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.
3.27: The Assiniboine Spillway, Assiniboine River, and a Cutoff
This low oblique photo, so called because no horizon is showing, is of one of the cutoffs 4 on the vertical air photo of Figure 3.26. The view is from the east, looking west across the Assiniboine spillway. This photo is unusual in that it was taken in winter when there was a thin snow cover that enhances the topographic details.
The wooded west wall of the spillway is seen in the foreground 1. The spillway runs across the photo with the river channel on the valley floor 2. The cutoff is in the centre 3, and another one is imminent 4. Meander scrolls emphasized by the woodland and snow cover are evident within the meander loop 5. The east side of the spillway is in the background, dissected by two steep-sided wooded gullies 6.
Figure 3.27: The Assiniboine Spillway, Assiniboine River, and a Cutoff
Low oblique air photo
Date: December 6, 1965
Scale: Variable-- larger in foreground, smaller in background.