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.
3.68: Assiniboine Delta Edge and Lake Agassiz Strandlines Near Miami
In this high-level, small-scale photo, the lakeward edge of the Assiniboine Delta deposited into glacial LakeAgassiz at the end of the ice age can be seen trending northwest/southeast across the area 1. This relatively steep slope is wooded and hence dark-toned. Three wooded stream valleys dissect the delta edge; from north to south they are South Tobacco Creek 2, an unnamed stream 3, and Shannon Creek 4. Thornhill Coulee 5 can also be seen in the southeast corner.
Northeast of the delta edge, several LakeAgassiz strandlines can be seen 6. The neutral term “strandline” is used because it is not clear whether they were deposited as beaches or offshore bars. LakeAgassiz retreated to the northeast, so strandlines in the west are at a greater elevation and earlier in origin than those further east. The strandlines stand out because of tonal variations that represent soil and land use variations. Although they are noticeable on the photo, their relief is only a metre or so, and they can easily be missed on the ground. East of the lowest strandline is typical very flat lake floor topography 7 with the only relief being the valleys of small intermittent streams 8.
The land is divided into the one-mile squares of the DLS system, and apart from the delta edge and some stream valleys, the land is intensively farmed, especially in the east where sections are divided into many fields 9. The hamlet of Miami 10 in the north is the only settlement in the area which is crossed by the east/west trending PTH 23 12. Elsewhere gravel roads follow section lines except where they cross the delta edge 13. The CN line 14 follows PTH 23 through Miami before veering off to the north to cross a strandline at an angle.
Figure 3.68: Assiniboine Delta Edge and Lake Agassiz Strandlines Near Miami
Vertical air photograph: A21852-10
Flight height: 24,100 feet a.s.l.; lens focal length: 85.55 mm
Date: August 12, 1970
Scale: 1:84,500 (approx.)
Location: Townships 3, 4, and 5; Ranges 5, 6, and 7 WI