About Structure Contour Maps
In contrast to contour lines on a topographic map showing the shape of the Earth's surface, structure contours are lines of equal elevation drawn on the surface of selected rock layers buried within the Earth (Fig. 1).
The shape of the surface is reflected in the orientations and spacing of the contour lines. For cases in which the lines are straight and parallel the surface is planar, as shown if Figure 1. Closer spacing of the lines indicates a steeper inclination of the surface.
If structure contour lines are curved it indicates that the surface is warped. In cases in which the structure contours form a horseshoe shape the upward arch of an anticline is indicated if the central portions are higher that the outer sides: a syncline has the central portions lower the outer sides.
Elevations of the selected rock units are constructed by finding the elevation of the surface (Fig. 1) by two means: 1) subtracting the depth drilled to reach the surface (in water wells and petroleum exploration drill holes) from the surface elevation shown by topographic contours on U.S. Geological Survey topographic quadrangle maps; 2) determining the elevation of the surface directly from the exposed location at the Earth's surface.
The accuracy of the map is generally greatest for the shallower depths, nearer the exposed formation. In such areas, accuracy is approximately one half of the contour interval (100 feet in this case). Because there is less information concerning the depth of the formation at greater depths, accuracy is less in those areas.