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 the Upper Limestone Rapids on the Nelson River about 80 kilometres southwest of Hudson Bay. “The Nelson has a steep gradient with numerous rapids, high water flows and high ice production.”[i] Here the river 1, which is about 0.5 miles (0.8 km) across at the southern edge of the photo and about 0.8 miles (1.3 km) across at the eastern edge, flows through three sets of rapids 2 with the upper one being at about 200 feet (61 m) above seal level. The Nelson is joined by the Limestone River 3 from the west and by Moondance Creek 4 from the south. The dark-toned Limestone River contrasts with the light-toned sediment-laden Nelson. The flow of the Limestone is deflected along the north bank of the Nelson 5 before eventually merging with the flow of the larger river. A well-defined terrace 6 can be seen on the north side of the Limestone. Most of the area is covered by a thin till layer on which muskeg has developed 7, but some dark-toned coniferous trees grow near the main river channel 8.
At the time the photo was taken the only evidence of human activity is the Hudson Bay Railway line 9 that leads north to Churchill. Two spurs lead into gravel pits 10. However, the Limestone power station has since been constructed 11 and the settlement of Sundance, built to house the workers involved in the building of the dam, was constructed 12.
Vertical air photograph: A14126-37
Flight height: 31,000 feet a.s.l.; lens focal length: 153.19 mm
Date: July 1, 1954
Scale: 1:61,600 (approx.)
Location: Townships 86 and 87; Ranges 21 and 22 E
Map sheets: 1:250,000 54 D Kettle Rapids
1:50,000 54D/9 Amery