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 eastward flowing Assiniboine River 1 clips the northern edge of the photo. In this area the Assiniboine River is etched into the Assiniboine Delta that is composed of sands. The meandering river shows the characteristic river cliff 2 on the outside of the bend with a grass and willow covered point bar 3 on the inside of the bend. Most of the land has been cleared for agriculture with the exception of a block of deciduous woodland in the north 4 and strips of woodland along creek channels 5. The cleared land displays the mottled tones 6 characteristic of sandy soils. Rain splash erosion results in light tones on slightly elevated areas 7. In other areas erosion is concentrated along minor rills 8. Some of these 9 lead towards a larger gully 10 that combines with another gully from the south 11 before reaching the Assiniboine. Water from these gullies has deposited a small delta 12 into the Assiniboine. Rather surprisingly the gullies are most obvious where they cut across wooded land 13. In two cases gullies are retreating from wooded areas into agricultural land by a process of headward erosion with a steep head wall 14 at the point of maximum extent.
Vertical air photograph: A16621-79
Flight height: 9000 feet a.s.l.; lens focal length: 6 inches
Scale 1:16,200 (approx.)
Date: July 18, 1959
Location: Township 9; Range 10WI
Map sheets: 1:250,000 62G Brandon
1:50,000 62G/10 Treherne