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
3.24: Meanders and Cutoffs on the Assiniboine River South of St. Lazare
The Assiniboine River occupies a glacial spillway, the floor of which ranges in width from about 1300 yards (1189 m) in the centre of the photo to 1700 yards (1554 m) in the north and south. Conditions are ideal for meander development; the river has a low gradient and the channel is cut into unconsolidated, but cohesive, surficial materials. The cohesiveness allows river cliffs to develop where the river erodes the outside of meander bends.
The modern river channel tends towards the western side of the spillway with clearly defined cutoffs east of the main channel at four locations 1. Two more cutoffs are imminent 2. In the south, a stretch of river rather than an individual loop has been abandoned 3. The main channel of the river 4 is light-toned because of reflection from suspended sediment in the water, whereas the cutoffs are dark-toned because there is no water movement to suspend sediments. Classic meander scrolls 5 indicate former positions of the riverbank. Trees (often willows) occupy the ridges of the scrolls whereas the intervening swales are marshy.
The spillway is incised into flat land to west and east 6 (Wattsview Plains). The western valley side is the more clearly defined of the two. Most of it is wooded, but some bare land exists where spring-fed gullies occur 7. The eastern side is less uniformly wooded; some parts are grass covered 8 because of the dry microclimate on the west and southwest facing slopes. A series of minor gullies exist 9, and in the north is the wooded valley of Snake Creek 10. Several small lakes can be seen 11.
Most of the eastern plain is used for agriculture, and, as is usual on hummocky ground, rain splash has resulted in light-toned areas at the tops of mounds 12. In contrast the plain west of the spillway is wooded or grass covered 13. Sandy outwash deposits underlie most of the area, and some of the sand has been blown into linear dunes aligned roughly northwest/southeast 14. Some ridges are devoid of vegetation where wind erosion has exposed the underlying sands 15.
Section lines of the DLS system can be seen in the east with a few gravel roads 16. These roads are straight, but within the spillway road direction is influenced by the topography 17. The CN line connecting St. Lazare (off the photo to the north) with Birtle (off the photo to the south) runs just above the level of the floodplain 18; slumping has disrupted the line and partially in filled a cutoff 19. The
Figure 3.24: Meanders and Cutoffs on the Assiniboine River South of St. Lazare
Air photograph: A15528-148
Flight height: 20,000 feet a.s.l.; lens focal length: 152.26 mm
Date: May 27, 1956
Scale: 1:36,200 (approx.)
Location: Township 16, Ranges 27 and 28 WI
Map sheets: 1:250,000 62K Riding Mountain
1:50,000 62K/6 Birtle