Topographic maps translate the three-dimensional surface of the Earth into a flat, two-dimensional representation using lines and symbols. These maps depict landscape features with precision, allowing users to understand elevation, slope, and terrain shape. A specific landform known as a depression requires a unique cartographic convention to distinguish it from a peak or a hill. This symbol is a specialized application of the standard contour line system, providing a visual interpretation of a closed, sunken area in the terrain.
Defining Topographic Depressions
A depression in a topographic context refers to a landform that is lower in elevation than all the surrounding terrain, essentially forming a closed basin or bowl shape. Water or other materials that enter a depression cannot flow out over the surface, meaning the area is internally drained. This geographical feature contrasts directly with a peak or a hill, which represents land that rises above the surrounding landscape.
Depressions can range from small kettle holes just a few meters across to large, complex geological basins spanning many kilometers. The defining characteristic is that the ground slopes downward from all sides toward the center. Recognizing these sunken areas is important for navigation and understanding local hydrology, as they often become areas where water collects.
Interpreting Hachure Marks on Contour Lines
Standard contour lines connect points of equal elevation and are the primary tool for representing vertical relief. To depict a depression, cartographers use a modified contour line known as a hachured contour. This is a closed loop with small, short lines drawn perpendicular to the contour itself. These hachure marks, or depression ticks, point inward toward the center of the loop, signifying that the elevation is decreasing in that direction.
A simple closed contour loop without these marks would be interpreted as a hill or peak, where elevation increases toward the center. The inward-pointing hachure marks indicate a reversal of the slope, showing a descent into a hollow rather than an ascent to a summit. The depth and elevation of the depression are determined by applying a specific rule related to the map’s contour interval.
The outermost hachured contour line is assigned the same elevation as the nearest standard contour line outside the depression. For example, if the standard line immediately outside is 500 feet with a 20-foot interval, the first hachured line is also 500 feet. Subsequent hachured lines located toward the center decrease by one contour interval (e.g., 480 feet, 460 feet). This system allows a map user to calculate the lowest possible elevation within the basin, which lies between the last hachured line and the value of the next theoretical line.
Examples of Natural and Artificial Depressions
Depressions appear across various landscapes and are formed by both natural geological processes and human activity. One common natural example is a sinkhole, prevalent in regions with karst topography where soluble bedrock, such as limestone, has dissolved. Volcanic craters and calderas also register as large-scale depressions, formed by the collapse of a volcano’s cone after an eruption.
Glacial action creates depressions like kettle lakes and ponds, which form when buried ice blocks melt and leave behind a water-filled hollow. Artificial depressions result from human intervention in the landscape. These features include open-pit mines, quarries for obtaining building materials, and gravel, sand, or borrow pits where material has been removed. If these artificial excavations are large enough to be represented by multiple contour lines, they are depicted using the same hachure mark convention as natural depressions.