270 ALCOCK AND BRUCE PRECAMBRIAN ROCKS OF MANITOBA 



areal extent. In parts of the country as much as one-fifth of the surface 

 is water. This youthful drainage in a country with such low relief is a 

 direct result of disorganization by Pleistocene glaciation. 



The present topography is largely an inheritance from pre-Ordovician 

 time and is a result of the stripping off of the Paleozoic sediments which 

 once covered the region. That the topography is really that of the pre- 

 Paleozoic surface is shown by the profile at the base of the Ordovician. 

 Both areas of Paleozoic rocks of the province offer an opportunity for the 

 study of the surface on which the advancing Ordovician sea deposited its 

 sediments. It was a surface of low relief, with minor irregularities much 

 like the present pre-Cambrian surface. It was also a surface nearly free 

 from debris, for in most places the basal Ordovician horizon is dolomite 

 resting on fresh pre-Cambrian rocks. Local clastic deposits are found at 

 the base of the Ordovician on the Lower Churchill Eiver,^ in the vicinity 

 of the Cranberry lakes,^ where a sandstone occurs, and at some other 

 places ; but these clastic beds are exceptional. 



General Character op the pre-Cambrian Rocks 



Granite and granite-gneiss form approximately 98 per cent of the rocks 

 of the pre-Cambriau belt of Manitoba. Here and there, however, are 

 areas of volcanic and sedimentary rocks which afford the only means of 

 deciphering the pre-Cambrian history of the province. These areas are 

 mere remnants which escaped the erosion of pre-Ordovician time. Some 

 of them are synclinal, caught between rising granite masses, and their 

 limited area shows the great extent of the erosion which stripped and 

 incised the batholiths. These rocks occur in ten main areas. The follow- 

 ing table shows the succession of the formations in each area, but it is 

 not meant to suggest that there is any definite correlation of types from 

 area to area. In neighboring districts similar formations may be ap- 

 proximately of the same age, but where areas are separated by long dis- 

 tances definite correlation seems now to be and may always be impossible. 



Description of Areas 

 athapapuskow lake area* 



The Athapapuskow Lake area lies 50 miles north of the Saskatchewan 

 Eiver and is part of a larger area extending westward beyond Amisk 

 Lake, in the Province of Saskatchewan. The following is the pre-Cam- 

 brian succession : 



" F. J. Alcock : Oeol. Surv. Can., Sum. Rept., 1913, p. 135. 



3 J. B. Tyrrell : Oeol. Surv. Can., Ann. Kept., vol. xili, part F, p. 39. 



* E. L. Bruce : Amisk-Athapapuskow map area. Geol. Surv. Can., Memoir 105. 



