50 THE ROYAL SOCIETY OF CANADA 



Notes on the Physiography of the Region 



The St. Lawrence lowlands occupy the area between the Lau- 

 rentian highlands to the north and the Appalachian Mountains to 

 the south and east. They extend from the Gulf of St. Lawrence to 

 Lake Huron and are divided roughly into two parts by a spur of the 

 Laurentians which extends across the St. Lawrence at the Thousand 

 Islands to the Adirondack Mountains in New York State. East of 

 this spur is a flat ridge which extends from the St. Lawrence River 

 at Coteau, to the Ottawa River at Hudson, while on the north shore 

 of the Ottawa is a group of hills known as the Oka Mountains or the 

 Two Mountains. A flat plain covered by the sands and clays deposited 

 by the Champlain Sea lies to the east of these elevations and extends 

 eastward to the foot of the Appalachian Range. A study of the rocks 

 underlying this plain shows that it is traversed by an extensive fault, 

 known as the Champlain Fault, which passes through the Lake 

 Champlain valley, and to the west of Yamaska, from which point it 

 turns more to the north-east and passes down the valley of the St. 

 Lawrence. West of this fault, the area is underlain by flat-lying or 

 gently dipping sediments, while to the east the sedimentary rocks 

 dip very steeply. The whole plain has been eroded uniformly to the 

 present level regardless of structure and through the plain thus de- 

 veloped the Monteregian hills rise abruptly as a series of prominent 

 hills. These intrusive bodies to-day form practically the only 

 elevations above the plain, and we have a series of bold precipitous 

 hills rising to heights of from 600 to 1,500 feet above the plain or 

 from 700 to 1,500 feet above sea level, which give the impression of 

 even greater height. 



These hills are more precipitous on their northern slopes than 

 to the south and usually the softer Palaeozoic sediments extend for 

 a considerable distance up the southern side as they were protected 

 from the scouring action of the glaciers by the harder rocks immedi- 

 ately to the north. In the vicinity of Montreal and to the west of the 

 city, small mounds and escarpments composed of igneous rocks 

 allied to the Monteregians are of common occurrence. These owe 

 their prominence above the plain to differential erosion. Usually 

 these mounds mark the only outcrops visible in wide areas, excepting 

 where streams have cut their way through the Quaternary sediments 

 and drift, and have laid bare the Palaeozoic rocks beneath. 



Geneïial Geology 

 In the area with which this paper is concerned, the following 

 succession is noted: 



