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.
In geological terms much of the land surface of southern Manitoba is very new, dating from the retreat of the last ice sheet no more that about 10,000 years ago. In the area shown, river channels are etching valleys in a till plain deposited during the Wisconsinan ice age. Snake Creek, a tributary of the Assiniboine—just off the photograph to the southwest—has already created a deep well-defined valley 1 wandering through the western side of this area. The steep-sided, flat-floored nature of the valley in the north 2 suggests that it used to carry much more water. Small creeks, tributary to Snake Creek, are gradually dissecting the till plain by headward erosion 3, and in at least two places, 4 small dams have been built either to prevent erosion or to catch water for agricultural use, or both. Several dark-toned lakes and sloughs have not been integrated into the drainage system 5. Much of the land in the south and west is still covered by deciduous forest, but in the north and east more land has been cleared for agriculture. Because of the irregular topography, fields are smaller than is often the case in southwestern Manitoba. The land is subdivided according to the DLS system, but roads do not follow all section lines, and the direction of some roads is clearly topographically determined 6. The CPR 7 line connects Birtle, just off the photo to the south and Foxwarren 8 in the north. As is commonly the case in small Manitoba settlements, the roads in the latter are parallel and at right angles to the railway line.
Vertical air photograph: A15530-50
Flight height: 20,000 feet a.s.l.; lens focal length: 152.26 mm
Date: October 9, 1956
Scale: 1:36,800 (approx.)
Location: Townships 17 and 18, Range 27 WI
Map sheets: 1:250,000 62K Riding Mountain
1:50,000 62K/6 Birtle