Chapter 3: Geologic Structure and Landforms
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
3.83: The Dand Spillway South of Hartney
Not all spillways result from water emptying from one glacial lake to another. In some cases they were created when glacial meltwater was deflected by ice. An example is the Dand spillway—named for a hamlet just off the southwest corner of the photo. Meltwater flowed in the depression now occupied by Chain Lakes 1, a small intermittent stream 2 and two sloughs 3. The spillway, about 50 feet (15 m) below the surrounding land cuts through hummocky ground moraine. Light-toned crests 4 that result from rain splash and wind erosion on exposed soils indicate minor elevations. A minor esker-like ridge runs across the southeast corner of the photo 5.
Most of the land in the area is devoted to agriculture. Lighter-toned fields 6 have been cropped whereas darker-toned fields 7 have been left fallow. In some of the cropped fields the farmer has been cultivating the land in preparation for next year’s crop 8. Dugouts 9 are ubiquitous in this dry area of southwestern
Figure 3.83: The Dand Spillway South of Hartney
Vertical air photograph: A20471-116
Flight height: 21,900 feet a.s.l.; lens focal length: 152.73 mm
Date: May 22, 1968
Scale: 1:41,700 (approx.)
Location: Townships 5 and 6; Range 22 WI
Map sheets: 1:250,000 62F Virden
1:50,000 62F/7 Hartney