|Activity 4: A Rule for Determining Attitude (Dip) from a Map|
|The shape of a rock unit plays an important role in determining its map outcrop pattern. The width of an outcrop pattern on a map is a function of the unit thickness, dip, and the dip of the land surface (Fig. 1). The effect of the dip of the bed on its width on a map can be investigated using the strike-and-dip applet. Clearly, the pattern will be most complex where a large number of units outcrop in the area and where the difference between the dip of the beds and the dip of the surface is great.|
Fig. 1: The interaction of a tabular
rock layer with the surface topography.
Fig. 2: Determining strike-and-dip direction for a tabular body from map patterns.
|The simplest and easiest to interpret patterns are those produced by tabular
dikes or sills,
and lava flows. When
such units have
been warped by deformation, the resulting outcrop patterns can become very complex and difficult to interpret solely from map
Clues for interpreting three-dimensional orientation can be gleaned from the way tabular bodies interact with the landís surface and the outcrop patterns they produce. Outcrop patterns of individual rock units combined with topographic information can be used to determine strike-and-dip direction. Crude estimates of dip magnitude also can be made this way.
|The strike of a tabular body can be determined by finding two points on its outcrop pattern that are at the same elevation (Fig. 2). You can find this from the map's contour lines. Because a line connecting these two points is horizontal, it parallels the unitís strike. Lines drawn at several elevations should be parallel if the body is tabular. The dip direction can be determined by simply noting the direction in which the elevations of these lines decreases (Fig. 2).|