96 Part I. Chapter 1. 
across. The Mackenzie River discharges into it with a great volume through a 
delta of intricate channels. Eastward along the arctic shore from the mouth 
of the Mackenzie, the coast is low consisting of soft rocks of the Cretaceous 
formation. An interruption of Devonian limestone occurs at Cape Parry from 
whence a long stretch of Cambrian rocks follows the shore to the eastern end 
of Coronation Gulf and Dease’s Strait, excepting Cape Barrow, which is Lau- 
rentian, and a cliff 1500 feet (456 m) high. This part of the coast is bold 
and high. 
With one exception (the Mississippi), the Mackenzie River is the largest 
in North America. It rises on the eastern slopes of the Rocky Mountains in 
the Vellow-head and Athabasca passes. The Peace and the Liard rivers are 
two of its great tributaries, while on the east, it receives the waters impounded 
by the Athabasca, Great Slave and Great Bear lakes. It is separated from the 
basins of the Churchill and Saskatchewan rivers by narrow and low water- 
partings. The Mackenzie valley is an alluvial plain bounded on the east by 
a more or less wooded region drained by the Coppermine and Great Fish 
rivers; on the west, it is limited by the Rocky Mountains and on the south by 
the divide of the North Saskatchewan. The soil is deep and well suited to 
the growth of crops wherever the climate permits. The elevation of the valley 
is very slight. Lake Athabasca is 690 feet (210 m) and Great Slave Lake in 
391 feet (119 m) above sea level. 
The Mackenzie river basin forms the northern portion of the great 
interior plain continuous with that of the United States. In the Dominion of 
Canada, it comprises the provinces of Mackenzie, Athabasca, Saskatchewan, 
Assiniboia, Manitoba and Alberta. The chief characteristic of the area is that 
of an immense ocean-like plain, treeless at the south, but gradually becoming 
forest-clad at the north. This plain slopes with a very gentle descent to the 
north. The initial elevation is slight and the distance to the oceans, whether 
of the north, or east, is great, hence the rivers flow over an almost level 
peneplain with a tranquil current and the low divides between adjacent water“ 
ways is a feature of the country. Two lines of elevation divide the plain into 
three parts called by physiographers the first, second and third prairie 
steppes, ascending from Lake Winnipeg to the Rocky Mountains. The region 
of the first steppe in quaternary time, geologists show, was covered by an im- 
mense glacial lake designated “the glacial Lake Agassiz”. It extended far 
into Minnesota and included all the basin of the Red River of the North and 
the basins of lakes Winnipeg, Manitoba, Winnipegoosis, Dauphin and Lake of 
the Woods. The outlet toward the north was blocked-by an ice cap and 
all the water drained southward into the Mississippi River. As the ice melted, 
outlets were opened to the north and the lake level was lowered. The water 
. withdrew into the lowest depressions and the existing lakes assumed their 
present proportions. 
The hard crystalline Laurentian rocks extend to the eastern shore of Lake 
Winnipeg. These are bordered on the west by a belt of Silurian and Devonian 
