GEOLOGICAL SURVEY OF CANADA DEPARTMENT OF ENERGY. MINES AND RESOURCES. OTTAWA Ch. 11 from Geolom and Economic Minerals of Canada, Economic Geology Report No. 1, fifth edition, published by Department of Energy, Mines and Resources, Ottawa. Canada 1970 Orders for twenty or more copies of this reprint for educational purposes will be accepted by the Geological Survey of Canada, 601 Booth Street, Ottawa, KIA OE8, Canada, price 50d per copy. A coloured map (Map 1254A. scale 1:5,000.000) showing the physiographic divisions of Canada is also obtainable from the Geological Survey of Canada, price $2.00. I INTRODUCTION smaller units. In the Borderlands physiographic boundaries I are to a great extent indicated by distinctive changes in Physiographically and geologically Canada is com- the topography and geology. For the Shield no such single posed of two great park a core of old, massive, Precam- controlling factor is evident, although the ages of orogeny brian crystalline rocks forming the Shield, and a sur- form a basis for geological subdivision (Stockwell, 1965). rounding crescent of younger, mainly stratified, rocks Despite this, there are features that enable a physiographic forming the Borderlands. In simple terms, the surface division of the Shield into major and minor units to be of the Precambrian core resembles that of an inverted --A- dinner plate with its flat, slightly depressed centre and an outward shelving rim terminated by a steep edge. The younger rocks of the Borderlands surround the northern, THE SHIELD western, and southern parts of the plate as segments of two concentric rings. The inner ring comprises a chain General Features of lowlands, plains, and plateaux of generally flat lying Viewed from some prominent summit the landscape sedimentary rocks overlying the gently shelving rim; the presents an even, monotonous skyline interrupted by outer ring is formed of discontinuous areas of mountains rounded or flat-topped monadnocks and ranges of hills. and plateaux in which the younger rocks are deformed. The smooth horizon is evidence of an old, almost all- On the northeast rim of the plate the Borderlands are subduing erosion surface; this is the most outstanding absent; there the edge of the Shield is tilted upward and characteristic of the Shield and gives a similarity to the drops abruptly into the sea. terrain whether it is seen in Labrador or the Northwest Part of the boundary between Canada and adjoining Territories. Only in scattered areas such as the Davis territories of the United States is natural, marked by lakes, and Lab dor Highlands, around Wager Bay, and Mealy rivers, or divides; much is, however, drawn at parallels Moun ti? ins is the relief mountainous, but even in these or meridians. Canada has immensely long and irregular places remnants of an old erosion surface form the coastlines that embrace great bays, inlets, and channels summits. Over the greater part of the Shield local relief within the general confines of her headlands and islands. is 200 to 300 feet. The greatest relief of more than 5,000 Parts of these coastal waters are shallow and cover the feet is found in the Davis and Labrador Highlands. continental shelves along the Atlantic, Pacific, and Arctic Many long periods of erosion have had a great coasts. influence in levelling the broad expanses of the Shield Physiographically the Shield and Borderlands are and reducing them to a nearly featureless surface even divided into regions or provinces each comprising many though parts are geologically distinct. Such features as 10 THE SHIELD II. Physiographic Subdivisions of Canada Introduction ----------.---_-_-------------------------------------10 The Shield .. 10 The Borderlands 17 Selected References --------------------------.------------.30 I I Physiographic Subdivisions of Canada H.S. Bostock Raised beaches on Hudson Bay Lowland. the East Arm Hills are no higher than the crystalline tortuous courses. In contrast to this heterogeneous uplands on either side of them. The eastern edge of drainage, where drift deposits are scarce the streams show Kaniapiskau Plateau is a marked scarp that overlooks a trellis drainage pattern, developed to various degrees, the sinuous ridges of the Labrador Hills, although some in which the main lines follow long straight lineaments, of their summits stand as high as the plateau surface. major fracture zones, or the dominant joint systems in the massive rocks. Differences in the resistance to erosion Drainage of the strata or gneisses may also impress a drainage The surface of the Shield varies in detail, but as a pattern guided by the sinuous twists of the folds with result of glaciation a great proportion of it is taken up by S-shaped lakes or concentric or radiating valleys. In other water as lakes, ponds, and swamps. Although the per- localities depositional features of glaciation may have been centage of area covered by water has not been accurately the overruling influence, imposing a strong parallelism measured, it is known to exceed 25 per cent in many upon the drainage in drumlin fields, the