4. 



PRECAM BRIAN 

 TECTONIC PROVINCES 



DISTRIBUTION OF PRECAMBRIAN ROCKS 



The continent of North America is made up in a broad way of a stable 

 interior and surrounding belts of deformed, intruded, and metamorphosed 

 rocks. The stable interior has been free of orogeny since a time in the 

 late Precambrian, or approximately for the last billion years. Before that 

 time, however, a number of intense and widespread orogenies occurred. 



The Canadian Shield is the greatest expanse of Precambrian rock 

 exposures. These same rocks are blanketed by Paleozoic, Mesozoic, and 

 Cenozoic strata over most of western Canada and the United States; only 



in areas of local uplift or doming have the old rocks been exposed. 

 The Crystalline Piedmont of the Atlantic margin of the continent con- 

 tains much rock of Precambrian age, and the western Cordillera exposes 

 the ancient rocks of several ages and complex relations in a number 

 of places. 



CANADIAN SHIELD 



Physiography 



The Canadian Shield is characterized by a vast expanse of Precambrian 

 rock. Its upland surfaces are uniform in height over large areas and, 

 although now dissected, represent an old age erosion surface as large 

 as. any in existence today. The extensive surface rises 1000 to 2000 feet 

 above sea level north of the St. Lawrence River and Lake Superior. 

 Around Hudson Bay, especially on the south and west, is a wide lowland 

 that ranges from sea level to 500 feet in elevation. In northern Labrador 

 along the coast just southeast of Ungava Bay, the surface rises to 5000 

 feet and is extensively dissected. Hudson Bay is a great modern epeiric 

 sea; it is a marine invasion from the north due to gentle subsidence in j 

 the heart of the shield in pre-Pleistocene or early Pleistocene time. The I 

 ice caps imposed such a weight on the shield in and around Hudson j 

 Bay that the area sank over a thousand feet in addition to the previous 

 subsidence, and then with the melting of the ice it has risen about 900 

 feet. 



Post-Proterozoic History 



Paleozoic strata lap upon the shield from the Canadian plains on the 

 west, and from the southwest in Saskatchewan and Manitoba. In 

 northern Minnesota the Precambrian rocks lie exposed and extend south- 

 ward into Wisconsin and eastward into northern Michigan. Paleozoic 

 rocks continue to overlap the Precambrian across southern Ontario and 

 Quebec to the Frontenac axis, where the Precambrian extends southeast- 

 ward and forms the Adirondack dome in New York. See the Geologic 

 Map of North America. For the most part, the Paleozoic rocks that skirt 



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