3: Geologic Structure and Landforms
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Geologic interpretation of images begins with the identification of lineaments, “regional linear features caused by linear arrangement of regional morphological features such as streams, escarpments, and mountain ranges and tonal features that in many areas are the surface expressions of fractures or fault zones.”[i] The emphasis in this section is on geologic structures and landforms, both of which can be easily identified on air photographs and other images.
The sequence followed here is that often found in geomorphology books. Geologic structures and structurally controlled landforms are illustrated first, followed by images of mass wasting and of the results of the agents of erosion—running water, ground water, ice, wind and the sea. The meandering rivers of southern Manitoba are excellent examples of that river form. Several of them have deposited deltas into lakes. The effects of ground water are less easily illustrated, but examples of spring sapping and artesian erosion exist in the south. As all of Manitoba was covered by ice during the latest glaciation, examples of glacial erosion, and especially of glacial deposition, are widespread. Equally impressive are the suite of landforms created by the large glacial lakes that appeared as the ice melted. Glacial spillways, glacial lake deltas, strandlines and flat lake floors are found in many places. Wind action has created dunes on some of the deltas and on glacial outwash deposits. The Hudson Bay coast is rising as a result of isostatic rebound, recovery from the weight of the ice. One result is the existence of strandlines many metres above the level at which they were created. Finally Manitoba’s great lakes are large enough to illustrate many of the landforms normally associated with sea coasts.
[i] Lillesand, T. M. and Kiefer, R. W. Remote Sensing and Image Interpretation (Third Edition). New York: Wiley, 1994, 179.
3.12: Landforms of Southwestern Manitoba
At first glance three dark-toned areas stand out: in the south, straddling the Canada/U.S.A. border, is Turtle Mountain 1; in the northeast is the Assiniboine Delta 2; and in the west is another area underlain by glaciolacustrine deposits with Oak lake 3 at the centre.
Turtle Mountain is dark-toned because it is mainly covered by woodland that contrasts with the light tones of the surrounding agricultural land. The international border across Turtle Mountain 4 stands out because more woodland has been cleared south of the border. The sharp northern edge of the woodland 5 is the boundary of Turtle Mountain Provincial Park in which little clearing has occurred.
The Assiniboine Delta, deposited into glacial Lake Agassiz, is not so uniformly dark-toned because the land cover varies. In the north and west rectangular agricultural fields can be seen 6. The darkest-toned areas are tree covered with aspen (Populus tremuloides), white spruce (Picea glauca), black spruce (Picea mariana), and tamarack (Larix laricina). One such area at the northern edge of the delta 7 follows a former course of the Assiniboine, a route that is now occupied by Epinette Creek. A very light-toned patch at the east side of the delta is the Bald Head Hills 8, an area of active sand dunes. The meandering Assiniboine 9 follows the southern edge of the delta.
The third dark-toned area is also a result of deposition into a glacial lake, but not in deltaic form. Again the land cover is varied, resulting in tonal variations: very dark-toned areas are woodland 10, less dark areas are grass covered 11, and some patterned cropped areas can also be seen 12.
The main upstanding areas are Turtle Mountain, a drift-covered Tertiary outlier which rises 700 feet (213 m) above the surrounding land, and two areas of end moraine, the Brandon Hills 13 and the Tiger Hills 14, both of which are part of the Darlingford moraine that extends across much of southwest and south central Manitoba. The dark-toned Brandon Hills are still largely wooded but more land has been cleared for agriculture in the Tiger Hills.
Two major river valleys of the Assiniboine 15 and the Souris 16 are the main areas of negative relief. West of Brandon, the Assiniboine flows in a glacial spillway 17 that carried water from glacial lakes in Saskatchewan to glacial Lake Agassiz, depositing the Assiniboine Delta in the process. The spillway is steep-sided with a flat floor approximately one kilometre across. It is best seen where it runs east/west 18 with a dark shadow on the south side. Also contributing to the dark tone is the fact that the south side, which enjoys a wetter microclimate, is wooded. The north side, facing into the sun, is drier and consequently supports less woodland.
Two former courses of the Assiniboine can be seen, one north and the other south of the present channel. To the north is a moderately dark-toned area 19. Probably the Assiniboine split and flowed on both sides of this almond-shaped area. To the south is a dark-toned area of lakes and marshes 20 now 200 feet (61 m) above the level of the Assiniboine. This route may have been followed by a proto-Assiniboine, flowing along the edge of the last ice sheet that covered the region. Below Brandon the valley loses its spillway form as it starts to flow across the Assiniboine delta.
The Little Saskatchewan River 21 joins the Assiniboine just above Brandon. Its valley is not a spillway, but it is much larger than the present flow of the river warrants, probably due to a large volume of water flowing from the Riding Mountain Uplands (off the image to the north) as the glaciers melted.
The Souris River enters southwest Manitoba from North Dakota. At the border it occupies a shallow spillway 22 that braids into several channels to the north 23. Farther northeast it opens out, and at Souris 24 it turns southeast and once again occupies a spillway-shaped valley 25. This spillway continues southeast and is occupied by a sequence of lakes: Bone Lake 26, Pelican Lake 27, Lorne Lake 28, Louise Lake 29, and Rock Lake 30. The Souris used to follow this route along the southern edge of the Tiger Hills moraine, but it now makes a 90-degree bend to cut through the Tiger Hills in a gorge 31 to join the Assiniboine below Wawanesa. This bend has been interpreted as an elbow of capture.
Southeast of the present Souris River is a valley only partly occupied by water—the Dand Spillway 32—that probably originated as an ice marginal channel.[i]
In addition to the lakes already named, Whitewater Lake 33 is located at the centre of a basin of inland drainage north of Turtle Mountain. The lake is intermittent; in wet years—which 1973 must have been—it is water filled, but in dry years such as the early 1980s it is almost completely dry. It is light-toned compared to Pelican Lake because of sediment stirred up in the shallow water. Numerous small lakes, very dark-toned, occupy hollows in the ground moraine on Turtle Mountain 34.
Much of the area exhibits the typical checkerboard pattern of the DLS system. The system dictates the road pattern with roads mainly following section lines running east/west or north/south. Roads show up most clearly where they cross wooded land, for example, PTH 10 across Turtle Mountain 35, PTH 5 across Spruce Woods 36, and Trans Canada Highway in the northeast corner of the image 37.
Brandon, the only city on the image; shows up as a light-toned patch 38, but the other small towns and villages in the region are not visible because the land cover is similar to that in the surrounding countryside. The airfield at Rivers 39 is very prominent but surprisingly Brandon airfield north of the city is not.
Reservoirs behind dams are obvious, particularly Lake Wahtopanah 40 northeast of Rivers and Upper Des Lacs Lake on the Souris River just south of the international border 41.
Figure 3.12: Landforms of Southwestern Manitoba
Skylab image: September 19, 1973
Scale: 1: 670,000 (approx.)
This is a photograph obtained from the Skylab spacecraft, one of the first “manned” satellites from which earth images were obtained.
[i] Brown, W.J. “Geography field excursion to southwest Manitoba.” in Field Guide for the Canadian Association of Geographers Annual Meeting. Winnipeg: University of Manitoba, Geography Department, June 1970, 43-71.