Chapter 3: Geologic Structure and Landforms
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
3.29: Meanders on the Assiniboine River North of Glenboro
East of Brandon the
The photo shows that the river erodes into the concave side of meanders producing low, steep river cliffs 1 and leaves deposits on the convex side to produce sandy point bans 2. As the river migrates laterally, it leaves behind meander scrolls indicating its former positions 3. Point bans are at first bare but are later covered by grass 4 and then trees 5. The scrolls are visible in fields devoted to agriculture 6 and where trees have been left on ridges 7.
In the east sand dunes can be seen 8. These are mainly stabilized by grasses and trees but some bare sand patches remain 9.
On the outside of one large bend, sapping caused by springs at the junction between the surface sands and the underlying clay has resulted in a series of amphitheatre-like embankments 10, and just to the north where the river is eroding into dunes, a high cliff has been produced 11.
The trees in this area are mainly deciduous, and in this fall photograph some have changed colour producing a light tone 12, while others, as indicated by their shadows, have completely lost their leaves 13. In one area coniferous trees, probably white spruce (Picea glauca),[i] predominate 14. There are also isolated patches of, and individual, coniferous trees scattered throughout the area. These are identified from their distinctive triangular-shaped shadows 15.
Some of the land is devoted to arable agriculture; three farms can be identified 16. Within the northeastern meander loop an intricate pattern 17 is caused by trails along which straw bales have been assembled in stacks 18. A north/south gravel road 19 along one of the section lines skirts around a meander in the north and leads to a now abandoned ferry 20.
Figure 3.29: Meanders on the Assiniboine River North of Glenboro
Vertical air photograph: A16405-13
Flight height: 10,500 feet a.s.l.; lens focal length: 152.63 mm
Date: October 19, 1958
Scale: 1:18:100 (approx.)
Location: Township 8, Ranges 13 and 14 WI
Map sheets: 1:250,000 62G Brandon
1:50,000 62G/11 Glenboro
[i] Kerr, G. D., Rounds, R. C., and Welsted J. “Use of panchromatic and colour infrared air photographs to produce a vegetation map for Canadian Forces Base Shilo,