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
3.6: Fault Scarp on the Precambrian Shield Northwest of Wabowden
The rocks in this part of the shield are mapped as “highly metamorphosed and partly migmatic derivative”,[i] but, as in the case of figure 3.4, they are mainly covered by surficial material. A striking linear feature trending north-northeast to south-southwest with a 26-degree bend at its southern end breaks the monotony of the topography 1. This is interpreted as a “fault scarp, post glacial”[ii] with the downthrow side to the southeast. The feature peters out in the south, but two other linear features at 2 and 3 seem to be a continuation of the same structure. A small meandering stream developed on the surficial material that offers uniform resistance to erosion crosses the area from north-northwest to south-southeast 4. In the southwest are extensive areas of string bog 5—strips of vegetation separated by water—that are perpendicular to the general slope of the land to the southeast. The string bog surrounds lenses of highland covered by coniferous trees 6. Streamers of light tone at the southern ends of the lenses 7 indicate slow movement of land to the southeast.
Figure 3.6: Fault Scarp on the Precambrian Shield Northwest of Wabowden
Vertical air photograph: A14982-154
Flight height: 30,000 feet a.s.l.; lens focal length: 152.91 mm
Date: June 14, 1955
Scale: 1:58,000 (approx.)
Location: Township 70, Ranges 11 and 12 WI (southern two thirds of photo)
Map sheets: 1:250,000 630 Nelson House