IDENTIFICATION OF RELIEF FEATURES USING THEIR CONTOUR FORMS

It is possible to identify specific relief features on topographical maps by examining the way their contour forms are arranged. This post discusses specific relief features and how their contour forms are arranged, especially on topographical maps.

 

Slopes and their Contour Forms

The various types of slopes that are found on topographical maps include uniform slope (which can be steep or gentle), convex slope and concave slope.

 

Uniform slope

This is shown on a map by contour lines that are more or less evenly spaced apart. This makes the degree of steepness of the slope almost even. A uniform slope is steep if the contour lines forming the slope are closely arranged.

Or gentle if the contour lines are widely spaced apart.

 

Convex Slope

A convex slope is shown on a map by contour forms that depict a pattern in which contour lines are closely spaced at lower height (which indicates steep rising of land and widely spaced apart as we approach higher heights. Therefore, the slope is steep at a lower height and gentle at a higher height.

If two places are intervened by a convex slope, then the two places are not intervisible.

 

Concave Slope

A concave slope is a slope that is gentle at lower height but steep at a higher height. On a map, such a slope is shown by contour lines that are widely spaced apart at a lower height but closely spaced at a higher height as shown in figure below.

 

Conical Hill

This relief feature is shown on a map by more or less circular contour lines with the inner-most contour representing the highest height as shown in the diagram below.

 

Knoll

A knoll is an isolated hill that is considerably lower in height than a conical hill. Therefore, a knoll has similar contour forms as a conical hill, but as a result of the low height, the number of concentric contours for a knoll is few. On a map, one or two concentric contours that indicate the rising of the land show a knoll.

 

Valley or Re-entrant

A valley is an area of low ground that penetrates into the land. It may contain a flowing stream or it may be dry. The former is called a river valley while the latter is a dry valley.

In the case of a river valley, three types of shapes are usually identified from its source to its base level. These shapes depend on the stage of the river. A river in the torrent stage shows a valley that is roughly V-shaped. This is shown on a topographical map by roughly V-shaped contour lines with the innermost contour line representing the lowest height.

This stage of a river is associated with the upper course unless rejuvenation has occurred in the middle or lower course.

The middle course or valley stage of a river is the stage where the river is said to be mature. The river flows through a wider and more open valley. On a topographical map, such a valley is shown by more open V-shaped contour lines that in some cases may not really look V-shaped as shown in the figure below.

 

The plain stage or old age is the lower course of a river where the river is about to enter its base level, the sea or a lake. The valley at this stage is shallow and broad with extensive flood plains. The contour lines representing such a valley hardly have semblance of a V-shape at all and the heights of the lines are considerably lower than those found in the upper and middle courses. It is at this stage of a river’s course that depositional features such as flood plains, braided channels, ox bow lakes, or pronounced meanders, etc.

 

Spur

A spur is a projection of high ground unto low ground. A spur separates one valley from another. On a topographical map, spurs are also shown by more or less V-shaped contour lines with the smallest V- shaped contour representing the highest height.

 

Ridge

A ridge is a strip of highland that is elongated and narrow. In other words, the ‘length’ is several times longer than the ‘width’. A ridge with a steep slope on one side and a gentle slope on the other is called a cuesta. The steep slope is referred to as the scarp slope while the gentle slope is known as the dip slope. On a topographical map, a ridge insert is shown by closely spaced ‘stretched’ contours which indicate the steep rising of the elongated land as shown below.

 

Col and Saddle

A col is a piece of low ground separating the peaks of a highland. A saddle is simply a large col. Both col and saddle are often associated with a ridge. On a map, a col is usually shown by at least one contour line running round two or more closed contours, which indicate the peaks of the high land as shown in the diagram of a ridge.

 

Pass

This is a col that is used or likely to be used as a route-way, perhaps a col that is not too high or too steep will qualify as a pass. This simply implies that a pass is always a col but a col (i.e. one that is too steep or too high) is not always a pass.

 

Plateau

This is a wide piece of undulating land that is considerably higher in elevation than the surrounding low land area. On a topographical map, a plateau is recognised by closely spaced contours (which indicate a steep rising of the land from the low land or general surface) and almost an absence of any contour at the centre (which indicate a level or gently undulating land surface). A plateau whose surface is heavily eroded by running water such as a river is termed a dissected plateau.

 

Undulating land

This is a piece of land that gently rises and falls. There may be low hills on such a piece of land.

 

Watershed

This is more or less a line on a highland where two or more streams take their sources and flow in a more or less opposite direction down a slope.