A depression on a topographic map refers to a specific geographic feature. Cartography uses these specialized maps to represent the three-dimensional surface of the Earth on a flat sheet. A topographical depression is an area of land that is lower in elevation than all the surrounding terrain, like a bowl or a pit.
Defining Topographical Depressions
A topographical depression is defined as a natural basin where the land slopes inward from every direction. Unlike a valley or gully, which has an outlet for water, a true depression is characterized by a closed perimeter. This means that if the feature were to hold water, the water would pool and not escape the boundary.
The boundary of a depression is marked by a continuous, highest point, or rim, which entirely encircles the lower area. This enclosed nature is the defining physical characteristic that sets it apart from other low-lying terrain. The depth and size of these depressions can vary dramatically, ranging from shallow puddles to vast, permanent lake beds. Geologists describe them as closed-contour areas because the lines of equal elevation must connect back upon themselves.
Reading Depressions on a Topographic Map
To accurately portray a depression on a map, cartographers employ a specific set of symbols to differentiate them from rising ground. The standard convention involves using small, short lines called hachure marks.
These hachure marks are drawn perpendicular to the contour line and point toward the lower elevation, indicating that the ground is dropping inward. This symbol is the direct opposite of a regular contour line, which indicates a rise in elevation toward the center of the closed loop.
The first contour line of the depression shares the same elevation value as the last regular contour line encountered before the terrain began to drop. This shared value marks the highest point of the depression’s rim.
Subsequent hachured lines decrease in elevation by the map’s contour interval. For example, if a map has a 20-foot interval, and the last regular line before the hachures is 500 feet, the first depression line is also 500 feet, and the next hachured line inward is 480 feet.
The lowest point of the depression is estimated to be between the value of the innermost hachured line and the value of the next theoretical contour line. If the last marked line is 440 feet, the bottom is lower than 440 feet but higher than 420 feet.
Common Natural Examples
Topographical depressions are formed through various geological processes, resulting in several distinct natural examples. Volcanic craters are a common form, created by the explosive eruption or collapse of a volcano’s cone, leaving behind a circular basin.
In regions defined by soluble bedrock, such as limestone, water erosion creates sinkholes, characteristic of karst topography. These features form when underground water dissolves the rock, leading to the collapse of the surface layer into the cavern below.
Glacial activity creates kettle lakes and kettle holes, particularly in areas once covered by ice sheets. These depressions form when large blocks of melting glacier ice are buried by sediment, eventually melting to leave a water-filled or dry pit.
Playas, or dry lake beds, are another common depression type found in arid climates. These are shallow basins that temporarily fill with water during rain events and then completely evaporate, leaving behind a flat, often salt-crusted floor.