 Changes in elevation on US Geological Survey or USGS Topomaps are illustrated by thin brown lines called contour lines. A contour line is an imaginary line on the ground along which every point is at the same elevation above sea level. Often every fifth line is darker than the others. These are known as index contour lines. They may also have a number printed on them which indicates the elevation along that line. The distance between one contour line and the next is known as contour interval. The contour interval of your map should be printed at the bottom margin of your USGS Topomap. There are some things to remember when interpreting contour lines. When contour lines are widely spaced apart, that represents a gentle slope. Lines that are closer together represent a steep slope. Topo lines that look like they're running together represent a cliff. When topo lines cross a stream or river, they make a V shape. The point of the V is always pointing uphill. Ridges or hills are represented with topo lines in a U shape. The bottom of the U points downhill.