The geographical term “spur” describes a common landform found in upland regions, particularly where mountains or hills meet valleys. This feature is fundamental to interpreting the topography of an area, revealing how erosional forces have shaped the landscape over long periods.
Defining the Geographical Spur
A geographical spur is a lateral ridge or tongue of land that projects outward from a larger mountain mass or the main crest of a ridge, typically descending toward a valley floor. This landform is characterized by a convex shape, meaning it bulges outward toward the valley, and is flanked by lower ground on three sides.
The sides of a spur are often steep, but the ridge itself slopes gently downward from the main upland area. Its formation occurs between two ravines or stream valleys, illustrating its relationship as the remnant high ground separating two parallel drainage features. When viewed on a topographical map, a spur is identifiable by contour lines that form a series of broad, concentric “n” shapes, with the open end of the “n” pointing downhill.
Geological Processes of Spur Formation
Spurs are the result of differential erosion, where water or ice selectively wears away softer or less resistant rock, leaving the harder rock projecting into the valley. In the upper reaches of a river valley, the water focuses its energy on vertical erosion, cutting a deep, V-shaped channel into the landscape. The river, however, is forced to flow around obstacles of resistant rock, which eventually become the projecting spurs.
The process involves the river following the path of least resistance, eroding zones of weakness in the bedrock like intersecting joints. As the river cuts downward, its winding course is established by these resistant rock formations, which remain as the high ground of the valley side. This constant material removal around the more durable core creates the distinctive protruding shape of the spur.
Classifying Types of Spurs
The two primary classifications of spurs, interlocking and truncated, are distinguished by the agent of erosion and the resulting valley shape. Interlocking spurs are characteristic features of young, V-shaped river valleys where the river meanders around the spurs as it flows downstream. When viewed along the valley, the spurs from opposite sides appear to overlap or “interlock,” much like the teeth of a zipper.
The river’s limited energy in its upper course means it cannot erode directly through the resistant rock of the projecting spur. Instead, it undercuts the outer bank of the spur on one side before swinging to undercut the spur on the opposite bank, creating the alternating, zigzag pattern.
In contrast, truncated spurs are found in U-shaped glacial valleys, representing a modification of the landscape by a much more powerful force. Unlike a river that flows around the projections, a glacier moves straight through the valley, using abrasion and plucking to erode the ends of the spurs. This process leaves behind steep, blunt, or cliff-like faces that drop sharply to the floor of the glacial trough.