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 Langs Valley 1, a large spillway occupied by a small misfit stream 2. Langs Valley[i], delimited by the dashed lines on the photo, is part of the Souris-Pembina spillway that carried water from glacial Lake Hind to glacial Lake Agassiz. After the glacial lakes disappeared, the spillway was occupied by the Souris River that flowed eastward to join the Red River south of the Canada/U.S.A. border. However, the Souris was captured just west of the photo and redirected northward through the Tiger Hills to join the Assiniboine near Treesbank. Now a large spillway is occupied by a tiny misfit stream 2 that flows west; i.e., opposite to the original flow, to join the Souris near the elbow of capture. It has eroded a steep-sided valley into the spillway floor. The spillway’s southern wall 3 is steep and well wooded. It is dissected by two steep-sided creek valleys 4 where some slopes are too steep to support woodland. The northern valley wall 5 is not as steep and has a well-marked terrace 6 into which two small gravel pits 7 have been excavated.
This is a large-scale, low-level photo on which section lines 8 are clearly defined. Roads follow section lines with some accommodation for the topography 9. Five forms 10 can be seen in this area of 6 ¼ square miles (16.2 sq km).
Figure 3.80: Langs Valley North of Dunrea
Vertical air photograph: A16182-84
Flight height: 10,500 feet a.s.l.; lens focal length: 152.63 mm
Date: July 22, 1958
Scale: 1:17,600 (approx.)
Location: Township 5; Range 18 WI
Map sheets: 1:250,000 62G Brandon
1:50,000 62G/5 Dunrea
[i] “Named after the Reverend James Lang who ran a popular campground in the valley in 1880” G. Holm ed. Geographical Names of Manitoba, Winnipeg: Manitoba Conservation, 2000, 143.