Sec. 14. Land Forms and Their Delineation Using Contour Lines
The features of the earth's surface are very diverse. The following are the principal land forms: a gentle slope, a mountain, a basin, a ridge, a gully and a saddle.
A slope is part of the earth's surface that has a uniform shape. There are gentle, medium and steep slopes. A very deep wall-like
Fig. 14
slope is called a precipice. A high precipice is a bluff (cliff). A platform on a slope is a shelf or a terrace. A flat slope on the ground is a sloping plane lying between curved surfaces. Contour lines show it on a plan (map) as a row of parallel, equally spaced straight lines (Fig. 14a). The slope of a different land form begins where contour lines pass from straight to curved lines.
Contour lines are spaced on a map or plan with different densities. The steeper a slope's surface the more crowded are its contour lines. Consequently, the distance between the adjoining contour lines as viewed in plan, described as a contour interval, characterizes the slope rate (gradient). For instance, where there is a dramatic change between neighbouring contour lines the slope must change suddenly. This points to a shelf or a terrace.
In modern plans and maps the downslope direction is shown by individual hachures (see Fig. 13). The heights of the contour lines are inscribed between the lines so that the tops of the figures are upslope).
A mountain is a landmass elevated substantially. It has a distinct foot, a line where the mountain's lateral slope passes to the surrounding terrain. A small mountain with smooth slopes is termed a hill. The highest point of the mountain is the summit. A flat summit is a plateau, while a pointed summit, is a peak. A mountain will be represented by closed contour lines, the individual hachures pointing outward (Fig. 14b).
A basin is a cup-like depression or a hollow. A basin with steep slopes is called a funnel, its lower part is the bottom. The line where the basin's slope passes into the surrounding terrain is the edge (or brow) of the basin. A basin has closed contour lines yet the individual hachures are directed inward (Fig. 14c).
Depressions mostly occur as gullies. A gully is an extended trough-like depression with a slope in one direction. A gully is typically a product of the action of flowing water. Slopes of a gully cross along the line of the water flow which is termed an aquifer or a thalweg. Minor gullies formed by water flowing across a land surface are termed washouts. Substantial washouts are ravines. A ravine is a narrow gully with steep slopes in a flat country. Ravines with well washed down slopes are said to be developing, while those with slopes covered with vegetation are fixed. Large ravines with fixed slopes and found in rugged terrain are described as balkas. A thalweg is often the bed of a river or of a stream.
A wide gully with a gently sloping bottom is a valley. A narrow steep-sloped gully found in mountainous country is a gorge or canyon. The steep banks of a mountain river are called canyon walls. A gully is shown by contour lines that bend sharply up to its highest point (Fig. 14d).
A land form that gradually slopes in one direction and one that is opposite to the gully is a ridge (Fig. 14e). The line where its slopes meet, the top of the ridge, is the ridge's axis, the watershed or the line of watershed. Two gullies are typically separated from each other by ridges. The slopes of ridges are wave-shaped as is a watershed. This latter connects the highest points of the ridge. If the slopes intersect at an acute angle, the top of the ridge is called a crest.
A saddle is a convex-concave land form lying between two adjacent mountains. It is composed of two ridges and two gullies pointing in opposite directions. The lowest part of a saddle's watershed is called a pass. They are commonly used as ways of communications between two sides of a saddle; in mountainous country they are called mountainous passes. The shape of the land surface at a pass resembles a saddle, hence the name (Fig. 14f).