of steeply 

 sloping land, descend from the Rocky Mountains toward 

 Hudson Bay. The lower terraces are covered with 

 boulder clay, and the terminal moraine of the great 

 Pleistocene ice -sheet has been traced in the form of a huge 

 ridge called the Grand Coteau des Prairies. This ridge 

 turns the Missouri River to the south, and the Saskatche- 

 wan, flowing from near Mount Brown in the Rocky Moun- 

 tains, to the north, thus separating the northern and 

 southern slopes. Upon the lowest terrace, where the 

 glacial remains are thickest, a line of wide shallow lakes 

 stretches from 49 N. to the Arctic Sea. Lake Winnipeg 

 in the south receives the Saskatchewan, and has an outlet 

 by the Nelson River to Hudson Bay. This lake is the 

 centre of a great but ill-defined drainage area, some of the 

 hundreds of small lakes surrounding it being connected 

 with several river systems, on account of the confused 

 ridges left by the melting ice-sheet. Traces remain of a 

 much larger ancient body of water, called Lake Agassiz, 

 which included Lake Winnipeg, and many smaller lakes 

 and river -valleys. The Athabasca, rising near Mount 

 Brown, flows north-eastward to Lake Athabasca, which has 

 an outlet northward to Great Slave Lake, whence the wide 



