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
3.48: Ground Moraine Topography South of Minnedosa
This ground moraine area is part of the Minnedosa-Reston Till Plain[i] four miles south-southeast of Minnedosa. It is typical of the Minnedosa pothole region: locally the depressions are referred to as potholes and the lakes within them, as sloughs 1. Many of the potholes contain sloughs surrounded by “emergent aquatic plants such as rushes, sedges, and whitetop” 2.[ii] In some cases this vegetation grades back into shrubs, principally willow (Salix spp) 3. The sloughs and surrounding vegetation zones are excellent waterfowl habitat, but many of the sloughs have evaporated, leaving only a dried out depression 4. There is no integrated drainage system: surface run off is to the nearest slough, which at times of high water may empty into a neighbouring slough.
In contrast to figure 3.47 most of the land has been cleared for agriculture, leaving only a few woodland patches 5. Much of the agricultural land has been left fallow 6, a standard practice at the time the photograph was taken, and in these areas the characteristic bald crests on higher land are very marked 7. They indicate a poorly defined ridge running from north to south through the area. In some parts they exhibit a circular pattern 8. A possible explanation is that stagnant ice broke into blocks; sediment within the ice slurped down the sides of the blocks as they melted forming a ridge surrounding the melting block. These circles are minor relief features; there is only one or two metres difference between the surrounding ridge and its depression.
Grain crops are being grown in several fields 9. The distinctive pattern in the fields results from the harvesting procedure: the grain was cut and left lying in rows 10. A combine then picked up the crop and thrashed it collecting the grain and spilling out straw at the back. The rows of straw are less sharply defined than those resulting from the original cut 11. There are six farms 12, two of which show a very typical arrangement of farm buildings protected from winds by shelterbelts of trees on the north and west 13 with dugouts 14 nearby to supply water.
Gravel roads run along several section lines 15, and sections are divided into quarter sections, sometimes with trees planted along the dividing lines 16. Somewhat unusually the section lines are at an angle to the photo border. Normally flight lines trend east/west or north/south so that section lines are close to the cardinal points of the compass.
Figure 3.48: Ground Moraine Topography South of Minnedosa
Vertical air photograph: A18621-114
Flight height: 9,500 feet a.s.l.; lens focal length: 152.13 mm
Date: September 13, 1964
Scale: 1:15,840 (approx.)
Location: Township 14; Ranges 17 and 18 WI
Map sheets: 1:250,000 62J Neepawa
1:125,000 62J/SW Neepawa