Table of Contents
2: Location, Borders, and Lakes
3: Geologic Structure and Landforms
6: Pre-historic and Early Historic Settlements
7: Survey Systems
8: Southern Hamlets, Villages, and Towns
9: Mennonite and Hutterite Settlements
10: First Nations Settlements
11: Northern Settlements
12: The Southern Cities
13: Mining and Oil Extraction
15: Industry / Manufacturing
16: Water Resources
17: Parks, Recreation, Sports
18: Transport and Communications: Past and Present
19: Legal Issues and Law Enforcement
3: Geologic Structure and Landforms
Click for chapter introduction
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.
3.75: The Assiniboine Glacial Spillway South of Shellmouth
At the end of the Wisconsinan glaciation, melting ice produced enormous quantities of water that accumulated in massive lakes in many parts of the prairie provinces. Once a lake reached an overflow level, it spilled into lakes at lower levels, possibly in as short a time as a few months or years. The erosive power of the overflow water was great, resulting in the creation of steep-sided, flat-floored valleys—spillways—usually cut into easily eroded glacial sediments but occasionally reaching down to the underlying bedrock. Spillways of various sizes are ubiquitous throughout Alberta, Saskatchewan, and Manitoba. This photograph shows a spillway, now occupied by the Assiniboine River, which carried water from glacial Lake Assiniboine in Saskatchewan to glacial Lake Agassiz.[i] At this location just south of Shellmouth, the flat spillway floor 1 is between 0.7 and 1.0 mile (1.1 to 1.6 km) across. The steep valley walls, over 200 feet (61 m) high, can be seen to the west 2 and east 3. The dark-toned west wall is more obvious because it is completely wooded, a result of the relatively wet microclimate on the east and northeast facing slopes. The eastern wall—west facing—is drier and therefore covered by scrub. Tributary valleys, short on the west side 4 but much longer, in the form of the valley of Thunder Creek 5, in the east, dissect both sides. Thunder Creek enters the Assiniboine River 6 near the southern edge of the photo. Its valley has the characteristic v-shape of recently eroded valleys, contrasting sharply with the flat-floored spillway. The freely meandering Assiniboine occupies the spillway floor. In three places the channel impinges on and runs along the valley sides 7. Recent cutoffs can be seen at several locations 8, and at one location a whole river segment has been abandoned 9.
West of the spillway is characteristic ground moraine topography with a definite north-northeast/south-southwest orientation with long narrow sloughs in depressions 10. In the east is a drier plateau with little surface drainage except two lakes 11 and with the characteristic ripple markings of outwash 12.
The imprint of the DLS system can be seen, but in this area with two major valleys, the topography influences road directions more than is usual in southern Manitoba 13. The same is true of the CN line 14, which crosses Thunder Creek at right angles 15, then swings south on the eastern plateau 16 before running obliquely down the spillway side 17. Two prominent cuttings can be seen along the railway’s route 18.
Figure 3.75: The Assiniboine Glacial Spillway South of Shellmouth
Vertical air photograph: A15537-20
Flight height: 20,000 feet a.s.l.; camera focal length: 6 inches
Date: October 9, 1956
Scale: 1:35,200 (approx.)
Location: Townships 21 and 22; Ranges 29 and 30 WI
Map sheets: 1:250,000 62K Riding Mountain
1:50,000 62K/14 Inglis
[i] For details see Klassen, R. W. “Wisconsin events and the Assiniboine and Q’Appelle valleys of Manitoba and Saskatchewan.” Canadian Journal of Earth Sciences, vol. 9, 1972, 544-560.