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.
3.70: Lake Agassiz Strandlines Along the Northeastern Edge of Riding Mountain National Park
On this photo Riding Mountain National Park (RMNP) is in the southwest, its boundary 1 marked by cutlines through the forest. At least five strandline zones 2, 3, 4, 5, 6 can be identified, the highest and oldest being in the southwest 2. As LakeAgassiz retreated to the northeast, it left behind strandlines whose prominence depends on how long the lake stood at a particular level; large prominent ridges indicate a long period of still stand. In the highest zone 2 are several individual lines 7, and below the lowest zone 6, tonal variations in cleared fields 8 indicate very low ridges which may have originated as offshore bars.
Within the park all the land is covered by coniferous/deciduous woodland, the darker toned patches 9 indicating stands of coniferous trees. Outside the park some land has been cleared for agriculture. In the fields the strandlines show up as light-toned areas 10 where dark-coloured soil particles have been removed by rain splash and wind erosion. The Ochre River 11 drains from the central part of RMNP to DauphinLake (off the photo to the northeast). It meanders and has deposited shale eroded from the highlands on the insides of bends resulting in light-toned areas 12. A straight drainage channel 13 has been excavated parallel to the general trend of the strandlines.
The boundary between townships 22 and 23 runs across the area. It is the sixth correction line in the DLS system, corrections being about three quarters of a mile (1.2 km) 14. Section lines 15 and quarter-section lines 16 can be seen. Several farms can be identified 17, but there are no nucleated settlements in the area.
Figure 3.70: Lake Agassiz Strandlines Along the Northeastern Edge of Riding Mountain National Park
Vertical air photograph: A15229-23
Flight height: 20,000 feet a.s.l.; lens focal length: 152.26 mm
Date: June 4, 1956
Scale: 1:37,300 (approx.)
Location: Townships 22 and 23; Ranges 17 and 18 WI