GEOLOGICAL MAPS AND THEIR INTERPRETATION 55 Figure Block diagram and map to show how outcrop traces of horizontal formations parallel topographic contours (as they must also be horizontal) In the example shown here, the geo logical map is deceptively intricate The outcrops are irregular, despite the simple, horizontal arrangements of the strata, because the landscape is highly dissected Figure Block diagram and map to show how outcrops cross valleys with a more open shape where the formation dip is greater As in Figure 3, the V in map view points in the direction of dip, here towards the W The outcrop of more steeply dipping formations such as c3 makes a more open V than move gently dipping units such as BO The superficial deposits (yellow) are approximate horizontal and make the tightest V shape, parallel to topographic contours Figure Block diagram and map to show the V shape made by the outcrop of a dipping formation crossing a valley The V in map view points in the direction of dip, here towards the southeast Note also the influence of land slope on outcrop width For example, the outcrop of formation a, at the steep escarpment of Wenlock Edge, is much narrower than that of formation b, at the flat land of Hopedale, even though they have similar thicknesses of surfaces – any geological surfaces, whether or not they are sedimentary strata Thus geological faults (see Tectonics: Faults) commonly appear on maps as more or less straight traces, not because they are faults but because fault surfaces are commonly very steeply dipping or vertical Gently dipping thrust faults, on the other hand, will give traces that are close to parallelism with the topographic contours (Figure 5) The relative widths of outcrops on a map of a given scale reflect the thicknesses of the formations Hence the outcrop width will vary if the material was deposited in different thicknesses at different places Other things being equal, thicker units give wider outcrops On larger-scale maps, however, the dip angle of the formation and the topographic slope will further influence the outcrop width In general, steeper dips give narrower outcrops, as steeper topographic gradients (Figure 3) Symmetrical, roughly mirror-image repetitions of outcrops indicate folding (see Tectonics: Folding) Older formations symmetrically flanking younger ones imply that the strata have been downwarped