TheLaurentian Upland (orLaurentian Highlands) is aphysiographic region which, when referred to as the "Laurentian Region" or theGrenville geological province, is recognized byNatural Resources Canada as one of five provinces of the largerCanadian Shield physiographic division.[1] TheUnited States Geological Survey recognizes the Laurentian Upland as the larger general upland area of the Canadian Shield.[2][3][4]
The Laurentian Region, as recognized by Natural Resources Canada, is part of theplateau and dissected southern rim of the Canadian Shield in the province ofQuébec. It is a western extension of theLaurentian Mountains, and continues across theOttawa Valley into Ontario as theOpeongo Hills. Viewed from thevalleys of theOttawa andSt. Lawrence Rivers, the south-facing escarpments of the Shield give the appearance of mountains 500–800 meters high; looking across the plateau, the relief is more moderate and subdued. These scarps mark the dramatic southern edge of this Upland region, of whichMont Raoul Blanchard is the highest peak.Although the other limits are less well defined, this Laurentian Region in Quebec may be considered to extend 100–200 km northward from thescarps and to stretch from theGatineau River in the west (mean elevation 400 m) some 550 km to theSaguenay River in the northeast. Here it attains its maximum elevation north ofQuebec City in theRéserve faunique des Laurentides (over 1000 m). Individual summits rise above the plateau surface:Mont Sir Wilfrid (783 m) and Mont Tremblant in the west,Mont Sainte-Anne (815 m) at Quebec,Mont Raoul Blanchard (1166 m),Mont Bleu (1052 m) andMont des Conscrits (1006 m) in Réserve faunique des Laurentides.Cap Tourmente (579 m) andMont des Éboulements (770 m) are dramatic examples of the scarp face as it drops precipitously to the St Lawrence River.
The more general Laurentian Upland Province may be considered to extend over a larger area of the Canadian Shield, into NorthwesternOntario and parts of NorthernMinnesota,Wisconsin,Michigan, andNew York State, and is recognized by theUnited States Geological Survey to include theSuperior Upland.[5] As a southern extension of the Canadian Shield, theAdirondack Mountains of New York State might also be considered an extension of the Laurentian Upland.[6]
The Laurentian Upland is primarily made up of ancientPrecambrianigneous,metamorphic, andsedimentary rock.[7] With the exception of the river valleys andlacustrine basins, it is a rolling to mountainouspeneplain that ranges from 800 to 1400 feet above sea level.[8]
TheSuperior Upland is theprovince of the Laurentian Upland which projects into the United States west and south ofLake Superior.[9] This upland, part of theCanadian Shield along with theAdirondacks, is a greatly deformed structure and is composed primarily ofigneous andmetamorphic crystalline rocks commonly associated with a rugged landscape. At some prehistoric period, this had a strong relief, but today the upland as a whole is gently rolling with the inter-streams surfaces being plateau-like in their evenness. Here they have elevations of 1,400 to 2,300 feet (700 m) in their higher areas, such as theMisquah Hills andHuron Mountains.[10] In this province, we find a part of those ancient mountains regions that were initiated by crustal deformation and then reduced by a long continued erosion to apeneplain of modern relief. A peneplain with the occasional moderately highmonadnocks left behind during the peneplanation of the rest of the surface. The erosion of the region must have been far advanced in prehistoric times, even practically completed, because the even peneplain surface is overlapped by fossiliferous marine strata from an early geological date,Cambrian. This shows that the depression of the region beneath an ancient sea took place after a long existence as dry land.[11]
The extent of the submergence and the area over which thePalaeozoic strata were deposited are unknown. Because of the renewed elevation without deformation, erosion in later periods has stripped off an undetermined amount of the covering strata. The valleys by which the uplands are here and there trenched to moderate depth appear to be, in part at least, the work of streams that have been superposed upon the peneplain through the now removed cover of stratified rocks.
Glaciation has strongly scoured away the deeply weathered soils that presumably existed here in preglacial time. It left behind firm and rugged ledges in the low hills and swells of the ground and spread an irregular drift cover over the lower parts, whereby the drainage is generally disordered being deposited in lakes and swamps and elsewhere rushing down rocky rapids.
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