swells and troughs large regions; in some parts, such as the Thelon Plain or of drumlinoid features, or the irregularly parallel, con- areas of Pleistocene lakes and marine submergence, the gruent system of ridges and hollows of ribbed moraines. percentage is relatively small. Indeed, in no other region of the world has glaciation The drainage pattern varies widely and as yet no more clearly placed its stamp upon the drainage pattern. preglacial system has been deciphered even for the main A comparison of the drainage patterns on each side streams. With a few exceptions such as Leaf River, the of the boundary between the Shield and the Borderlands main streams flow in the direction of general slope of the commonly reveals decided differences despite the over-all land surface. The details of the tributary drainage, how- masking effect of glaciation. On the Shield the drainage ever, are disrupted by the local effects of glaciation generally shows a multitude of lakes and streams, their including the tilting of the land surface, the grouping of pattern controlled by the Precambrian bedrock; whereas the bedrock, and the deposition of surficial materials. on the adjacent Borderlands lakes are less numerous and Stream channels commonly follow elements of bedrock often display bulbous outlines, and the Paleozoic bedrock structure such as persistent fracture zones, major joint imposes different stream patterns. patterns, or the traces of soft strata; in places they are diverted across former divides dammed by glacial debris. Old Peneplains In many areas glacial movement transverse to bed- The general surface of the Precambrian reeks of the rock topography and scattered deposition of glacial drift Shield dips at low angles under the bordering Paleozoic have formed a mosaic of ponds interspersed with small strata of the Arctic and Interior Plains, the southwest part ridges and hummocks amongst which streams flow along of the St. Lawrence region, and particularly around CH. II/PHYSIOGRAPHIC SUBDIVISIONS OF CANADA 11 Hudson Bay Lowland and Foxe Plain. In some areas below the surrounding Precambrian upland surface with6ut drilling has shown that the surface is undulating but in evidence of tectonic movement to bring them to their general it shelves gradually to great depth beneath the present position. From the distribution of the Paleozoic rocks of the Borderlands. Where the character of the outlied appears that the Shield was peneplaned and Precambrian surface is well exhibited along the border the valley system cut in the bedrock before the Paleozoic. its hills and valleys continue beneath the overlying It seems that the Precambrian surface has been little Paleozoic strata. The strata fill hollows on the Precam- changed since the removal of the Paleozoic cover, and brian surface, encroach on the ridges, partly mantle them, that glaciation has only superficially modified its character and leave only the summits projecting as inliers along (Ambrose, 1964). the continuations of the ridges. The present surface of Although the surface of the Shield seen today is the Shield, at least in many areas near its borders, appears mainly an exhumed surface once buried under Paleozoic to be an exhumed pre-Paleozoic erosion surface. strata many areas of Proterozoic sedimentary rocks show Outliers of Paleozoic strata are widely scattered in the similar relationships to the surface of the crystalline Shield (some lying far within its borders) generally in Archean rocks beneath them (Ambrose, 1964). The depressions or grabens. Some are in valleys such as that @ surface of the Shield has had an ancient and complex Lake Timiskarning (Hume, 1925) where the Paleozoic strata history indeed. First there was the development and were deposited on a relief of more than 200 feet, or in deep partial dissection of a peneplain on the Archean rocks basins such as that of Manicouagan Lake (Rose, 1955) in preproterozoic time, then the burial and exhumation where several outliers are preserved at 1,200 feet or more of this surface once or perhaps twice during the 12 THE SHIELD J Proterozoic, followed by the exhuming of these several coastal plain of little relief mantled largely by reworked surfaces from beneath the cover of Paleozoic strata, and drift deposits that obscure nearly all the bedrock. finally the scouring of the surface during the surges of Athabasca Plain forms a large embayment into the Pleistocene glaciation. western part of Kazan Upland. The plain consists of nearly flat lying sandstones and has a rolling hummocky, Subdivisions of the Shield lake-spotted, forested surface that rises from 900 feet Despite the general uniformity of its terrain, geologi- elevation on the west to 2,000 feet on the east and has a cal structure and evidence of orogenies show that the local relief of 200 to 300 feet. Shield is composed of different parts that were once Thelon Plain lies in the central part of the region probably as distinct physiographically as the Cordilleran and includes areas of nearly flat lying sandstones and Region and Interior Plains are today.