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.
Often vegetation obscures the bedrock geology, but in some cases the vegetation is a reflection of the geology. The photo shows an area of flat, mainly wooded land between South Moose Lake and Cedar Lake in central Manitoba. “Lines of trees disclose the location of joint zones (joint zones are numerous closely-spaced individual parallel and on echelon joint planes, which, because of their number and close spacing are accentuated by longtime weathering and erosion). Because the closely spaced joints weather and erode differentially in flat-lying Silurian dolomite terrain, they offer a better environment for growing trees than nearby areas where the joints are widely spaced.”[i] Two prominent lines of joint zones trend west-northwest/east-southeast 1 and northeast/southwest 2 intersecting at a 45° degree angle 3. Part of the south shore of Lamb Lake 4 coincides with the west-northwest/east-southeast trend. In the northwest is Ranchers Creek 5.
Figure 3.8: Joint Zones South of Lamb Lake
Vertical air photograph: A19767-87
Flight height: 8,920 feet a.s.l.; lens focal length: 152.16 mm
Date: September 16, 1966
Scale: 1:16,500 (approx.)
Location: Township 54, Range 19 WI
Map sheets: 1:250,000 63F The Pas
1:50,000 63F/9 Lamb Lake
[i] Mollard, op. cit., plate 10.19a