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 spillway can be seen trending northwest/southeast and deeply entrenched (300 feet, 91.4 m) into the surrounding landscape. It differs from spillways shown previously in that although it is steep-sided, it has a narrow floor into which the Pembina River 1 is incised. In the northwest the spillway has migrated northwards, leaving behind a wide terrace 2. Land here has been cleared for agriculture, whereas both the north side 3 and south side 4 of the spillway are covered by dark-toned deciduous woodland. The Pembina River meanders but is not freely meandering from side to side of a flat spillway floor. It is joined from the north by the Little Pembina River 5 which is also in a steep-sided, wooded valley; several minor creeks 6 join from the south. Land both north and south of the spillway is typical hummocky ground moraine topography.
This photograph shows the Canada/U.S.A. border 7 with North Dakota south of the line. Similar, but different, land division systems exist in the two countries. Many different baselines are used in the U.S.A., whereas only one—the 49th parallel—is used in Canada. The baseline used for this part of North Dakota results in only partial sections in the northern tier south of the Canadian border 8. The principal meridians used in the U.S.A. are also different from those in Canada resulting in sections in Canada being offset from those in the U.S.A. 9. This accounts for the bend on a north/south road (PTH 31 in Canada) as it crosses the border 10. In Canada roads follow section lines except where they cross the Pembina Valley (11 and 12).
Vertical air photograph: A21821-61
Flight height: 24,000 feet a.s.l.; lens focal length 85.55 mm
Date: August 10, 1970
Scale: 1:79,200 (approx.)
Location (in Canada): Townships 1 and 2; Ranges 7 and 8 WI
Map sheets: 1:250,000 62G Brandon
1:50,000 62G/1 Morden