Table of Contents
2: Location, Borders, and Lakes
3: Geologic Structure and Landforms
6: Pre-historic and Early Historic Settlements
7: Survey Systems
8: Southern Hamlets, Villages, and Towns
9: Mennonite and Hutterite Settlements
10: First Nations Settlements
11: Northern Settlements
12: The Southern Cities
13: Mining and Oil Extraction
15: Industry / Manufacturing
16: Water Resources
17: Parks, Recreation, Sports
18: Transport and Communications: Past and Present
19: Legal Issues and Law Enforcement
3: Geologic Structure and Landforms
Click for chapter introduction
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.
3.26: Glacial Spillway and the Meandering Assiniboine River North of Virden
The Assiniboine River occupies a steep-sided, flat-floored glacial spillway about one to one and a half miles (1.6 to 2.4 km) across at its base. Cutoffs can be seen at several locations 1, and a segment of the river has also been abandoned 2. Another small cutoff is imminent 3. The two most clearly defined cutoffs 4 were in existence when the original DLS maps were made in 1873. Neither now receives water from the main river channel, but in both cases when they contain a lot of water it flows out from them to the river. In both cases enough time has elapsed since abandonment for trees to grow across the meander neck 5.
Both sides of the spillway are wooded, and steep-sided mainly wooded gullies dissect both the west bank 6 and east bank 7. Woodland (dark-toned) can also be seen skirting the river channel 8, but further back on the floodplain land has been cleared for agriculture 9.
On either side of the spillway land is flat. On the west side there are extensive areas of scrubland with tree clumps 10 developed on sandy outwash deposits. Further west and also east of the spillway, land is devoted to agriculture 11. Light-toned hummocks can be seen in many of the fields 12.
The section lines of the DLS system are obvious with north/south 13 and east/west 14 gravel roads along some of them, although in some locations the road direction is influenced by the topography 15. One road runs obliquely down the valley side and then along the spillway just above the floodplain 16. The CP line, totally unrelated to the survey system, can be seen at the western edge of the photo 17.
Figure 3.26: Glacial Spillway and the Meandering Assiniboine River North of Virden
Vertical air photograph: A24966-242
Flight height: 15,433 feet a.s.l.; lens focal length: 85.32 mm
Date: June 18, 1978
Scale: 1:52,800 (approx.)
Location: Township 12, Range 25 WI
Map sheets: 1:250,000 62F Virden
1:50,000 62F/15 Virden