14 



Near Darwin the Pre-Cambrian shield disappears at a 

 very low angle under the alluvial plain representing the 

 bed of glacial Lake Agassiz. Between this station and 

 Winnipeg the old lake bed gradually changes from a slightly 

 rolling, heavily forested country to level, treeless prairie. 

 The Ordovician limestone of the interior plains regions, 

 which laps over the Pre-Cambrian shield from the west, 

 is hidden beneath the Lake Agassiz silts and clays except 

 near Tyndall and Garson where quarries may be seen at 

 son"ke distance from the railway. 



Winnipeg is the gateway to the Great Plains of Western 

 Canada. It is situated in the basin of glacial Lake 

 Agassiz, an extinct lake which drained southward to the 

 Mississippi, and deposited a thick sediment of silt and 

 clay on a bed rock of Paleozoic limestone. The bed of 

 Lake Agassiz has an elevation of 800 feet (243 m.) above 

 sea level and forms the first prairie level of the Great 

 Plains. 



The western border of Lake Agassiz is the Manitoba 

 escarpment which crosses the Canadian Pacific railway 

 about Austin, where the surface of the plain rises to the 

 second prairie level. This line of escarpment is coincident 

 with the eastern edge of a wide band of Cretaceous which 

 rests on the Paleozoic rocks and extends westward to the 

 Rocky mountains. 



The second prairie level has an average elevation of 

 about 1600 feet above the sea and continues westward 

 on the line of railway for about 280 miles (450 km.) to a 

 point a short distance beyond Moosejaw. It is underlain 

 by ffat-lying Cretaceous rocks which, however, are so 

 uniformly covered with a thick soil that outcrops of them 

 are rarely exposed except in the river valleys. 



The Missouri Coteau a few miles west of Moosejaw 

 rises somewhat abruptly for about 500 feet (152 m.) from 

 the second prairie level and forms the eastern boundary to 

 the third prairie level which then stretches without any 

 notable breaks to the foot of the Rocky mountains. To the 

 south of the railway line between Moosejaw and Dunmore 

 irregular flat-topped hills rise 1000 feet (304 m.) or more 

 above the general level of the plain as remnants of a once 

 higher plain since largely destroyed by erosion. These hills 

 are built of undisturbed shales, sandstones and conglomer- 

 ates of Oligocene age deposited after the period of crustal 

 disturbance in which the Rocky mountains were elevated. 



