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.
The photograph shows the Souris spillway just north of the Canada/U.S.A. border in southwestern Manitoba. It carried water from glacial Lake Souris to the south to glacial Lake Hind to the north.[i] As with previous examples, it has a flat floor and steep sides, but in this case they are only 50 feet (15.2 m) high. Both sides are picked out by patches of scrubby trees (1 and 2) in slightly wetter areas of this dry part of Manitoba. The Souris River 3, which has obviously been straightened, flows from south to north. A light-toned embankment 4 can be seen along the east side of the river. Several small intermittent streams 5 flow towards the main river. Land on either side of the spillway is extremely flat.
Figure 3.77: The Souris Spillway South of Coulter
Vertical air photograph: A16181-17
Flight height: 10,500 feet a.s.l.; lens focal length: 152.63 mm
Date: July 20, 1958
Scale: 1:18,100 (approx.)
Location: Township 1; Ranges 26 and 27 WI
Map sheets: 1:250,000 62F Virden
1:50,000 62F/ 2 Waskada
[i] For details see Kehew, A. E. and Clayton, L. “Wisconsinan floods and development of the Souris-Pembina spillway system in Saskatchewan, North Dakota and Manitoba” in Glacial Lake Agassiz eds. J. T. Teller and L. Clayton. St. Johns: Geological Association of Canada, Special Paper 26, 1983, 187-209.