A spur in geography is a ridge of high ground that extends outward from a hill or mountain, sloping downward on three sides and connecting to higher ground on the fourth. Spurs are among the most common landforms you’ll encounter on both maps and trails, and understanding them is useful for everything from reading topographic maps to navigating while hiking.
How Spurs Form
Most spurs are shaped by rivers cutting downward into a landscape over thousands of years. In a river’s upper course, where the gradient is steep, water erodes primarily downward rather than sideways. As the river encounters bands of harder rock that resist erosion, it bends around them rather than cutting straight through. Over time, this carving leaves fingers of high ground jutting out between the curves of the valley. The result is a series of spurs on alternating sides of the valley that, when viewed from downstream, appear to interlock like the teeth of a zipper.
These are called interlocking spurs, and they’re one of the signature features of a V-shaped river valley. The river winds back and forth between them, and if you were standing at the bottom of the valley looking upstream, you’d see spur after spur overlapping on either side.
Interlocking vs. Truncated Spurs
Not all spurs keep their original shape. When a glacier moves through a valley that was previously carved by a river, it doesn’t follow the winding path the water took. Glaciers are massive and powerful, and they grind straight through obstacles using abrasion (scraping) and plucking (pulling rock away). The glacier shears off the ends of interlocking spurs, leaving behind steep, cliff-like faces called truncated spurs.
Truncated spurs are a telltale sign that a glacier once occupied a valley. Where a river valley has a narrow V shape with interlocking spurs zigzagging along its length, a glaciated valley is broad and U-shaped, with truncated spurs forming abrupt walls on either side. If you’ve ever looked at a wide mountain valley and noticed steep, flat-faced rock walls partway along its sides, you were likely looking at truncated spurs.
How Spurs Look on a Map
On a topographic map, spurs show up as V or U shapes in the contour lines that point toward lower ground. This is the key detail that distinguishes them from valleys. A valley’s contour lines also form V shapes, but those Vs point toward higher ground. If you can remember that one rule, you can reliably tell spurs from valleys on any contour map.
A spur typically extends outward from a ridge, so on a map you’ll often see it branching off a long line of higher elevation. The contour lines will be closer together on steeper spurs and more widely spaced on gentler ones. In practical terms, if you trace the contour lines and they bulge outward from the high point of a hill or mountain, that bulge represents a spur.
Spurs vs. Ridges and Re-Entrants
Spurs are easy to confuse with two related landforms: ridges and re-entrants (also called draws). All three involve sloping ground, but they differ in shape and position.
- Ridge: A long, continuous line of high ground. A spur often branches off from a ridge, extending outward and downward like a limb from a trunk. Ridges are the main high features; spurs are subsidiary ones.
- Re-entrant (draw): Essentially the opposite of a spur. A re-entrant is a small valley or indentation in a hillside. On a map, its contour lines point toward higher ground, while a spur’s contour lines point toward lower ground. In the field, a re-entrant funnels water downhill, while a spur sheds water to either side.
The simplest way to keep these straight: if you’re standing on high ground that drops away on three sides, you’re on a spur. If you’re standing in a low area with higher ground rising on three sides, you’re in a re-entrant.
Identifying Spurs While Hiking
In the field, a spur is recognizable as a feature that slopes downward on three sides and upward on only one. You can think of it as a nose of land sticking out from a mountain or hillside. Walking along a spur feels like walking along a narrowing path of high ground, with the terrain falling away to your left, right, and ahead.
Spurs are genuinely useful for navigation. Because they’re prominent, linear features, they serve as natural handrails you can follow uphill toward a summit or downhill toward a valley. Many hiking routes follow spur lines for this reason. If you’re trying to match your map to the landscape around you, spotting a spur and confirming its direction on the contour lines is one of the most reliable ways to fix your position. Look for the ground that rises on one side only, compare it to the V shapes on your map pointing toward lower elevation, and you’ll have a solid reference point.