What is a depression contour line?

A depression contour line is a marking on a map indicating a low point of land completely surrounded by more elevated land. The area is represented by closed concentric lines. The inner depression contour lines indicate lower elevations, with the elevation increasing with each outer layer.

Sinkholes and various types of craters, including volcanic, pit and impact, are types of depressions marked by these lines. Small marks, called "hachures," appear on the depression contour lines and point toward the areas of lower elevation. The contour intervals, or the distance between the various lines, is consistent within each individual map and is marked on the bottom of the map in terms of feet or meters.