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
3.49: Corrugated Ground Moraine South of McAuley
The area shown is located about six miles (9.7 km) south of McAuley near the Manitoba/Saskatchewan border. It exhibits a more definite patterned topography than that shown in figures 3.47 and 3.48. Numerous sloughs emphasize the corrugated appearance of the ground. The sloughs are mainly dark-toned 1 with somewhat lighter tones where emergent vegetation is growing along the edges 2. In the south sloughs are light-toned 3 because the relationship between the angle of the sun, the water surface, and the camera produces the specular reflection effect. Sloughs exist in hollows with low ridges between; the amplitude of relief is only a few meters. “The pattern of ridges is convex to the southeast, indicating that the lobe of ice flowed from the northwest. Intermittent creeks follow the hollows between ridges , further emphasizing the transverse orientation of the moraine. Several smaller longitudinal ridges (northwest-southeast) parallel the direction of ice flow and cut across corrugations, creating a reticulate pattern…The corrugations probably formed by pushing and thrusting of material into subglacial zones of weakness near the glacier margin, where the ice was thin and heavily crevassed. The longitudinal ridges are probably the result of the deposition of till in zones of weakness parallel to ice flow, similar to the radial splay crevasses that occur near the front of an ice lobe. Both sets of ridges were preserved by stagnation and melting of the glacier not long after they were formed.”[i] In the west a series of light-toned patches indicate the crest of a discontinuous esker 5.
Much of the land is used for agriculture, and as was the practice at the time, numerous fields are left fallow 6, and in these light-toned bald crests are obvious. Grains probably occupy the light-toned cropped areas. 7 The area lies in townships 13 and 14, ranges 29 and 30 WI. The Manitoba/Saskatchewan border runs along the western edge of range 29 WI, but there is no evidence its existence on the photo. PTH 41 8 runs north/south across the area, three miles east of the provincial border. Section lines can be clearly seen with gravel roads along some of them 9, and quarter sections are picked out by land use differences 10.
Figure 3.49: Corrugated Ground Moraine South of McAuley
Vertical air photograph: A15528-51
Flight height: 20,000 a.s.l.; lens focal length: 152.26 mm
Date: October 17, 1956
Scale: 1:36,600 (approx.)
Location: Townships 13 and 14; Ranges 29 and 30 WI
Map sheets: 1:250,000 62K Riding Mountain
1:50,000 62K/3 Miniota
[i] Mollard and Janes. op. cit., 1984, 48.