A depression on a topographic map looks like a set of concentric closed contour lines with small tick marks (called hachures) pointing inward toward the center. These tick marks are the key feature that distinguishes a depression from a hill, since both appear as closed loops of contour lines. Without hachures, you’d have no way to tell whether you’re looking at ground that rises or ground that sinks.
How Hachure Marks Work
On a standard topographic map, contour lines connect points of equal elevation. When those lines form closed loops, they could represent either a hilltop or a basin. To solve this ambiguity, mapmakers add short tick marks called hachures to the contour lines of depressions. These ticks extend at right angles from the contour line and point downslope, toward the lowest point of the depression.
Think of the hachures as tiny arrows pointing “down into the bowl.” If you’re looking at a sinkhole, for example, the outermost hachured contour sits at the highest elevation, and each ring moving inward drops lower. This is the exact opposite of a hill, where the innermost ring represents the peak.
Depressions vs. Hills on the Map
A hilltop and a depression can look nearly identical at first glance because both show up as nested circles or ovals. The difference comes down to one detail: hachures. A hill’s contour lines are plain, with elevation values increasing as you move toward the center. A depression’s contour lines carry those small inward-pointing ticks, with elevation values decreasing toward the center.
If you’re ever unsure which you’re looking at, check the elevation labels on nearby contour lines. On a hill, the surrounding terrain sits lower than the closed loops. On a depression, the surrounding terrain sits higher. The hachures confirm the direction, but reading the printed elevation numbers is a reliable backup.
Reading Elevation Inside a Depression
The first hachured contour line in a depression shares the same elevation as the regular contour line just outside it. This is an important convention that trips people up. If the last normal contour line around a depression reads 100 feet, the first hachured line also represents 100 feet. The next hachured line inward then drops by one contour interval (if the map uses a 20-foot interval, that second line sits at 80 feet).
If you need to estimate the elevation of a specific point inside the depression that doesn’t fall on a contour line, you can interpolate. Draw a straight line from the point to the nearest contour on each side, measure those distances, and use a simple proportion to calculate where the point falls between the two known elevations. For instance, if a point sits roughly two-thirds of the way between an 80-foot contour and a 60-foot contour, its elevation is roughly 67 feet.
What Natural Features Create Depressions
Several types of landforms show up as hachured depressions on topographic maps. The most common include:
- Sinkholes: Closed depressions formed when underground rock (usually limestone or dolomite) dissolves and the surface collapses into the void. These are especially common in karst landscapes like those found across Missouri, Florida, and Kentucky.
- Volcanic craters: The bowl-shaped opening at the top of a volcano. On a topo map, you’ll see regular contour lines climbing up the flanks of the volcano, then hachured lines marking the crater rim and dropping into the interior.
- Kettles: Depressions left behind by melting blocks of glacial ice, common in previously glaciated areas of the northern United States and Canada.
- Dry lake beds and playas: Shallow basins in arid regions where water occasionally collects and evaporates.
Any enclosed area where the ground surface dips below the surrounding terrain and has no external drainage will appear this way on the map.
Color and Line Weight on USGS Maps
On official U.S. Geological Survey topographic maps, depression contours follow the same color scheme as regular contour lines but with the added hash marks. Index depression contours (the heavier lines labeled with elevation) appear in a darker brown, while intermediate depression contours use a lighter brown. Supplemental depression contours, used for extra detail in flat areas, are lighter still. All three types use the same hachure tick style, just in different shades, so you can distinguish major elevation breaks from minor ones at a glance.
The hachure ticks themselves are drawn at a standardized size and always point perpendicular to the contour line, directed toward the center of the depression. On printed maps, they’re small enough that you may need to look closely, especially in areas with tight contour spacing. On digital USGS topo maps, zooming in makes them easier to spot.
Quick Tips for Spotting Depressions
When scanning a topographic map, closed contour loops are your starting point. Most of the time, these represent hilltops or ridges. To find depressions, look specifically for the tiny inward-pointing ticks on closed loops. They’re easy to miss on a busy map, particularly in rugged terrain where contour lines are packed close together. If you’re working with a paper map, a magnifying glass helps. On a digital map, zoom to a scale where individual contour lines are clearly separated.
Context also helps. If you’re looking at a region known for karst geology, expect sinkholes. If you’re examining a volcanic peak, look for a crater depression near the summit. Flat glaciated plains may have scattered kettle depressions. Knowing the landscape gives you a head start on what those hachured circles represent on the ground.