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.
3.18: Soil Erosion in the Little Saskatchewan River Valley
The Little Saskatchewan River occupies a large valley eroded into the ground moraine of the Minnedosa-Reston till plain.[i] Both the valley floor 1 close to the river and the valley sides 2 are covered by deciduous woodland, whereas much of the plain above the valley 3 has been cleared for agriculture. In some fields rain splash erosion has produced the characteristic light-toned hillcrests 4.
In this area, just below Minnedosa, the river meanders as it flows from northeast to southwest. Several abandoned meanders can be identified 5. Some are relatively new and still contain standing water 6, whereas others have been filled in and overgrown by trees 7. Other cutoffs are close to being created 8. Meander scars indicating previously higher river positions can be seen 9. South of the river a major river terrace has been preserved (between 10 and 11). Most of the terrace has been cleared for agriculture resulting in severe soil erosion. A series of southeast/northwest trending gullies can be seen 12, and sediment eroded from the top of the slope is deposited in fans lower down 13. Some of the fields have been left fallow, and (at 14) the farmer has started cultivating a field, either to deter soil erosion or to eliminate weeds. A north bank tributary enters the Little Saskatchewan depositing sediment near the junction of the two streams 15.
Settlement in the area is limited to two farms 16. Two railway lines cross the area 17, 18 and in the case of the more northerly line, a train can be seen on the line in the northwest corner of the photo 19. It is stationed close to an elevator that is identified by its distinctive shadow 20. A gravel road runs along the north valley side just above the floodplain 21, and other minor roads follow section lines of the DLS system.[ii] An east/west gravel road 22 is the northern boundary of township 14 that is a correction line in the system. The jog 23 is the extent of the correction at this location.
Figure 3.18: Soil Erosion in the Little Saskatchewan River Valley
Vertical air photograph: A16065-32
Flight height: 9,500 feet a.s.l.; lens focal length: 152.08 mm
Date: June 10, 1958
Scale: 1:15,500 (approx.)
Location: Townships 14 and 15, Range 18 WI
Map sheets: 1: 250,000 62J Neepawa
1:50,000 62J/4 Moorepark
[i] See Corkery, T. M. “Geology and Landforms of Manitoba” in J. Welsted, J. Everitt, and C. Stadel. eds. The Geography of Manitoba: Its Land and Its People. Winnipeg: The University of Manitoba Press, 1996. Figure 2.9, 21.
[ii] The road system in this area has changed considerably—associated with PTH 10 bypassing Minnedosa.