What do closed contour lines with thick marks pointing toward low ground represent on land navigation maps?

Closed contour lines with thick marks pointing toward low ground represent depressions on land navigation maps. These features indicate that the elevation decreases as you move toward the center of the closed contour. The thick marks, called hachures, show the direction of the slope—pointing toward the lowest area—providing a clear visual cue that distinguishes it from other terrain features.

In mapping, depressions are defined by a series of closed contour lines that help visualize the sinking area in the landscape. Each contour line indicates a specific elevation, and the presence of hachures emphasizes the drop in elevation, making it apparent that the area in question is lower than the surrounding terrains, such as hills or ridges. This unique representation is crucial for navigational purposes, helping navigators determine potential hazards or significant changes in elevation.