![]() ![]() Because the section is vertical (and the unit has dip), the apparent vertical thickness of 20 m must be shown (not the true thickness).The cross-section is oriented along the 180 m strike line for the top of the sandstone and drawn at a scale of 1/1 (i.e., there is no vertical exaggeration). ![]() Along this line of section, the map unit base is exposed only in one small area. The section locates areas of (potential) exposure. The cross-section (true scale) plots the intersection of strike lines at 180 m (blue dots – top of unit) and 160 m (green dots – base of unit) with topography (red dots). Beginning with the location where the top is exposed, plot the strike line (everywhere along this line will be 220 m asl), and note where the line intersects the 220 m topography contours at other locations (dashed blue line and dots in the diagram below). ![]() We need to determine where this dipping unit intersects the surface topography. True thickness is calculated as shown in the diagram below. What is its map extent?īecause the unit is inclined, the 20 m thickness we determine from the two outcrops must be an apparent thickness. The structural contours correspond to lines of strike. In this scenario, the sandstone unit strikes 045 o and dips 25 o NW. If the unit is folded (and plunges vertically), then the map trace will correspond to a cross-section profile of the fold. The structural contours in this case correspond to lines of strike. If the unit is flat, the map extent of the top and base will plot as straight lines. The outcrop trace of a vertically dipping sandstone unit intersects all topographic contours along its line of strike (in this case strike is NE). Geological maps of a flat sandstone bed for the simplest case of a horizontal unit (left) where contacts coincide with topographic contours, and a vertical unit where the contacts cross all contours. In this case, the structural contours coincide with the relevant topographic contours. True thickness is 20 m and we assume this applies across the map area (until we have information that tells us otherwise). In this case, the top and base everywhere will be at an elevation of 220 m and 200 m respectively. Incomplete sections of the sandstone crop out at two locations: the top at one location at 220 m (above sea level – asl), the base at a second location close by at 200 m asl. The three examples worked through here take the simple case of mapping the geological contacts of a flat sandstone unit. Structure contour maps are used widely in surface geological mapping because they provide basic information on outcrop patterns, subsurface extent and structure.
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